For many people, the prospect of snow is exciting, especially if they have young kids, but getting snow in Northern Ireland is rare and often difficult to forecast exactly.
Met Office forecasters have now explained just why snow is so hard to forecast and why it's pretty rare to get an influx of snow here.
A spokesperson explained: "Due to the chilly temperatures high up in the atmosphere, most precipitation either starts as snow or supercooled raindrops. As it descends to earth, it usually melts in the warmer air.
"Depending on the temperature near the ground, we might see rain, sleet, or hail. However, the freezing level (the boundary at which precipitation will fall as snow rather than rain) doesn't remain constant every day, or even within a day - it can change hourly, across the country, or even a few miles down the road."
This is due to our surrounding relatively warm seas, which often keep our temperatures up. In winter, land cools more quickly than the sea, so areas with a lot of land and very little sea, such as the vast interior of continental Europe, Canada, or the United States, get cold enough for frequent snowfall.
However, for the UK, being an island surrounded by milder water, the air often warms up slightly before reaching our shores, and we often see rain rather than snow, or a mix of rain, sleet, and snow, which is even trickier to forecast. So, what do we need for snow to fall in the UK?
Essentially, we need the air to be cold enough and a supply of moisture.
Let's tackle the cold bit first
For the UK to experience cold air, we need winds coming from the north or east. Northerly winds, which travel from north to south, bring arctic air over a cold sea to reach us.
In winter, easterly winds, moving from east to west, are chilly because they originate from the frosty continental interior of mainland Europe. However, there's another method that requires minimal wind - high pressure that settles across the UK for an extended period in winter.
As long as the skies are clear, temperatures can gradually decrease day by day due to the weak sun and lack of cloud cover to retain heat at night. The most common wind direction in the UK is south-westerly, so more often than not, we receive relatively mild air from the Atlantic bringing rain, rather than this cold air from the north and east which frequently turns any rain into snow.
The moisture content also plays a significant role in this scenario. Typically, cold easterly winds tend to bring dry air due to their passage over vast expanses of dry land, resulting in limited moisture available for snow formation and instead yielding crisp winter sunshine.
For snow to occur, one of two conditions must be met: either the cold air must intersect with a rain-bearing weather front, causing the rain to turn to snow, or the cold air must accumulate sufficient moisture during its brief traversal of the North Sea to generate showers. Additionally, the phenomenon of air ascending hills and mountains is another crucial factor.
As air rises, it cools, eventually condensing into clouds and precipitation. The nature of this precipitation - whether rain or snow - hinges on the air's temperature and the altitude of the "freezing level".
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Ever wondered why snow sometimes only covers the top of hills, while the ground below remains untouched?
This phenomenon occurs when the air at higher elevations is cold enough for snow to form, but the air closer to the ground is too warm, causing the snow to melt into sleet or rain. Meteorologists refer to this boundary as the "freezing level" – the point in the atmosphere where the temperature is at 0 °C.
The freezing level can vary greatly, sometimes dipping as low as 200ft above sea level. This means that not just hilltops, but also many towns and cities, which are often situated at elevations above 200ft, can experience snow.
To determine whether you would have seen rain or snow on a particular day, you can check the altitude of your location.
So, why is snow forecasting in the UK particularly challenging?
There are several reasons:.
1. Air origin: The direction from which the air has come can significantly impact the temperature.
For instance, if the air has travelled a slightly longer distance over mild water, it may be warmer, causing the freezing level to be higher.
2. Heavy precipitation: Intense precipitation can lower the freezing level, bringing snow to lower elevations.
Conversely, prolonged heavy rainfall can turn to snow.
3. Collision of warm and cold air: When warm and cold air masses meet, predicting snow becomes especially difficult.
This clash of air masses can lead to complex weather patterns, making it challenging for meteorologists to accurately forecast snowfall. Fronts mark the boundary between cold and warm air, so when a weather system moves in, it brings with it warm air and moisture.
While we need the moisture for snow to form, the warm air makes it very tricky to forecast if it will turn to rain. As the warm air collides with the cold air, it rises above it (since warm air rises).
The precipitation falls into the colder air below, but over time the air mixes together, making the cold air slightly warmer and the warm air slightly colder, which then makes rain more likely. You'll often find that there's a fine line between who sees snow and who sees rain.
Sometimes, a fraction of a degree is the difference between rain and snow. That's what makes forecasting snow difficult (and often frustrating for forecasters and weather watchers alike.
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