IWO Weather Glossary

From our 2015 Autumn A-Z, a guide to some of the lesser-known terms used in meteorology.

| A |

ABSOLUTE TEMPERATURE: temperature using the Kelvin scale. It is absolute because it is directly related to Absolute Zero and allows two temperatures to be directly compared.

ABSOLUTE ZERO (0 K): zero degrees on the Kelvin temperature scale. The lowest temperature possible, where all molecular motion stops.

ABSORPTION: the process by which radiation is absorbed by a substance, e.g. the air. Ozone absorbs harmful UV-C radiation while water vapour (H2O) and carbon dioxide (CO2) absorb longwave radiation (thereby causing the Greenhouse Effect).

ADIABATIC: meaning no energy transfer between surroundings, e.g. a parcel of air cooling as it rises.

ADIABATIC LAPSE RATE: the rate at which a parcel of air cools or warms as it rises or descends adiabatically, respectively. Dry air cools and warms at the Dry Adiabatic Lapse Rate of 9.8 °C/km, whereas moist air (e.g in clouds) cools and warms at the slower Moist Adiabatic Lapse Rate of ~6.5 °C/km.

ADVECTION: the transport of some property of the air into a location, e.g. cold air advection by a northerly wind in winter.

ADVECTION FOG: fog formed by the advection of warm and moist air over a cold surface, e.g. a cold lake.

ALBEDO: a measure of how reflective a surface is. It is the ratio of how much radiation it reflects compared to how much it receives. Values near 0 mean that it absorbs almost all of the radiation whereas values near 1 mean it reflects almost all of it. Fresh snow has an albedo of around 0.8 as it reflects a lot of the sunlight (which is why skiers wear shades). Calm deep water has an albedo of <0.1.

This is the problem with the melting polar ice cap. As more ice melts it exposes more dark sea, which absorbs more radiation, warming up the sea, which melts more ice, and so on, like a chain reaction.

ALTO-: a prefix that signifies high, e.g. altocumulus, altostratus.

ANEMOMETER: an instrument that measures windspeed and direction.

| B |

BALL LIGHTNING: a very rare ball-shaped glowing discharge, up to 30 cm in diameter, that can float eerily for a few seconds before disappearing.

BAROCLINIC: (not a clinic for treating alcoholics) an area that has a temperature gradient, i.e. fronts. Our storms are baroclinic in that they form along the polar front.

BAROTROPIC: the opposite to baroclinic, meaning no temperature gradient, fronts, etc. A hurricane is a barotropic system but converts to a baroclinic depression before reaching Ireland.

BAROMETER: an instrument for measuring pressure.

BLOCKING HIGH: an anticyclone that stays almost stationary, deflecting or blocking the normal path of depressions.

BOUNDARY LAYER: the layer of atmosphere nearest the ground that is affected by daily heating, convection, turbulence, etc. It can be just a few tens of metres deep in cold stable weather to a couple of kms deep in unstable weather.

| C |

CAPE (Convective Available Potential Energy): a measure of how unstable the atmosphere is, i.e. how fast the atmosphere cools with height. The faster it cools the more buoyant rising warm and humid air parcels will be, leading to showers and thunderstorms. Measured in Joules/kg, with >1000 being a very rough guideline for Ireland.

CLOUD: a visible collection of tiny water droplets or ice crystals suspended in the air. Types of cloud include CUMULUS (heaped cotton wool cloud) and CIRRUS (high wispy cloud made of ice crystals).

CEILING: the height of the lowest layer of broken or overcast cloud cover (>5/8ths of the sky covered). Measured visually or using a CEILOMETER (an instrument that sends a laser beam upwards and measures the time for its reflection from the cloud).

COL: an area of slack weather between two highs and two lows on a weather chart.

COLD FRONT: the dividing line between advancing cold air and retreating warmer air.

CONVECTION: the process of warm air rising due to its lower density than its surroundings. The same as boiling water bubbling in a pot.

| D |

DENDRITE: those classic hexagonal snowflakes that we all know, with some very complex patterns, each of which is unique. Dendrites form when the cloud temperature is between -10 to -20 °C, and the more complex patterns occur where humidity is greatest.

DEW: condensed water that forms on a cold surface due to the air near the surface cooling to the dewpoint.

DEWPOINT: the temperature to which air must be cooled so that its relative humidity is 100% (saturated) and further cooling will cause condensation (cloud/fog/dew). The more humid the air the higher the dewpoint.

DENSITY: the mass of something divided by its volume. At sea-level, the average air density is 1.225 kg/m³ and falls by around 9%/100 m altitude in the lower atmosphere, however, density varies with temperature, pressure and moisture content.

DOBSON UNIT: a unit of concentration of ozone in the atmosphere (normally stratosphere). It has a tricky definition. 1 D.U. = 0.001 atm-cm of ozone. An atmosphere-cm of ozone means that if you took all of the ozone in a vertical column through the whole depth of the atmosphere and brought it to a pressure of 1 atmosphere (1013.25 hPa) at 220 K (-53 °C) then the layer of ozone would be 1 cm thick (i.e. 1000 D.U.). In reality the ozone layer contains around 0.3 atm-cm (300 D.U.).

DRIFTING SNOW: (who remembers January ’82?). Snow that is unevenly distributed in piles by the wind.

DRIZZLE: tiny (0.2-0.5 mm) water droplets slowly falling.

DROUGHT: an extended period of reduced or no rainfall, long enough to cause a problem (to e.g agriculture). There is no strict definition, as the effects vary from country to country. In Ireland, however, we’ve seen that 2 weeks without rain in summer can cause grass to go yellow.

DUST DEVIL: a small rotating whirl of air that is unconnected to a cloud and indeed forms from the surface upwards on hot sunny days.

| E |

ECMWF: European Centre for Medium-range Weather Forecasting.

EDDY: a small circulation of air that behaves totally differently to the main flow. Turbulent eddies form, for example, as wind is disturbed as it flows over/around buildings, etc. A tornado is also an eddy. You can also see eddies in a river as the water forms little swirls as it flows around rocks.

ELECTRICAL STORM: a thunderstorm. Electrical because of the lightning.

EL NIÑO: warmer than normal waters in the tropical east Pacific, cooler in the west, with an associated breakdown of the easterly trade winds. This can have wide-ranging effects on the global circulation. We are now in a strong El Niño, part of the ENSO (El Niño Southern Oscillation). The opposite is La Niña.

EVAPORATION: the conversion of a liquid to a gas, i.e. “drying up”, due to the molecules receiving enough energy to escape from the surface tension of the liquid surface. Water evaporates off your washing on the line. The opposite is Condensation.

EXTRATROPICAL: referring to outside the tropics, e.g. our latitude. Tropical Cyclones (e.g. hurricanes) transform into Extratropical Cyclones as they become caught up in the prevailing westerlies and move northeastwards over cooler waters. This is why we cannot get hurricanes in Ireland.

EYE: of a hurricane. The clear area at the centre of a hurricane that is cloud-free and almost calm. Surrounding the Eye is the EYE WALL, which is a big wall of thunderstorma and is where the strongest winds are found.

| F |

FAIR WEATHER: those nice days with less than half the sky covered by cumulus cloud, no rain and good visibility.

FALLSTREAK HOLE: another name for a Punch Cloud. A spectacular, almost perfectly circular or oval, hole in an altostratus or altocumulus stratiformis cloud layer. It is caused when supercooled droplets in the clouds are made to instantly freeze at a point, turning them to ice crystals, which fall out of the layer. This freezing process quickly spreads outwards to neighbouring droplets to form the hole. A common cause is an aeroplane climbing or descending through the layer, acting as a trigger for the process.

FEW: an official term to describe cloud-cover to an extent of 1 or 2/8ths.

FLIGHT LEVEL: abbreviated FL. The altitude of an aircraft when its altimeter is set to a sea-level pressure of 1013 hPa (29.92 in Hg). This is done for altitudes above 3000 ft in Ireland, and it ensures that all aircraft will be showing identical altitudes if they are indeed at the same height. If they didn’t use FL and had different pressure settings set then they could be showing the same altitude even though in reality it would be different, risking a collision. FL370 means 37,000 feet, etc.

FLURRY: a short fall of light snow, giving at most just a dusting.

FOEHN (FÖHN) WIND: a warm and dry wind that forms on the leeward downslope of a mountain. As moist air rises on the windward side of the mountain it cools at a rate of 6.5 °C/km, losing some of its moisture through rainfall. Crossing the peak it starts to sink down the leeward side, now warming at a faster rate of 9.8 °C/km (through compression, like a bicycle pump heats up). By the time it’s back to the original altitude it is warmer and also drier (as the moisture has fallen out).

We get some Foehn winds to the north of high ground, although our mountains are not high enough to give more than a couple of degrees of extra heating.

FOG: tiny water droplets that have condensed out at ground level due to cooling or addition of moisture. It is basically cloud at ground level. Fog gives visibility <1 km whereas Mist is >1 km

FRACTUS: a ragged type of cloud that forms in unsettled weather. Two types are Cumulus fractus and Stratus fractus.

FROST: ice crystals on a cold surface caused by deposition of water vapour.

FREEZING FOG: fog composed of supercooled water droplets (liquid drops below 0 °C) which freeze on contact with a surface.

FREEZING RAIN: supercooled rain droplets that freeze on contact with a cold surface. It can occur at the end of a cold spell when rain from a warm front falls on surfaces that are still below 0 °C.

FRONT: the boundary between two different airmasses. There are cold, warm and occluded fronts. FRONTOGENESIS/FRONTOLYSIS is the strengthening/weakening of a front.

FUJITA SCALE: a scale for classifying the intensity of tornados based on the extent and type of damage sustained. F0 is weak while F5 is devastating.

FUNNEL CLOUD: a rotating column of air extending from the base of a cumulonimbus cloud. If it touches the ground it is a tornado.

| G |

GALE: sustained wind of 34-47 knots (63-87 kph).

GEOSTATIONARY SATELLITE: a satellite stationed at ~35,800 km above the Earth, which makes it in GEOSYNCHRONOUS ORBIT (i.e. moving with the rotation of the Earth so that it is always above the same spot).

GLACIATION: the transformation from liquid to ice particles. The upper part of a thunderstorm (the anvil) is that flat wispy top that is composed of ice crystals.

GLAZE: a layer of smooth clear ice that forms on solid object during freezing rain.

GRASS-MINIMUM: the minimum temperature of a grass surface. It is measured by a thermometer placed along the top of the blades of grass. Grass-minima can be several degrees colder than the air temperature, which is measured at around 1.5 metres above the grass. This can lead to grass frost even when the air temperature is a few degrees above zero.

GRAUPEL: snowflakes covered by a layer of rime (ice). They look like little white balls of polystyrene and will crush easily to reveal the snowflake inside. They form when strong updrafts push a falling snowflake back up into the supercooled area of the cloud (such as during those Lake-effect snow showers along the east coast in December 2010).

GRAVITY WAVE: a disturbance in the flow in which buoyancy restores an air parcel back to its equilibrium level. In other words, air flows over a mountain, rises but is more dense than its surroundings so falls back down and oscillates about its original level. We can see Gravity waves from a regular pattern of lines of cloud on a satellite picture.

GREENHOUSE EFFECT: the natural warming of the atmosphere due to the trapping of longwave radiation by certain gases (e.g. water vapour, carbon dioxide, dinitrogen oxide, or any molecule with 3 atoms). Water vapour is by far the strongest greenhous gas. Without the greenhouse effect the atmosphere would be about 33 °C cooler than it actually is (about -18 °C instead of about +15 °C)!

GROWING SEASON: the time between the last destructive frost of Spring to the first destructive frost of Autumn.

GUST: a sudden increase in windspeed by at least 10 mph (16 kph) and lasting <20 seconds.

GUST FRONT: the leading edge of a downdraft of a thunderstorm, i.e. that rush of cool air that falls out of a thunderstorm and shoots ahead of it, bringing cooler and gusty winds.

GUSTNADO: a weak and short-lived tornado that forms along a gust front, usually visible as a swirl of dust or debris.

| H |

HAAR: fog that forms over the sea during Spring and Summer as warm and moist air flows over a cold sea surface. We can get this along east and south coasts, especially April-June, when warm air off the continent flows over the still-cold Irish and Celtic Seas. It can cause a disappointing cool and murky day in these areas while the rest of the country enjoys nice warm sunshine.

HAIL: hard spherical or conical particles of ice precipitation. If cut open they have a layered appearance (like an onion) because they have been repeatedly flung back up into the supercooled area of the cloud by the strong updrafts, each time acquiring a layer of rime ice.

HALO: a ring encircling the sun or moon caused by the refraction of light through tiny hexagonal ice crystals way up in the thin cirrus or cirrostratus clouds. We can get these clouds a few hours ahead of the arrival of a warm front.

HAZE: a suspension of tiny dry particles of dust, salt, pollen, pollution, etc. that reduces visibility and gives the air a bluish whitish hue.

HEAT ISLAND: a dome of warm air over a large urban area due to the heat of the buildings, roads, pavements, etc. That’s why built-up areas generally don’t get as cold as rural areas and why the thermometer in your garden or your car is not a good representation of temperature.

HEAVY: a qualifier used to describe the intensity of rain. Heavy rain reduces visibility to 400 m or less.

HIGH: an area of high pressure (anticyclone) caused by a mass subsidence (sinking) of air from upper levels. This subsidence compresses and warms the air (like a bicycle pump), so in general clouds are reduced or eliminated, however not always (we can get a stubborn layer of stratocumulus forming just below the subsidence inversion (a warm and dry layer of air) or even haar (see above)).

HORSE LATITUDES: subtropical areas dominated by high pressure (anticyclones). These extend to around 30-40 ° north and south of the Equator (e.g. The Canaries, Australia).

HUMIDITY: the amount of water vapour in the air. Specific Humidity is the actual concentration, in g/kg, and Relative Humidity is how much is in the air compared to much that air could hold, in % Warm air can hold more vapour than cold air.

HURRICANE: a severe Atlantic tropical cyclone with sustained windspeeds of at least 64 knots (74 mph/119 kph). They are warm-core barotropic (see letter B) systems with an eye in the centre and the strongest winds concentrated around the eye. They form from clusters of thunderstorms over an area of warm seas (at least 26.5 °C) and low vertical wind-shear (change of wind with height). We can NOT get hurricanes in Ireland because the seas are too cold and the jet stream tears the structure apart, however we can get storms with hurricane-force winds (e.g February 2014). Debbie in 1961 is the nearest we got to an actual hurricane. Charlie in 1986 was NOT a hurricane when it reached us. It had long since transitioned into an extratropical system before reaching us.

HYDROMETEOR: a particle of precipitation (liquid or solid) that forms in the sky (e.g. cloud droplets, rain, ice crystals, etc.).

| I |

ICE AGE: A period during which the polar ice caps extend to lower latitudes and the global temperature decreases. The last ice age ended around 10,000 years ago but part of it still remains as the Greenland ice sheet.

ICE CRYSTALS (DIAMOND DUST): tiny individual crystals of ice suspended in the air, giving it a glittery appearance. The form when the air is very cold, below -15 °C. I saw some during the deep freeze around Christmas 2010.

ICE FOG: another name for Freezing Fog (see the letter F).

ICE PELLETS: precipitation in the form of frozen rain drops < 5mm diameter. They form when rain from warmer upper levels falls through a deep-enough layer of cold air near the surface.

INFLOW BANDS: those lines of low cloud that you see feeding into a thunderstorm or heavy shower.

INFRA-RED RADIATION: electromagnetic radiation of wavelengths greater than visible light. Also called Longwave radiation (in meteorology. It is not the same as Longwave radio, which has much larger wavelengths). The earth absorbs shortwave (Ultraviolet) radiation from the sun and re-emits it as longwave (heat). This longwave heats the air, which the rises to form clouds, rain, etc. Infra-red satellite pictures are useful for seeing different temperatures clouds and therefore how high up they are, etc.

INDIAN SUMMER: a period of warm weather occuring in Autumn, usually after the first air frost. No one really knows where the term came from but we sure know we like to hear it forecast!

INSTABILITY: a setup in which the air cools quickly with height. It is unstable because if air rises it will continue to rise as it will be warmer than its surroundings. Unstable weather is what gives us towering cumulonimbus clouds, showers and thunderstorms.

INTERTROPICAL CONVERGENCE ZONE (ITCZ): the region near the Equator where northeasterly and southeasterly winds converge, generating a band of showers and thunderstorms. The ITCZ migrates northwards by a few degrees during summer and back southwards during winter. Many tropical storms and hurricanes start off in the ITCZ.

INVERSION: a warm layer of air lying over a cooler one. Normally temperature decreases with height, but sometimes it can actually increase. This happens when air near the surface cools on a clear night or when subsiding air in an anticyclone (high pressure) warms as it compresses. A layer of cloud can form as air rises and hits the inversion, spreading outwards instead of upwards. Inversions can trap pollution, etc. below them, leading to haze and lower visibility.

IONOSPHERE: a layer of the atmosphere above around 80 km where temperature increases with height due to chemical reactions. The ionosphere is used to bounce radio waves off to large distances, which is why you can sometimes pick up MW radio stations from countries huge distances away (especially at night).

IRRIDESCENCE: patches of green or pink light formed by sunlight passing through medium or high clouds.

ISOPLETH: a line on a chart connection places of an equal property, e.g. pressure. Isopleths include Isobars (pressure), Isodrosotherm (dewpoint), Isotach (windspeed), Isotherm (temperature), Isoyet (rainfall), Isoyel (sunshine hours).

| J |

JET-EFFECT WIND: a local wind formed by acceleration of airflow through a mountain pass, valley, etc. It is why sometimes you feel a stronger wind when you’re out walking through the mountains, even though you may not be up that high.

JET STREAK: a core of maximum winds within the Jet Stream.

JET STREAM: a meandering and sometimes discontinuous stream of strong (generally westerly) winds in the upper troposphere at mid-high latitudes, at a height of roughly 8-12 km. It is formed in both hemispheres by the thermal difference between cold air towards the Poles and warm air towards the Equator. There is the Polar Jet Stream, formed along the Polar Jet (at about our latitude) and also the Sub-tropical Jet Stream, which forms at 20-30 ° N and S, but is mostly concentrated over southeast Asia.

The jet stream’s strength and position varies widely from day to day, depending on the largescale setup around the globe, however its average position is to our south during winter and to our north during summer. It is the driving force of many of our weather systems, from depressions to anticyclones, and can cause depressions to deepen rapidly, especially in winter. It is also the reason why flights westward usually take longer than flights eastward.

JOULE: a unit of energy or work. It is the energy required to move 1 kg a distance of 1 metre by a force of 1 Newton.

1 Joule of energy is required to:

– Lift a 100 g apple 1 metre into the air.
– The heat required to warm 4 g of water 1 °C.
– Light a 1-Watt L.E.D. for 1 second.
– Sit on your ass for 17 milliseconds (1 Joule is released from your body as heat). 1 calorie (dietary) is 4,180 J.

In meteorology, CAPE values (see the letter C) are measured in Joules per kg of air (J/kg), with typical values of a few thousand J/kg for severe thunderstorms.

| K |

K-INDEX: an instability index calculated from the amount and depth of the moisture in the lower atmosphere and how fast the temperatures falls with height. Higher values indicate the potential for severe thunderstorms.

KATABARIC or KATALLOBARIC: a term referring to decreasing atmospheric pressure.

KATABATIC: a wind blowing down a downslope of a mountain. It can form when the upper part of the mountain cools and this cold air, being more dense, sinks and accelerates down the slope. Greenland gets some extreme katabatic winds coming down off the glacier.

KATA FRONT: a cold front in which the warm air descends down the frontal surface. This causes clouds to be shallower and rain to be less intense and ahead of the surface front. The opposite is an ANA FRONT. We mostly get Kata fronts in Ireland.

KELVIN: an absolute temperature scale in which zero represents the temperature where all molecular motion stops, i.e. the lowest temperature possible. One degree Kelvin is the same as one degree Celsius, however a Kelvin temperature is always 273.15 degrees less than the Celsius. 0 K = -273.15 °C.

KELVIN-HELMOLTZ CLOUDS: a row of regularly-peaked clouds with their tops fallen over, a bit like the way the upper point of the ice cream of a 99 flops over. It is caused by a marked change in windspeed with height (the cloud, not the ice cream!!).

KNOT: a unit of speed equal to 1 nautical mile per hour (1.15 statute miles per hour (mph), 1.85 kph). It is used in navigation and reporting windspeeds.

KINETIC ENERGY: the energy that something has due to its velocity. It is equal to its mass (kg) times its velocity squared. If you double the speed the kinetic energy increases fourfold.

KÖPPEN CLASSIFICATION: a climate-classification system devised by the German climatologist Wladimir Köppen. It is based on average temperature and precipitation, and also takes into account the type of vegetation (dessert, etc.). Ireland’s climate is classified as Cfb (C=temperate oceanic, f=precipitation spread over all 4 seasons, b=warmest month averages below 22 °C but at least 4 months average above 10 °C).

| L |

LAKE-EFFECT SHOWERS: showers (of rain or snow) caused by the passage of a cold airmass over a relatively warm sea or large lake (such as the Great Lakes in the US or the Irish Sea). Certain conditions must be met:

– the temperature at 850 hPa (around 1500 m up) must be at least 13 °C colder than the sea-surface temperature,
– there must be a long enough fetch over the water, i.e. there must be enough distance for the heat of the water to take effect. If the fetch is too short then the air won’t pick up enough heat to generate the clouds,
– there should be less than 60 ° difference between the direction of the wind at the surface and at 700 hPa (around 3000 m up).

The showers that hit the east coast in December 2010 were formed by this process. A very cold northeasterly airflow from Scandinavia had enough sea-fetch between Cumbria and tge east coast for showers to form. The Isle of Man, however, did cast a “shadow”, breaking up the fetch and causing a shower-free band for some while a few km away others were getting a pasting!

LAMINAR FLOW: smooth flow, the opposite to turbulent flow. Layers of a fluid moving independently of eachother without causing turbulent eddies to form. We can see this flow in the form of lenticular clouds over mountains. An example of laminar flow is when you turn your kitchen tap on slowly. The water first comes out in laminar flow but after a point turns turbulent as it picks up speed.

LA NIÑA: a cooling of the equatorial waters of the east Pacific. The opposite to El Niño. During La Niña events hurricane activity in the Atlantic increases.

LENTICULAR CLOUDS (LENTICULARIS): lense-shaped clouds that form above mountains when a strong laminar airflow is forced to rise and fall. Altocumulus lenticularis and Altostratus lenticularis are two forms.

LAND BREEZE: a breeze blowing from the land to the sea, the opposite to a sea breeze. It occurs when the land is colder than the sea, such as a clear night in early winter.

LANDSPOUT: the land-based version of a waterspout. A tornado that does not form from a supercell (rotating thunderstorm) but more from a cumulonimbus or towering cumulus cloud.

LAPSE RATE: the change in temperature with height in the atmosphere. The standard lapse rate is around 6.5 °C/km or 2 °C/1000 ft but it varies widely from day to day. Steep lapse rates are associated with unstable weather, and vice versa.

LATENT HEAT: the heat required to transform water from its liquid to vapour state. This heat is released when vapour condenses to liquid, such as air rising to form clouds. Latent heat is a huge factor in thunderstorms and is the food devoured by hurricanes. Without this heat they quickly die out.

LIFTED INDEX: for a parcel of air lifted from the surface to the 500 hPa level (~5500 m), L.I. tells how much warmer or colder this parcel is compared to its surroundings. If it is warmer (negative L.I.) then it will continue to rise, and vice versa.

LIFTING CONDENSATION LEVEL: the level at which a parcel of air lifted from the surface starts to condense out and form clouds. You can estimate the LCL by adding 400 ft for every degree difference there is between surface temperature and dewpoint, e.g. temperature 14, dewpoint 8, difference=6, so we would expect cumulus to form at 6×400=2,400 ft.

LIGHTNING: an electrical discharge (huge spark) caused by voltage differences in a thundercloud. The voltage difference can be millions of volts and the current 100,000 Amperes, yielding trillions of Watts of power. This heats the air to sometimes 50,000 °C explosively expanding it and causing a shockwave we hear as thunder.

LITHOMETEOR: any dry particles suspended in the atmosphere. Examples include dust, haze, smoke, sand, etc.

| M |

MACKEREL SKY: a layer of cirrocumulus or altocumulus clouds that resembles the scales on a mackerel. If the individual cloudlets are small (cirrocumulus) then the weather could deteriorate in the next 12 hours and if they’re bigger (altocumulus) then the weather could be geyting better as a front is breaking up.

MAMMATUS CLOUDS: from the Latin for breast/boob. Rounded pouch-like blobs of cloud that protrude from the underside of the anvil of a thunderstorm.

MARINE LAYER: a shallow layer of an airmass that has been modified by its presence over the sea. It can show a marked change in dewpoint (humidity) and temperature in its bottom few hundred metres. As this layer moves in over the coast it can be clear up to a point but then the warm land can cause cumulus and showers to pop up. This is why coastal areas can be clear while further inland can be cloudy.

MEAN SEA LEVEL (MSL): the average height of the sea surface, defined at some location. For Ireland the location is Malin Head. All pressure readings from stations at various elevations are reduced to MSL using a mathematical formula based on the station’s elevation and its average temperature over the past 12 hours. In general, pressure falls 1.0 hPa for every 8.2 m increase in elevation.

MERIDIONAL FLOW: flow (wind) that has a mostly southerly or northerly direction. The opposite is Zonal, which is more westerly or easterly.

MESOCYCLONE: the rotating part of a large supercell thunderstorm from which tornados extend.

MESOSCALE: a feature that has dimensions of a few tens of kms; smaller than synoptic scale but larger than individual clouds.

METAR: a routine coded weather report from an airport, issued every 30 minutes in Ireland. It shows current conditions and any possible changes expected within the next 2 hours.

MIE SCATTERING: scattering of electromagnetic radiation by small particles of uniform dimensions. This is the reason why the sky looks a paler blue or white as you look towards the sun or down towards the horizon. Mie scattering is not wavelength-dependent, so all wavelengths are scattered equally to give the white hue. Compare it to Rayleigh Scattering.

MILLIBAR (mbar): a unit of pressure, interchangeable with the more commonly used hectopascal (hPa). Standard atmospheric pressure at sea-level is 1013.25 mbar (14.7 psi) and falls by 1 mbar for every ~8.2 metres increase in altitude in the lower atmosphere.

MIRAGE: an optical illusion which makes it seem that there is a body of water in the distance when in fact there is not. It is caused by refraction (bending) of light from the sky through the hot (and less dense) layers of air nearest the ground.

MIST: a cloud of condensed minute water droplets at ground-level that reduces visibility to not less than 1000 m. If visibility is less than 1000 m then it is fog.

| N |

NACREOUS CLOUDS: also called Mother-of-pearl or Polar Statospheric clouds. Wispy cirrus clouds occuring very high up (20-30 km) over certain areas (Antarctica, Alaska, northern Europe). They show brilliant colours of the spectrum during and after sunset and before sunrise. They are composed of nitric acid hydrates and form on sulphuric acid particles originating from volcanos. Keep an eye out for them this winter.

NANOMETER: one billionth of a metre or one millionth of a millimetre. The wavelengths of visible light range from about 400-750 nm.

NAUTICAL MILE: a unit of distance equal to 1 minute of latitude (1.852 km or 1.15 statute miles). There are 60 NM in 1 ° of latitude, so our rough latitude of 53 °N is 53×60=3180 NM (5883 km) from the Equator. Way too far…😐

NEWTON: the unit of force. 1 newton is the force required to accelerate a mass of 1 kg by 1 metre per second squared. It is the force you exert against gravity in holding a 100 g apple in your hand.

NEWTON’S LAWS OF MOTION: 3 laws that describe how forces act on an object (e.g. air molecules hitting your face as wind, snooker balls, gravity, etc.).

– An object at rest or at a constant velocity will remain so unless acted on by a force.
– Its change in momentum is equal to the force applied.
– For every action (force) there is an equal and opposite reaction.

NIMBOSTRATUS: Ns, the bane of our life in Ireland. A large thick, grey, featurless cloud the brings rain or snow. Nimbo- comes from the latin for “dark cloud” and is used to mean “rain-bearing cloud). This cloud occurs in our typical frontal systems where cirrus thickens to cirrostratus, then altostratus and then nimbostratus as the warm front approaches.

NOCTILUCENT CLOUDS: from Latin, meaning “lit at night”. Silvery clouds that are visible during the night in summertime north of around 50 ° latitude. They form extremely high up (around 85 km) and are made up of tiny ice crystals that form around particles of dust from meteorites.

NOR’EASTER: a term you may hear for a strong winter storm that form along the east coast of the US. It’s called nor’easter because it brings strong northeasterly winds to coastal areas.

NORTH ATLANTIC OSCILLATION (NAO): a see-saw of atmospheric pressure between the Iceland low and the Azores high. When the Iceland low is strong and/or the Azores high is strong then this forms a tight gradient and strong winds along which storms can form. This is a positive NAO.

A negative NAO is when is when pressure is higher over Iceland and lower over the Azores, meaning weak or even no westerlies, possibly bringing us cold snowy northerlies or easterlies in winter.

NUMERICAL WEATHER FORECAST: a forecast state of the atmosphere as calculated by a computer model. There are many computer models out there, the main ones being the ECMWF, UKMO (UK Met Office), GFS (US Global Forecasting System), HiRLAM (Hi-Resolution Limited Area Model).

| O |

OASIS: an area of vegetation in a desert. It is caused by a local rise in the water table or an underground spring.

OBSCURATION: anything in the atmosphere that reduces visibility (e.g. fog, haze, blowing snow, sand, etc.). Rain is not an obscuration.

OCCLUDED FRONT: a front formed by the cold front catching up with, and undercutting, the warm front in a typical depression. When this happens the depression will stop deepening and will start to move to the left of its current track.

OFFSHORE BREEZE: another name for a land breeze. A breese that blows from land to the sea, the opposite to a sea breeze. Surfers like offshore breezes.

ONSHORE BREEZE: the opposite to an Onshore breeze. In other words, a sea breeze.

OROGRAPHIC LIFT: air that is forced to rise over orographic features (i.e. mountains). As it does so it expands and cools, condensing into clouds and rain. This is what happened in Kerry and Connemara over the past week and is why annual precipitation increases by 100-200 mm for every 100 m rise in elevation in Ireland, highest in the west.

OUTFLOW: air that flows out from a thunderstorm. It is that wave of cooler and gustier air that you feel, formed by dense rain-cooled air falling out of the the cloud and spreading along the surface, like spilled milk does on your kitchen floor.

OUTFLOW BOUNDARY: a boundary between the cool outflow air and the warmer air. It’s like a mini cold front.

OVERCAST (OVC): greater than 90% of the sky covered in cloud. Don’t we know it!

OVERRUNNING: warm moist air overrunning cooler denser air at the surface to form stratus clouds and light rain/drizzle.

OVERSHOOTING TOPS: very strong updrafts in thunderstorms that break up through the relatively flat anvil of the cloud and form bumps, visible from a distance and from satellite. They indicate the potential for severe weather.

OZONE: a form of oxygen with 3, not 2, atoms in its molecule. There is a layer of ozone in the stratosphere which filters out most of the sun’s harmful UV rays. This layer forms when UV breaks normal oxygen molecules into very active single atoms, which the react with other oxygen molecules to make a triatomic ozone molecules.

Ozone is also a pollutant at ground level.

OZONE HOLE: a hole in the ozone layer over Antarctica which forms every Spring due to the reaction of fluorocarbons with nacreous cloud particles.

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PANCAKE ICE: sea or river-ice in the form of flat circular sheets a couple of metres in diameter and with raised edges. The raised edges are caused by collisions with other ice.

PANNUS CLOUD: an accessory cloud (cloud that only forms with another main cloud) that forms as ragged shreds below the main cloud. We see them regularly in wet, dreary weather, when rain falls out of a cloud and partially evaporates, increasing the relative humidity below the main cloud until it reaches saturation.

PARCEL: an imaginary “bubble” of air that can be used to represent what happens air when it rises (i.e. how fast it will cool, saturate, etc.). Parcel theory says that a parcel of dry air will cool at 9.8 °C/km as it rises adiabatically until it saturates, then it will continue to cool at the slower rate of roughly 6.5 °C/km as it rises and forms clouds.

PARHELION: a coloured halo that forms on either side of the sun. It is caused by the refraction of light through vertically-aligned hexagonal ice crystals in cirrus or cirrostratus clouds.

PARTIAL PRESSURE: the pressure exerted by each individual gas component in our atmosphere. Nitrogen (78%) and Oxygen (21%) are the main gases, but there are others, such as water vapour, CO2, argon, etc. The total atmospheric pressure is the sum of all the partial pressures.

PERIHELION: the nearest point of the Earth’s orbit to the sun. This occurs in our winter, or the souther summer. Solar radiation is around 7% stronger at this time, which is why summer sunbathing in Australia is a little more dangerous than sunbathing at a similar latitude in the northern hemisphere.

PERLUCIDUS: a layer of stratocumulus or altocumulus containing distinct gaps or holes through which the sun or sky can be seen.

PERMAFROST: a sub-layer of soil that remains permanently frozen. Summer melting of the topsoil fails to penetrate down to the permafrost. It occurs in Tundra locations where the average annual air temperature is -5 °C or less.

POLAR AIRMASS: a cold and dry airmass that forms in polar regions, Greenland, etc. Many of our cool and showery weather comes from polar maritime northwesterlies, where polar air becomes unstable as it passes over the Atlantic between us and Greenland/Iceland.

POLAR FRONT: the semi-permanent front around our latitude that divides polar air from tropical air. Most of our low pressure systems form along kinks in this front.

POLAR STRATOSPHERIC CLOUDS (PSC): another name for Nacreous clouds.

POLAR VORTEX: this has become a popular but mis-used term in the media. It is a circumpolar circulation that forms every winter due to the very cold and dense air over the Pole. When it is strong it normally holds this cold air near the Pole, but it can get disrupted and split under certain circumstances, causing part of this cold air to flow south as cold wintry outbreaks.

This is nothing new, but some of the media have recently latched onto this buzz term to sensationalise cold weather events.

POPCORN CONVECTION: convection in which scattered cumulus forms in a random pattern, with no real organisation. It forms in slightly stable conditions during the heat of the afternoon.

PRECIPITATION: rain, hail, snow, etc. that falls from the sky.

PRESSURE: the pressure exerted by the weight of the atmosphere above a point. As you go higher and higher there is less atmosphere above you and the pressure is therefore lower. It falls by around 12 hPa (millibars) for every 100 metres.

PRESSURE-TENDENCY: a trend and extent of the pressure change over the past 3 hours (e.g. rising slowly, falling rapidly). This can give an idea of the weather to come. Falling rapidly means a strong depression is arriving, etc.

PREVAILING WIND: the wind direction that occurs most frequently at a location. In Ireland it is southwesterly.

POSITIVE VORTICITY ADVECTION (PVA): the movement (advection) of air with positive vorticity into an area. Vorticity means spin, which can enhance upward motion of air, leading to enhanced precipitation and deeper storms, etc.

PUNCH CLOUD: also called a Fallstreak Hole. A spectacular, almost perfectly circular or oval, hole in an altostratus or altocumulus stratiformis cloud layer. It is caused when supercooled droplets in the clouds are made to instantly freeze at a point, turning them to ice crystals, which fall out of the layer. This freezing process quickly spreads outwards to neighbouring droplets to form the hole. A common cause is an aeroplane climbing or descending through the layer, acting as a trigger for the process.

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QFE: in aviation, a code that represents station pressure, e.g. QFE1012. If a pilot sets QFE in the altimeter it will show him his height above the station, not above sea-level. His altimeter will read 0 when he is on the ground.

QNH: another code used in aviation to mean the pressure reduced to sea-level. If a pilot sets QNH in the altimeter is will show her her altitude above sea-level, not the airport. Her altimeter will show the airport elevation when she is on the ground. QNH is the pressure reported in metars, e.g. Q1022.

QUASI-BIENNIAL OSCILLATION (QBO): a flip in the winds in the equatorial stratosphere that occurs roughly every 13-14 months. It can affect the northern hemisphere winter through promotion of Sudden Stratospheric Warmings (SSWs), which in turn can disrupt the Polar Vortex and bring cold air southwards.

The QBO is currently in the same position as it was this time 2010, so we’ll see if our winter turns out the same!

QUATERNARY PERIOD: roughly the last 2 million years in geological terms.

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RADAR (RAdio Detection And Ranging): a system that sends out a beam of electromagnetic radiation and measures the time for receipt of its reflection off objects (e.g. rain drops, hail, etc.). By rotating the emitter and receiver the location of the objects can be determined.

Radar is very useful for showing the current location and intensity of rainfall. It is done on a log scale as the intensity of reflection of the beam increases with the 6th power of the diameter of the raindrop. That means that if one drop is twice as big as another then its reflection will be 64 times stronger (2 to the power of 6). Rainfall intensity is proportional to not only the diameters, but also the number, of drops, so the rainfall rate (in mm/hr) can be determined and displayed as a colour scale.

RADIATION: energy emitted as photons by electrons of an atom. The sun emits huge quantities of radiation due to a nuclear reaction in its core. The energy of the radiation depends on its wavelength. Shorter wavelengths have more energy (higher frequency). Ultraviolet (UV) has shorter wavelengths than visible light, which has shorter wavelengths than infra-red, which has shorter wavelengths than say radio waves, etc. For UV to infra-red, wavelengths are from a few hundred to a couple of thousand nanometers (millionths of a millimetre), while radio waves have wavelengths of up to a few kms.

The ground absorbs UV radiation and remits it as infra-red radiation, heating the air above it. This is how our weather works.

RADIATION FOG: fog formed on cold clear nights by the ground cooling quickly and
cooling the air near it to below its dewpoint.

RADIATIONAL COOLING: the ground losing its heat quickly on clear nights. Its infra-red radiation emitted gets lost to Space because there is no cloud to reflect it back down. This cooling can lead to fog and frost.

RADIOSONDE: a package of instruments attached to a weather balloon. The instruments measure pressure, temperature and relative humidity as the balloon ascends up to the stratosphere. By tracking it with GPS its height and velocity can be determined, and therefore also the winds at different heights. In Ireland two radiosondes are sent up twice a day, one from Valentia, Co. Kerry, and the other from Castor Bay, near Craigavon, Co. Armagh.

RAIN: liquid drops that fall out of a cloud. They have a diameter >0.5 mm. Most of the rain in Ireland is actually melted snow from several thousand feet up.

RAINBOW: we all know what it is. It is formed by the refraction and reflection of sunlight by drops if liquid water (e.g. rain, spray from your garden hose, etc.). Each drop acts like a prism, refracting the light into all the colours of the spectrum. Different colours (wavelengths) are refracted by different angles, which is why they all appear beside eachother in the form of a coloured band.

RAYLEIGH SCATTERING: scattering of radiation by particles much smaller than the wavelength of the radiation. Scattering is inversely proportional to the 4th root of the wavelength (if you halve the wavelength the scattering is 16 times stronger. Nitrogen molecules in our atmosphere scatter out the blue (shortest) wavelength of sunlight 10 times more than the longer red wavelength, which is why the sky appears blue. Compare it to Mie Scattering.

REFRACTION: the bending of light as it passes through two substances of different densities (e.g. air and water).

RELATIVE HUMIDITY: the actual amount of water vapour in the air compared to how much the air at that temperature could actually hold if it was just saturated. R.H. of 60% means the air is 60% on its way to saturation. Add more vapour or reduce the temperature and this value will increase, and vice versa.

RETROGRESSION: weather systems moving in an east->west direction, as opposed to the normal west->east direction. This happens when the number of large waves in the upper pattern around the globe is small (e.g. 2 or 3). It can mean that our weather changes relatively slowly, so we don’t get a barrage of depression after depression.

RIDGE: an elongated area of high pressure, without a closed circulation. The opposite is a Trough. Ridges form behind a depression and bring some finer weather, but they normally don’t hang around for long.

RIME: ice formed when small drops of supercooled water freeze on contact with a cold surface (e.g. an aircraft wing as it flies through a cloud below 0 °C).

ROPE FUNNEL: a thin, rope-like funnel that is usually formed as a tornado is dissipating or “roping out”.

RUNWAY VISUAL RANGE (RVR): the slantwise visibility that a pilot has as he is approaching to land. It is reported in METARs when it gets below 2000 metres. It is measured by instruments located at the two ends and also the middle of the runway. You may see these instruments at the airport. They are like two security cameras looking at eachother. One is sending a beam to the other, which receives it and measures its intensity. The loss in strength is proportional to the number of particles in the air, so the visibility of the air can be calculated.

Have a look for these instruments the next time you’re at the airport.

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SAFFIR-SIMPSON HURRICANE SCALE: a scale of hurricane-intensity devised by Herbert Saffir and Robert Simpson. Category 1 is from 64-83 knots while Category 5 is >135 knots.

ST. ELMO’S FIRE: an electrical discharge that occurs on objects, e.g. an aircraft flying near a thunderstorm. When the electric field-strength is 100,000 V/m or more the charge build-up becomes too much and it is discharged as an eerie glow.

SANDSTORM: a vast cloud of sand particles carried aloft by the wind. These frequently occur in the middle east and the US mid-west.

SATURATION: when air is holding the maximum amount of water vapour possible for that temperature. Any more water vapour will result in condensation. Its relative humidity is 100%.

SEA-BREEZE: a breeze from the sea that starts up during the morning as the land becomes warmer than the sea. This causes the air over the land to rise, with air from the sea rushing in to fill the void. Sea-breezes can be a pain, especially along the east coast in early summer, as they can ruin an otherwise nice day at the beach.

SEA-LEVEL PRESSURE: pressure of a location expressed as if that location were at sea-level. A calculation is applie to the actual pressure measured at the station, taking into account its altitude and average temperature over the past 12 hours. Pressure falls by 1 hPa for roughyl every 8.2 metres increase in altitude. Expressing all station pressures as sea-level pressures allows them to be directly compared on surface charts.

SHORTWAVE RADIATION: the sun’s radiation of shorter wavelengths that are abosorbed by the surface and re-emitted as warmer longwave radiation.

SHORTWAVE TROUGH: a disturbance in the upper atmosphere that can cause showers and thunderstorms to kick off.

SHOWER: a short, sharp fall of rain, hail or snow from a cumulonimbus or towering cumulus cloud. It starts and stops abruptly. April showers are famous in Ireland as cool airmasses from our northwest become destabilised as the sun’s heating of the land becomes effective during April.

SLEET: in Ireland and the UK it is taken to mean “a mix of rain and snow”, or “semi-melted snow”. It occurs when the temperature is slightly too warm for snow to survive its freefall to the surface in its original frozen state.

SLUSH: partially melted snow.

SMOG: from the words “SMoke” and fOG”. Pollution from motor vehicles or the burning of fuel, sometimes mixed with fog. In Dublin there was a big problem with smog up to the 1980s until smokeless fuel was introduced. Smog can become bad in times of high pressure, where the pollution becomes trapped under the temperature inversion.

SNOW: frozen precipitation in the form of flakes of normally hexagonal ice-crystals. These crystals form by deposition of water vapour onto a growing crystal. The temperature and humidity is crucial in forming the shape. Simpler shapes (discs, needles) form when the humidity is dry, while more complex patterns form when the humidity is high. The complex dendrite pattern forms below -20 °C, while colums and needles form between -40 and -10 °C and also below -20 °C. In Ireland we get mostly large flakes of snow because the temperature is never too cold. Some of the flake melts, causing individual flakes to stick to one another, forming large fat flakes.

SNOW GRAINS: tiny white grains of ice, formed by the freezing of drizzle.

SNOW PELLETS: (another name for Graupel). White, opaque grains of ice with diameters of around 2-5 mm. They form when supercooled water freezes around a snowflake.

SOUNDING: a vertical plot of temperature, dewpoint and wind throughout the atmosphere, as measured by a radiosonde attached to an ascending weather balloon. Soundings are extremely important to forecasters as they show subtle features in the atmosphere that models may not pick up, especially when forecasting snow or thunderstorms.

SQUALL: a sudden increase in wind to at least 22 knots for at least one minute.

SQUALL LINE: a non-frontal line of thunderstorms.

STABILITY: a condition in which temperature falls slowly with height, meaning it is difficult for air to rise and form deep clouds. We get stable weather with high pressure and on cold clear nights, wher the temperature near the surface can be colder than higher up.

STANDARD ATMOSPHERE: an internationally-agreed representation of the atmosphere based on average values. Aircraft are calibrated using standard atmospheric values. Standard surface temperature is 15 °C and pressure is 1013.25 hPa.

STORM: a depression with windspeeds of at least 48 knots (Beaufort force 10).

STRATOCUMULUS: a common cloud in Ireland in the form of a relatively flat layer of broken cumulus elements. It is a sign of relatively stable weather.

STABILITY: a condition in which temperature falls slowly with height, meaning it is difficult for air to rise and form deep clouds. We get stable weather with high pressure and on cold clear nights, wher the temperature near the surface can be colder than higher up.

STRATOSPHERE: the stable layer of atmosphere above the troposhere, from about 15-50 kilometers. The ozone layer is in the stratosphere.

SUPERCOOLED WATER: water that is still liquid even though the temperature is below zero. Many clouds are composed of supercooled water, which can be down to -10 °C. This is a hazard to aircraft unless the aircraft has anti-icing equipment installed.

SUPERCELL: a severe thunderstorm with a rotating updraft. Many tornados form from supercells.

SYNOPTIC CHART: a surface chart showing fronts, pressure systems, etc., over a large area. Synoptic stations report observations at least every 6 hours so that these charts can be generated. Ireland has 26 such synoptic stations.

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TAF: Terminal Area Forecast. A coded forecast produced for an area surrounding an airport. They can be valid for 9, 24 or 30 hours and are issued at least 4 times per day.

THERMAL: a warm rising parcel of air that forms due to solar-heating of the surface. These thermals can form cumulus clouds and are also what gilder pilots use to stay airborne. Some thermals can be stronger than other due to differences in the surface (e.g. a forest versus a ploughed field).

THETA-E: theta is the Greek letter for T. Equivalent Potential Temperature is the temperature an airmass would have if it lost all of its moisture and it sank and warmed adiabatically to 1000 hPa. The 850 hPa level is usually used. This shows how “tropical” an airmass is and how likely it is to form thunderstorms, snow, etc.

THUNDER: the sound generated by the shockwave as lightning heats the air to thousands of degrees. The closer the lightning is the “cracklier” (is that a word?!) and shorter the thunder will be. Long rumbles are heard when thunder produced at different points (and hence distances) along the lightning channel arrive to our ears.

THUNDERSTORM: a cumulonimbus cloud that has generated enough of a voltage difference to produce lightning. We don’t get many severe ones in Ireland because the temperature and moisture values of our airmasses just don’t get high enough often enough.

TORNADO: a violent rotating funnel of air extended from a cumulonimbus and in contact with the ground. If it is not in contact with the ground then it is a Funnel Cloud, not a tornado. We get several small tornados in Ireland every year and in fact the UK has the most tornados per sq. km of any country in the world!

TRADE WINDS: persistent winds that blow from sub-tropical latitudes towards the Equator. They are generally northeasterly in the northern hemisphere and southeasterly in the southern hemisphere.

TROPICAL STORM: a low-pressure system that forms in the tropics and has sustained winds of at least 34 knots. If it strengthens further it can become a hurricane (>63 knots).

TROPIC OF CANCER: the latitude of 23.5 ° North, the northernmost extent of the sun (occurs at the summer solstice, June 22nd).

TROPIC OF CAPRICORN: the latitude of 23.5 ° South, the southernmost extent of the sun (occurs at our winter solstice, December 22nd).

TROPICS: the area between the Tropic of Cancer and the Tropic of Capricorn.

TROPOSPHERE: the lowest layer of the atmosphere, from the surface to around 8 km at the Poles and around 15 km at the Equator. All of our weather happens in the troposphere. Above the troposphere is the stratosphere.

TROUGH: an elongated area of lower pressure, the opposite to a ridge. It can cause showers and thunderstorms to form.

TYPHOON: the name for a hurricane in the western Pacific.

| U |

ULTRAVIOLET RADIATION: electromagnetic radiation of wavelengths shorter than visible light  (from 5-400 nm). UV accounts for about 9% of the total energy from the sun.

UV-A: UV radiation with wavelengths from 320-400 nm. This causes tanning of the skin.

UV-B: UV radiation with wavelengths from 290-320 nm. This can cause sunburn and skin cancer. It is only partially filtered out by the ozone layer.

UV-C: UV radiation with wavelengths from 200-290 nm. This is extremely harmful to us but is luckily almost all filtered out by the ozone layer.

UPDRAFT: an upward current of air, very strong in thunderstorms (can reach speeds of over 150 kph in severe storms).

UPSLOPE FLOW: flow that is forced to rise up along high terrain. In doing so it can condense to form clouds and rain.

URBAN HEAT ISLAND (UHI): a dome of wamer air that surrounds a built-up area or city. Cities are warmer due to the building trpapping, reflecting and emitting extra heat compared to more open areas. Some calm nights in large cities can be 10-12 °C warmer than the surrounding rural areas!

UTC: Universal Coordinated Time (the same as GMT). In aviation it is given the letter Z (e.g. 1430Z = 1430 UTC/GMT). All flights in the world use UTC, even if they are 12 timezones away. Ireland is UTC during the winter and UTC+1 in the summer.

| V |

VAPOUR-PRESSURE: the pressure exerted by the water-vapour in the air. Typical vapour-pressures in our climate are from 10-20 hPa, making up around 1-2% of the total atmospheric pressure.

VEERING: a clockwise shift in wind-direction, e.g from southerly to westerly. This usually occurs behind a front. In Ireland our winds usually start off southeasterly ahead of the warm front, then veer southwesterly in the warm sector, finally veering to westerly after the cold front clears. The weather behind the cold front is initially clear and settled. The opposite to veering is Backing. If the winds backs then expect rain.

VENTURI EFFECT: how wind speeds up as it is forced to pass through e.g. a mountain valley.

VERTICAL SHEAR: the change of wind-speed and/or direction with height. This is important in forecasting thunderstorms. The stronger the shear the more severe the storm can become.

VIRGA: also called Fallstreaks. A visible tail of precipitation that falls from a cloud but evaporates before reaching the surface. Altocumulus virga looks like an array of jellyfish.

VISIBILITY: the farthest distance from which an object can be clearly seen. It can be measured by a human observer or calculated by instruments.

VON KÁRMÁN VORTICES: spectacular vortices or swirls of cloud that form in the wake of a mountain or island. You can see them a lot just south of the Canaries. Google-image them!

VORTICITY: a measure of rotation in a fluid. In meteorology, vorticity at upper levels is an important factor in generating bad weather, as the stronger it is the more air is forced to rise. Anticlockwise rotation is positive vorticity, and vice versa.

VORTEX: a rotating column of air, e.g. a tornado, dust devil, etc.

| W |

WALKER CELL: a circulation in the equatorial Pacific in which cooler air flows westwards towards Asia, is heated by the warmer seas there and rises, flowing back eastwards and sinking near South America. During El Niño the sea temperatures are reversed and this circulation is disrupted.

WALL CLOUD: a lowering of the base of a cumulonimbus cloud. Tornados can spawn from wall clouds.

WARM FRONT: the boundary between advancing warm air and retreating cooler air. It generally brings warmer and more humid air with stratus clouds, light rain or drizzle and poor visibility, especially on hills.

WATER VAPOUR: water in its gaseous state (evaporated). Although in general it only comprises around 1% of the gases in the atmosphere, it is the main greenhouse gas. In Ireland the air contains around 5-15 grams of water vapour per kg of air, depending on the season (more in warmer humid air). 1 kg of air occupies around 850 litres (around 5 bathtubs) so this volume of air contains around 1-3 teaspoons of water (if the vapour condensed out to liquid).

WEATHER BALLOON: a helium or hydrogen-filled balloon that carries a Radiosonde to high levels of the atmosphere (up to around 25 kms) before bursting. The radiosonde falls back to earth by parachute. Weather balloons are launched twice a day in Ireland, at both Valentia and Castor Bay.

WET-BULB TEMPERATURE: the temperature an air parcel would have if it was cooled by evaporating moisture into it. The latent heat of vaporisation would cool the air. The WBT is important when forecasting snow as it describes what happens as a snowflake melts and water evaporates off its surface. If the WBT is low enough (<0.5 °C) then melting will stop and the snowflake will survive.

WIND: the movement of air. We feel it as trillions of air molecules hit our face. Wind forms for a variety of reasons, such as a pressure-gradient between high and low-pressure systems, the warming of the land relative to the sea to form a sea-breeze, cooling of the upper slopes of a mountain to form a katabatic wind, or even a fart!

WIND-CHILL: a cooling of the skin due to wind, making the temperature seem lower than it actually is. For example, if the air temperature is 10 °C and the wind is only 10 kph then the air will feel like 9 °C, but if the wind increases to 40 kph the air will feel like it is -1 °C.

WIND-SHEAR: an abrupt change in wind-speed and/or direction in a short distance (usually vertically). This causes turbulence in a plane and makes for tricky landings. Windshear is reported by pilots and added to METARs as a warning to others.

| X |

X-RAYS: electromagnetic radiation of wavelengths shorter than UV but longer than Gamma radiation. They are emitted by the sun but can also be generated by a machine to examine patients. Solar flares of X-ray radiation can affect the our upper atmosphere and knock out communication systems.

XENON: pronounced “zenon”, it’s an unreactive gas found in the atmosphere at a very low level of just 0.0000087% (8.7 parts per billion).

| Y |

YOUNG ICE: a flat layer of newly-formed lake or sea ice between 5 and 20 cm thick. We got this forming in many of our lakes during the big freeze of 2010 (remember people walking on the Mullingar lakes?).

YOUNGER DRYAS: an abrupt period of rapid cooling around Greenland by 5-7 °C in the space of a few decades and similarly fast recovery. It occured between 10800 and 9600 BC and is thought to have been caused by a change in the ocean currents.

| Z |

Z: Zulu time, the same as UTC and GMT. It is added to METAR and TAF times and referred to as “Zulu”, e.g. 1400Z (“one four zero zero zulu”, 1400 GMT/UTC).

ZENITH: the point directly above you in the sky.

ZEPHYR: a soft, gently breeze, from Zephyros, the ancient Greek name for a light westerly wind.

ZONAL: meaning along latitude, i.e. east-west, as opposed to Meridional, which is more north-south.