Image by Wilson 'Snowflake' Bentley Collection c.1880-1920
We've been having an office reminisce about winters past. The more recent 2009/10 and 2010/11 but also childhood memories from early/mid 80s. Proper snow. Some people can't stand the disruption, the cold and the upheaval it can cause but for others, it is still totally magical. The excitement as we head towards Christmas, the beauty of whirling snowflakes or a still, white scene, that perhaps sits better with a day off or the holiday season, rather than rush hour on the M11 on a Friday evening.
The UK is just in a tricky position. Surrounded by sea, not a guaranteed large cold land mass. Our prevailing winds come from the mild, damp SW but we can get a cold NW'ly, the raw northerly or occasionally a long lasting properly cold spell from the east or NE. All this keeps us on our toes.
Snowflakes - How do they form?
Ice crystals form in a cloud around a tiny nucleus, a bit of airborne dust, soil or decayed plant material, and then grow as super cooled water vapour adds on. These new ice crystals build out as the six arms, around the main starter ice crystal, forming the shape of a snowflake. Different air temperatures result in different shapes, and how much moisture there is in the air affects their formation too. It can't be too cold to snow, you just get different shapes at different temps.
They stay in the cloud and grow a bit more until they become too heavy. They can then fall towards the ground and land as snow. If the air is very cold and very dry, the snowflakes stay distinct and separate and you end up with powder snow. If the air is hovering around zero and has more moisture in it, the flakes can stick together on their way down. So, you get larger clumps or flakes, not quite sure about the dinner plates, but those good-sized ones that children want to catch on their tongue
That is all fine if the air is sub-zero, or up to about +2C. Above that and the snowflakes can melt a bit. The snowflake can melt a bit and then freeze again so the snowflake has a covering of ice and is known as an ice pellet (or sleet). In our moderate climate we often get a flurry of snowflakes, then some cold rain with maybe icy sleety bits which are more obvious on a car windscreen.
What can happen is that a shallow zone of cold air develops right at the surface. The snowflakes fall through a deep warmer layer and melt, then land on a cold surface within the shallow layer of colder air. The water then instantly freezes and creates treacherous conditions on the roads and runways and in other countries accretes ice onto powerlines causing them to snap under the extra weight. It doesn’t happen very often in the UK and is difficult to forecast. As a warm front comes in against existing cold air, it will be forward sloping, so the warm air aloft will overtake the colder denser air at the surface. This can result in freezing rain in winter, just ahead of the surface warm front and more particularly in cold continental setups. Severe icing can occur on aircraft very quickly in this situation
Will it settle?
That depends on the surface. A dark road which had sunshine on it in the morning maybe a bit warmer than the grass next to it. Heavier showers can pull the air temperature down and keep adding more snow, enough to finally settle. Overnight it is usually colder and so there can be more chance of snow settling in the cold night air, ready for that “It’s snowed “ moment when you open the curtains.
Snow is recorded in cms. You should look for an even open area with flat, level snow cover, which is representative of the snowfall. You can take 3 readings and then an average, just stay away from any drifting areas, or where snow is just collecting, say by your doorstep.
Intensity is classified as Slight/Moderate/Heavy:
Slight snow- sparse flakes that are usually small. Rate of accumulation not usually above 0.5cm/hour – dusting really
Moderate snow – large flakes falling thickly enough to impair visibility substantially. Snow cover increasing in depth at a rate of up to 4cm/hour
Heavy snow - reduces visibility greatly (as a guide to less than 250m). The snow cover increases at a rate exceeding 4cm/hour
It can be reported in the Station circle observation below the low cloud data as SNnn, for example 2cms of snow would be SN02.
Blizzards, Drifts and Blowing Snow
Drifting snow is snow which is moving below the observers eye level. Blowing snow is generally at or above the observer’s eye level.
The UK Met Office defines a blizzard;
as moderate or heavy falling snow (either continuous or in the form of frequent showers) with winds speeds of 30 mph or more and a reasonably extensive snow cover reducing visibility to 200 metres or less. BLIZZARD
Even when it does eventually snow, there are different types, intensities, movements, never mind if it will actually lie.
Next time; a look at forecasting setups and what the snow warnings mean.