Interesting Science Of How Snow Accumulates

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28 cm of lying snow in Tallaght on December 2nd, 2010. Image: Author

Snow is an extremely complicated form of precipitation, and as such leads to a very tricky time for a forecaster. A fraction of a degree either way can tip the balance between a proper snowflake and a more melted form, especially here in Ireland. There is a whole lot of physics going on there, much of which is still poorly understood, but let’s take a look at the stuff we do understand.

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Blast From The Past: Hurricane Debbie (1961)

Hurricane Debbie over the Atlantic on September 13th, 1961. Image from Wikiwand
Hurricane Debbie over the Atlantic on September 13th, 1961. Image from Wikiwand

Of all the Irish windstorms in living memory, Debbie in September 1961 set many records that still stand to this day. With a total death-toll of 78 (18 in Ireland and 60 in a plane crash in Cape Verde), it left a path of destruction in its wake. It has been claimed that this storm was the only true hurricane to survive as far north as Ireland, but this is not the case. Hurricanes, in the true sense of the word, simply cannot hold onto their tropical characteristics throughout the long trek this far north, and Debbie was no different. It is possible for us to get regular storms with hurricane-force winds, but they are a different beast altogether. So what was Debbie then?

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Layer Of High Cloud All We Got From Joaquin

Altostratus cloud from ex-hurricane Joaquin partially blocks out the sun over Kildare on Saturday morning. Image: Author

For a week we had been wondering if hurricane Joaquin would send anything our way as it died a death somewhere over Europe this weekend. As it turns out, the main depression is filling off the northwest coast of Iberia, but one fragment of upper moisture from the former Category 4 hurricane has made it north and is doing its best to block out the sun.
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Ozone Hole Largest Since 2008

earth-stratosphereThe ozone layer is a layer of ozone gas around 20-30 km up in the stratosphere, and is of crucial importance in protecting life on Earth from harmful UV-B radiation from the sun. Every southern spring a large hole develops in this layer over Antarctica, however this year’s hole is the largest since 2008. How does the hole form and how important is this year’s growth?

Ozone is a gas consisting of three oxygen atoms in a molecule instead of the usual two. It is formed in the stratosphere when incoming ultraviolet radiation from the sun breaks up normal oxygen molecules (O2) into two single oxygen atoms. These atoms are very reactive and quickly react with another oxygen molecule to form an ozone molecule (O3).

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High Pressure Firmly In Charge This Week

Our fine and settled spell of weather continues this week as an intense area of high pressure refuses to shift. Hour after hour of long-awaited sunshine, tempered only by some wisps of high cirrus clouds and contrails. We could have done with this back during our real summer, but better late than never. But why does high pressure mean sunny weather?

Firstly, high pressure (anticyclones) usually means sunny weather, but not always. The reason for this will come later, but first we’ll look at why this one is giving us our sunniest spell since June. Continue reading

Irish Weather Online’s A-Z of Weather Terms

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

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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).

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Our Atmosphere In Numbers

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Our atmosphere. Image from NASA.

Our atmosphere is an envelope of gases surrounding the globe, extending up to a height of around 500 km. It is mostly made of nitrogen and oxygen, but also contains water-vapour, argon, carbon dioxide, ozone and others. It may seem uninteresting at first glance, but delving into it in a little detail can reveal some remarkable and unexpected findings.

The atmosphere is divided into several layers, as shown above. As it extends to around 500 km its volume is around 275 billion km3. Its density at sea-level is 1.225 kg/m3, decreasing exponentially to practically 0 at roughly 500 km.
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