Aviation Weather: A Guide To METARs and TAFs

Automatic Weather StationOf all the activities carried out by man, flying a plane obviously ranks right up there when it comes to the importance of knowing what the weather is doing, or is likely to do. Whether flying along alone on a light aircraft, or across the Atlantic with 250 others on a large airliner, there’s nowhere to go only down if you find yourself in weather that is beyond the limits of the aircraft. It’s highly important, therefore, that the pilot has a sound knowledge of what kind of weather that’s likely to be encountered along the route.

It is therefore not a coincidence that most of the routine-reporting weather stations around the world are located at airports. Even the smallest airfield will have a wind sock, probably the simplest type of weather instrument there is,

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Our Atmosphere And How It Makes Weather

AtmosphereWe breathe it. We feel it. We see its effects. Without it we wouldn’t be here.

The atmosphere is a layer of gases (78% Nitrogen, 21% Oxygen, 0.9% Argon, 0.04% Carbon Dioxide, plus others) that blankets the earth, extending to a depth of a few hundred kilometres. It contains many different layers, such as the Troposphere, Stratosphere, Mesosphere, Thermosphere, etc., but the layer we are most interested is the one nearest the earth – the Troposphere – as this is the one we live in and where all our weather happens. It extends from the surface to a height of around 16 km in the tropics, and 8 km near the Poles. The top of the troposphere is called the Tropopause, and it marks the boundary between the troposphere and the next layer above it, the Stratosphere. Continue reading

Comet ISON may be spectacular in December

If we are fortunate, Comet Ison (C2012 / S1) may put on quite a show in the December skies. Right now, the comet is approaching the inner solar system from quite a high angle above the north poles of the Sun, earth and most other planets (Uranus does its own thing by rolling along poles generally pointed in the same directions as the orbital plane).

Comet ISON is expected to hurtle very close to the Sun’s south pole around 28 November and then emerge into a more visible position on the earth’s side of the solar system, heading back into the north (which means above the Sun’s north pole) all through December. At some point early in the month, we’ll know if it survived that close encounter intact, or broke apart, and just how bright it becomes. Some estimates continue to promise a very bright object even brighter than Venus and considerably brighter than Jupiter which will also be prominent in clear December skies. Our closest approach to the comet is 26 December (0.42 A.U.) at which time it will be about the same distance from the Sun as is the earth (1 A.U.). If it is a bright object, Comet ISON should have a long tail pointing up away from the Sun and towards the pole star (Polaris) which it will be aiming towards on its exit from our solar system. By January, a much less prominent comet would be only viewable in binoculars but the earth will go through the remnants of its tail which might produce a minor episode of shooting stars or meteors.

So it’s fingers crossed that Comet ISON will produce unlike some other hyped comets of past decades, and put on a glorious show in December. Then there’s the matter of finding clear enough skies in the cloudiest month of the year in many locations around the northern hemisphere.

To see an orbit diagram for Comet ISON, go to this link:

http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=C%2F2012+S1&orb=1

How The Weather Affects Your Golf Shots

Shane Lowry of Ireland protects himself from the rain as he walks off the course during a practice round for the British Open golf championship on the Old Course in St. Andrews


Golfers are famous for being imaginative when it comes to making up excuses when things go wrong for them out on the course, with weather being one of the obvious candidates. When it comes to weather, however, give the golfer the benefit of the doubt. Different weather conditions do have a bigger effect on golf shots than you might think, and this is something that should be considered when playing at different times of the year.

Consider two scenarios; a cold, dry frosty morning in winter (say air temperature 0 °C, RH 80%, with frost on the ground) and a warm, humid afternoon in summer (say 25 °C, RH 70%, strong sunshine). Assume no wind in both cases and we’re playing the same course (so the same altitude, say at sea level).

In this case air density and ball temperature are the two factors directly affecting the flight of the ball (if we neglect the effects of the heat or cold on the player and club shaft!).

Air density, in kg/m³, varies from day to day, being dependent on air temperature, specific humidity, pressure and altitude. Higher pressure leads to higher density, but if we assume that altitude and pressure are the same in both scenarios then only temperature and specific humidity are different. By specific humidity I mean the actual amount of water vapour in the air (in grams per kg of air) as opposed to relative humidity, which is the percentage of water vapour in the air compared to the maximum it could hold at that temperature (warmer air can hold more water vapour than colder air). 

The air densities for the two scenarios are

Winter: 1.286 kg/m³
Summer: 1.170 kg/m³

The summer value is about 10% lower than the winter one.

The aerodynamic drag on a golf ball is a force that opposes motion and is directly related to the air density (i.e. lower density producing lower drag). The percentage difference between the drag on the balls in our two scenarios is simply the difference in densities (i.e. around 10%). That means a 10-yard difference for every 100 yards of carry, so a wintertime carry of 220 yards could be around 245 yards in summer, all other things being equal.

But all other things aren’t equal. The temperature of the ball has a big effect on its flight. They say the optimum temperature of a golf ball is around 27 °C, as that’s where the compression of its elastomeric core is greatest. It takes a ball several hours to fully warm to this temperature to its core, so storing it indoors overnight prior to a round can gain you a few extra yards!

During a round of golf the ball spends most of its time lying on the ground and is only in the air a tiny fraction of the time. It is therefore the ground temperature, not air temperature, that will most influence the ball’s temperature. In winter, with frost on the ground (assume they didn’t close the course!) the ground temperature could be as low as -10 °C and temperature of the ball will fall quickly during the round, reaching its coldest well before the 18th hole. You could of course keep a second ball in your pocket and alternate balls every hole to slow down this cooling process, but they will still cool down overall. In summer, with strong sunshine, the ground temperature could be 30 °C or more, so the ball should hold its optimum temperature right throughout the round, giving you that advantage, particularly on the later holes. As the core materials are more elastic at this temperature, and the ground will probably be harder, the bounce and roll will also be much bigger than in winter.

So we have the 10% increase in carry due to different air densities plus the effect of a warmer ball and harder ground (not to mention the physical state of the player). We could be looking at 20-30% longer shots and using around 1-2 irons less in summer versus winter. 

Taking all of the above factors into consideration, the longest distances will be acheived in afternoon rounds in warm, muggy low pressure systems, while the shortest ones will be in early morning rounds in cold, dry high pressure systems.

But let’s take just air temperature as the variable, as this is the easiest one to check. All other things being equal, an increase in air temperature from 0 to 25 °C yields a 7.4% decrease in density, which is 0.3% per degree. This means that for every degree warmer the air is the ball will fly 0.3% further. This is only about 1 extra foot per 100 yards, which is very little. In Ireland we don’t get a wide range of temperatures throughout the year, most rounds probably being played between 5-20 °C. If we only look at air temperature and assume pressure, humidity, etc. are the same then these extra 15 degrees will add about 10 yards to a 220-yd drive. Now add in different ball and ground temperatures, air pressure, humidity and ground hardness and it might be interesting to compare your distances now to those in a few months’ time (assuming wind conditions are the same) to see how your club choices vary.

In the meantime, work on the rest of those excuses!