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.
There are two “types” of snow – wet snow and dry snow – and the particular type we get is important in forecasting how it will react 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 or slightly above 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. In other words, if 1 mm of liquid-equivalent precipitation falls as wet snow it will form a layer of snow around 5-10 mm (0.5.- 1 cm) thick.
Wet snow falls in large flakes because the liquid content causes multiple flakes to stick together as they fall. This type of snow will not drift to well in wind but is the best for making snowballs and is what we usually get in our infrequent snowy spells each year. I call it cheap Atlantic muck.
Dry snow occurs when the PBL is a lot colder (and hence drier). The partial melting is inhibited because any attempt at melting removes latent heat of fusion from the air, cooling it and preventing further melting. The drier the air the greater this effect. This means this snow is less dense and contains smaller, more powdery flakes. The lower liquid content makes this snow difficult to make snowballs but it 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 (i.e. 1 mm water-equivalent will form a layer of snow around 1.5 – 3 cm thick). This is a more continental type of snow and one you will probably get if you go skiing in the Alps.
So now we know a bit about what types of snow we get, what will determine whether or not it will melt or accumulate on the ground?
Obviously if it falls onto frozen ground then as long as the air temperature also remains around freezing or below, the snow will remain intact and accumulate. However, even of the ground is above zero the snow will still accumulate if it is falling at a fast enough rate. The first snow that falls will melt, a process that removes heat from the ground. As more snow falls the ground cools further until reaching 0 °C, after which melting will stop and the snow will accumulate.
A layer of dry snow contains lots of air, which is a good insulator. As more snow falls on top it builds up a layer that will then further help to keep the ground temperature low. It will also insulate any newer snowfall from any warmth radiating up from deeper in the ground. This is also why snow accumulates more on grass than on concrete pavements, roads, etc. The blades of grass 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, preventing melting and allowing the snow to build quicker. Of course, as explained above, the more it builds the greater its mass, therefore the greater the energy input required from the ground to raise its temperature and cause melting. A single snowflake requires very little energy to melt it, but hundreds of flakes clumped together are now one large mass and therefore require a lot more energy to melt fully. It is true that there is strength in numbers.
A layer of wet snow, on the other hand, contains more water and will compact quickly under its own weight. The higher water content makes melting more efficient, but under clear skies at night it can refreeze to form a hard crusty layer of ice. Wet snow will easily turn to slush under the pressure of footsteps or vehicle tyres, therefore it is more dangerous than dry snow when it comes to traffic, presenting a higher risk of aquaplaning.
Another factor to consider is whether snow falls during the daytime or night-time. With night-time snow there is no ultraviolet (UV) radiation present to cause sublimation of the snow (turning the water directly from ice to vapour, skipping the liquid phase) as it falls, therefore more of it reaches the ground intact than when it falls in daylight. Daytime UV radiation also heats the ground and other surfaces, reducing the chances of the snow sticking, but if there is a covering of snow already present it will reflect this radiation, preventing it from being absorbed by the ground. So even if the temperature and humidity of the atmosphere are exactly the same, snow will melt easier during the day than at night.
However, if a patch of ground does become exposed to sunlight it will heat up, causing melting of the snow to increase around its edge. Before long the snow cover can become more patchy. This is most pronounced on solid surfaces such as footpaths, roads, carparks, etc. and is less notable over grass. This melting is least effective during December and January, when the sun is at its weakest, but as we go through February and March we can see how much quicker snow can melt as the sun gets higher in the sky.
What about the saying “it’s too cold to snow”? Does this hold any water (pun intended)? The answer is yes, but it is more to do with the depth of the cold air. If the air is very cold higher up in the atmosphere (2-4 km), where the ice crystals form, then there will be less moisture available and as a result less crystals to make snowflakes. The heaviest snowfalls occur when the temperature and dewpoint are not much below zero throughout this deeper layer, which can happen as a warm front approaches an area of cold surface air.
So it is the air up high that determines how much snow will fall, and the air down low that determines in what state it will arrive at the ground.
So as we head into the business end of winter and the chance of snow increases, take a look at how snow falls in your area.