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The Physics of snow accumulation; Why does snow stick sometimes and other times not?
Topic Started: February 4 2012, 02:49 PM (276 Views)
Fergal (IWO)
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Let me explain what factors are at play when it comes to snow.

Snow is an extremely complicated form of precipitation, and as such leads to a very tricky time for a forecaster. There are two "types" of snow - wet and dry, and the particular type we get is important in how it reacts when it hits the ground. Wet snow occurs when the temperature in the Planetary Boundary Layer (roughly the lowest 500-1500m of the atmosphere) is fairly constant and near zero. Partial melting means it contains some liquid water, and is therefore more dense than dry snow. It will compact quicker, so for the same amount of liquid-precipitation, it will form a thinner layer than dry snow, with a liquid equivalance of between 10:1 and 5:1. It will also fall in larger flakes, because two or more flakes will agglomerate as they fall, and it will not drift to well in wind. This type is the best for making snowballs.

Dry snow occurs when the PBL is a lot colder, inhibiting this partial melt, and therefore yielding a less dense and smaller flake. The lower liquid content makes this snow difficult to make snowballs with, and will easily drift as it blows in the wind. Its lower density means it will form deeper layers than wet snow, with a liquid equivalent of between 15:1 and 30:1.

So what will determine whether or not snow will remain on the ground? Obviously if snow falls onto frozen ground then as long as the air temperature remains around freezing or below, the snow will remain intact and accumulate. As more snow falls, it builds up a layer that will then further help to keep the ground temperature low, but also insulate any further snowfall from any radiating warmth from deeper in the ground. A dry snow layer means it contains lots of air, which is a good insulator, adding to the effect. This is also why snow accumulates more on grass than on concrete pavements, roads, etc. The grass blades are more exposed to cold air than concrete, which will stay warm for longer as it's in full contact with the ground's internal warmth. The air pockets around grass insulate the snow from the this warmth, allowing it to build quicker. Of course, the more it builds, the greater its bulk, therefore the greater the energy input required from the ground to raise its temperature and cause melting.

Posted Image
Grass blades are surrounded by air, which is a good insulator and prevents snowmelt.


Another factor is whether snow falls during the daytime or nighttime. With nightime snow there is no uv radiation present to cause sublimation of the snow (from solid directly to water vapour) as it falls, therefore more of it reaches the ground than when it falls in daylight. Dayime uv radiation also heats the ground and other surfaces, reducing the chances of it sticking, but with a covering of snow present, this radiation gets reflected and doesn't get a chance to heat the ground, therefore air temperatures remain low. However, if a patch of ground does become exposed then the sunlight can get to work heating it, causing melting to increase around its edge, and before long the snow cover can become more patchy. Again this is most pronounced on solid surfaces such as footpaths, roads, carparks, etc. and is less notable over grass.

So with an icy easterly wind affecting much of the country this week, why didn't we see some snow showers form over the Irish Sea? The reason is that there was high pressure very close by, and its subsidence inversion (sudden layer of warm dry air at around 4000 ft) put a cap on convection. Also, the very low water content of the continental airmass meant that the lowest layers did not have enough sea track (or fetch) to pick up enough moisture and heat from the sea to allow deep convection to occur. This is why we got such clear skies, and at best could only manage some cumulus humilis and stratocumulus. If the airmass had been coming from a slightly more northeasterly direction then it would have had a lot more fetch over the North Sea to allow it to generate some showers.

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