Not an everyday question – but quite relevant for alpine outdoorsportsmen (or women) in about every season: at which rate does snow melt? When hoping to perform winter sports, you want snow to accumulate. In the summer season, generally, you don’t want snow fall to block your routes and limit possibilities. But how fast does snow actually melt?
This depends on a number of variables:
- Soil temperature
- Air temperature
- Wind speed
- Presence of clouds (in the night)
- Sunshine & its angle of inclination relative to the surface
- Inclination of the terrain
- Snow temperature
- Age of the snow and the type of snow that has fallen
When the first snow flakes start falling, they fall directly on the ground. When the ground is frozen, the snow starts to accumulate. If the ground isn’t frozen yet, it will first thaw away. This thawing process depletes a lot of energy from the surrounding environment. The air will cool down, but so will the soil.
Due to the melting of snow, essentially a phase-change from frozen water to liquid water, the temperature of the surrounding objects will start to reach the freezing point. Only then accumulation will occur and a pack of snow starts to appear.
Imagine that it’s been a very bright night, with a soil temperature already below the freezing point. The snow falls in the morning on a frozen surface. This freshly fallen snow starts to form an insulatin blanket on the soil which prevents heat from the soil to escape. This can cause the snow to melt from below. This melting depends on how fast the melting water can flow away. The melting process will be killed quickly if the ground is porous or if the water can flow away quickly through streams – such as on slopes.
When water can’t flow away, for example if the snow has fallen on a frozen swamp or lake, the melting will continue for a longer period of time and the pack will be very rich in water and mushy.
As a matter of principle, snow only melts at temperatures above 0 °C, depending on humidity. Higher temperatures will lead to faster snow melting. However, this air temperature in turn is influenced by this melting process. The melting process extracts a lot of heat from the air, which will cool down. If this air isn’t replaced with freshly warmed air, the process will slow down.
As discussed above, snow melts at temperatures above 0°C. You’d think. But not really. In very dry air, the so-called wet bulb temperature is lower. This can pervent the snow from melting, even if the (dry bulb) temperature is (well) above 0. The evaporation of moist is cooling down the snow. This is called sublimation: water can go from it’s solid state to a gas state, without necessarily going through it’s liquid state.
Conversely, with temperatures below 0°C and a milky sun in the sky, the snow can start dripping. This is due to the absorption of radiation in impurities in the snow (stones, sooth, branches).
Already stated under the paragraph of air temperature: air must be moved (and thus replenished) to get rid of the absorbed heat from the melting process – or the air will cool down itself. If the wind blows harder, this replenishment happens at a higher speed.
Under so-called Föhn-conditions, moist air is adiabetically heated by its decend. The decend heats up the air and dries it out simultaneously. Especially south-föhns in clear skies and thus a lot of sunshine and low humidity can cause very rapid snow depletion. Partially due to actual melt, partially due to sublimation. This can be multiple tens of centimeters per day. Certain valleys are more prone to föhn winds: mainly north-south oriented valleys are sensitive. Also the higher mountains are more sensitive to the dry air. In the value it could be windstill and in the side valleys cold air could be trapped as a result.
Presence or absence of clouds
During daytime, clouds protect the snow cover from solar radiation. During the night time, clouds prevent the radiation into the sky and nightly cooling is prevented – or even night time heating. Clear skies are by far the best to maintain a snow cover.
You can experience this yourself in winter: just look at outside temperatures when there is a snow cover. When the skies clear, the temperature plummets. Especially in dry air, as there is no moisture that prevents the cooling and the dew point (at which temperature stops dropping and the air is saturated with moist) is much lower.
The sun, obviously, heats the atmosphere. 3% of all solar energy is in the UV-bandwidth, 47% is in visible light and 50 percent is in infrared radiation. Objects absorb radiation (both visible and invisible), and radiate heat as it absorbs light. This heated air rises and starts to cool down as the air expands. Each object absorbs infrared radiation (it’s what’s making the sun feel so warm in the spring time). The extent of absorbance in the visible spectrum is defined by the color: red objects for example absorb all colors except red. The red color is reflected and thus the object appears to be red.
Snow absorbs light too, but only about 10%. The remaining 90% is reflected and as such, the temperature doesn’t do much under sunshine conditions. When the snow gets older though, the reflection drops to 60% and thus more energy is absorbed by the snow – and thus snow melt increases, leading to lower snow cover and might reveal rocks or trees under the snow. Objects in the snow, such as houses, skilifts and trees have very different absorption levels and are great at absorbing solar heat and thus melting snow.
A steep north face doesn’t get a lot if any solar radiation in winter time (on the northern hemisphere). A south facing slope on the other hand does. If this angle of inclination comes closer to 90° (perpendicular) to the surfcae, more radiation is received per square meter.
The sun is in the winter at an inclination of about 23° above the horizon in the European Alpine region. A slope with an inclination of 67° is thus perpendicular to the sun (when south facing). In the valley, the sun is just 23° above the horizon (or might still be behind the mountains, effectively not reaching the valley at all). The snow melt on the south facing slope will be heavily influenced by this – and the valleys might become pools of cold air.
The snow temperature doesn’t need to be 0°C. In many cases it will be much less. Before snow starts to melt, it must first heat up to 0°C, before melting starts to occur. It will be 0°C until all of it is gone. However, as snow has a low mass of itself (most of it is air), this heating can go quite rapidly. Flowing water will speed up the process considerably.
How fresh is the snow and what type of snow is it?
Fresh snow is the perfection of white. There is no artificial color that’s more white than fresh snow. It reflects 90% of all radiation and radiates a lot of energy during the night. When snow gets older, this whiteness is lost and more heat is absorbed and melting occurs more quickly.
It is also important to consider what type of snow it is. Fresh, dry poweder snow consists mainly of air and has little mass. There is not a lot of energy needed to let this melt or evaporate. Powder snow can disappear very quickly due to this. But not just this: it also becomes more compact due to its own (even limited) weight and starts to sink in. It becomes much more compact and less sensitive to abrupt temperature swings.
The oldest part of the snow (so the closest to the ground) may be compacted so far that if becomes ice or firn after a year: it’s how glaciers are grown.
When all the fresh snow is fallen as compact, wetter snow, the process of sinking in goes much quicker and ice is formed much faster. This melts much more slowly.
Conclusion: how fast does snow melt?
In the end it depends on many variables. A big föhn storm could melt 20cm of fresh snow within hours. However a big pack of snow will sink in a lot and last long. High up in the Alps, up to 10 meters of snow could fall in the winter period. In these regions it could take until August before all snow has melted before the new winter season starts.
Snow melt in the reality
December 10, 2017. A low-air pressure system caused 15 centimeters of snow to fall in the morning. However, after the cold side of the depression, the temperature rose sharply and snow was turned into ordinary rain fall. The air temperature rose to 7°C within a few hours.
At 5pm there was just 9cm of snow left. Partly due to sinking of the cover (due to the rain, it got very heavy), and partially due to actual melting. The wind picked up in the evening and temperature dropped to 4°C. Gusts above 90km/h avoided the cold air above the melting snow to stand still: it was all replaced with warmer air in a matter of seconds. At 11pm, there was just 2cm of snow left.
In februari 2021 (the 8th) , we had a lot of snow again. Over 20 centimeters and snow dunes of over 50cm – locally even above 100cm. Below picture is from the street next door.
Obviously, we spend a lot of time in the snow ourselves and made a snow castle. This castle had a court yard and consits of multiple cubic meters of snow. The bottom of the castle would become firn due to the pressure – we helped nature in compacting the snow considerably…
This hump of snow is located in the middle of my garden, which is sheltered against most of the winds and is in full sunshine. Since this picture was taken, an additional 10cm of snow had fallen.
We expected the castle to last for weeks. It’s difficult to see perhaps, but it’s quite big. Reality? February 23 it was gone.