Snow: from millimetres of precipitation to centimetres of snow

When meteorologists talk about precipitation, they always mean the amount of water, whether it falls as rain or snow. One millimetre of precipitation is one litre of water per square metre, and that applies equally to snow as to rain – the snow is simply melted before the measurement is counted. That's why a forecast can state "5 mm precipitation" for a snowfall without directly saying how many centimetres will end up lying on the ground.

The link between water content and snow depth is often called the snow-to-water ratio. A common rule of thumb is 1:10 – that is, one millimetre of water equals one centimetre of snow. But that's only an average, and in practice the ratio varies enormously depending on temperature and the snow's structure.

Why the same precipitation gives different snow depths

Snow density is determined mainly by the temperature in the cloud where the crystals form and in the air they fall through. When it's really cold, ideally below about −10 °C, light, well-branched crystals form with a lot of air between the flakes. Such snow can give a ratio of 1:15 or even 1:20 – five millimetres of precipitation then becomes 7.5 to 10 centimetres of fluffy powder snow.

If the temperature is instead close to zero, which is common in southern and central Sweden for much of the winter, the crystals partly melt on the way down and clump together into large, wet flakes. That snow packs tightly as soon as it lands, and the ratio can drop towards 1:5. The same five millimetres of precipitation then becomes only two-and-a-half to three centimetres of heavy, wet snow – but that snow still weighs just as much per square metre as the light powder snow would have, since the water content is the same.

The weight of snow on a roof

What matters for a roof isn't the number of centimetres but the weight, and there the water content is what counts, plain and simple. Freshly fallen, dry snow weighs roughly 50–150 kilograms per cubic metre. Snow that's been packed by wind or has settled for a few days typically weighs 200–350 kilograms per cubic metre. Wet snow, and especially snow that has partly melted and then frozen to ice, can weigh 500 kilograms per cubic metre or more – almost as much as water itself (1000 kg/m³).

That explains why twenty centimetres of freshly fallen powder snow rarely worries a structural engineer, while twenty centimetres of wet snow after a couple of degrees above freezing can approach the limit of what an older sheet-metal roof or a carport is designed for. As a rough comparison: a 20 cm layer of dry fresh snow weighs on the order of 20–30 kg per square metre, while the same depth of wet snow can weigh 80–100 kg per square metre or more.

Wet snow, fallen trees and power cuts

Wet snow is also the type most often responsible for fallen trees and power cuts, despite rarely producing the deepest snow layers. It sticks and clumps onto branches and power lines in a way that dry, powdery snow doesn't – the latter simply blows or sifts off. When wet snow is then followed by frost and freezes solid, the load on branches and power lines becomes especially heavy, and that's when the most extensive forest damage and power distribution outages occur.

Wind amplifies the effect further by adding an extra sideways force on already loaded branches. It's the combination of wet snow and a brisk breeze, rather than an extreme snow depth on its own, that lies behind most storm-related power cuts in the Swedish winter. More on the forces of wind is in the wind guide.

Snow showers, snowfall and drifting snow

Just as with rain, meteorologists distinguish between organised snowfall, which comes from a continuous frontal system and can last for hours at a steady intensity, and snow showers, which are short-lived, often intense bursts from individual cloud towers. Snow showers can produce heavy, dense precipitation and almost zero visibility for a few minutes, before clearing completely – a pattern common in air masses passing over open water, for example over Lake Vänern or the Baltic Sea with northerly winds early in winter.

Drifting snow is a different phenomenon and doesn't involve any new snowfall at all, but rather already-lying snow being blown up and moving along the ground. At sufficiently strong wind, ideally above 10–12 m/s in open terrain, drifting snow can produce visibility just as poor as heavy snowfall, while SMHI's precipitation radar shows no precipitation at all – all the swirling snow has already fallen previously.

What it means for road conditions

For traffic, it's rarely the raw centimetre figures that decide how dangerous the road surface becomes, but the combination of temperature and precipitation type. Snow around 0 °C that gets packed by traffic turns into smooth, very slippery ice within an hour or so. Dry snow at clearly sub-zero temperatures often gives better grip, since the crystals don't melt and pack into ice in the same way. The rapid swing between above- and below-freezing temperatures, common in autumn and spring, is therefore often more dangerous than a stable, cold snowfall.

Also keep an eye on whether the forecast states precipitation amount in millimetres or centimetres of snow – many weather apps automatically convert using an assumed 1:10 ratio, which for wet snow can underestimate how heavy the snow becomes, and for very cold powder snow can underestimate how deep it becomes.

Why is the forecast snow depth sometimes completely wrong?

Most models convert precipitation to snow depth using a standard ratio, often 1:10. If the actual temperature is close to zero, the snow becomes heavier and shallower than calculated; if it's unusually cold, it becomes lighter and deeper than calculated.

How do I know if the roof is at risk?

Look at the weight rather than the depth. As a very rough rule of thumb, start paying attention at around 30–50 kg of snow load per square metre on older or lightweight structures, but the exact limit depends on the building's design – if in doubt, clear the snow or contact a professional.

Why does snow stick to some trees but not others?

Conifers with dense canopies catch more wet snow than leafless deciduous trees, and branches with a broad, flat surface collect more snow than narrow branches. That's why spruce and pine forests are often hit hardest by snow breakage during wet snow.

Is drifting snow more dangerous than fresh snowfall?

It can produce worse visibility because it arises quickly and often in already windy weather, and it doesn't show up on the precipitation radar. The combination of poor visibility and strong wind makes drifting snow an underestimated risk in open, flat terrain.

Try setting different temperatures and precipitation amounts in the visualiser to see how the snow type changes from light powder snow to heavy wet snow.