SnowFlurry 2 2017/18 | Humidity and temperature - an emancipated marriage

Looking for a partner

We don't need to talk much about air temperature and its direct influence on snow temperature and therefore on the properties of snow. Actually. We all know that the formation and transformation of snow is heavily dependent on the air temperature. That with the supercooled droplets, the crystallization nuclei in the cloud or the temperature gradient on the ground and so on.

But behind the supposedly dominant groom called "temperature" there is a lot we should know beyond that: Snow can be 0°C "warm", but when it falls from the sky it can also be -30°C "cold". In both cases, it is relatively "hot". Why? At 0°C, snow melts and turns into the liquid phase. That is why it is never actually far from its melting point. And much colder snow practically never falls to earth in our region, or hardly cools down any more once it is already on the ground. Materials that are just below their melting point are very versatile.

What does that mean? Let's compare the snow with a lump of steel: with a melting point of around 1,400°C, steel starts to glow at around 600°C, at 1,300°C it glows brightly and glistens and, to an observer not affected by the Austrian metalworkers' wage negotiations, appears to be on the verge of exploding. Annealing changes the properties of steel. Annealing is therefore used specifically to process it. This is referred to as "recrystallization annealing". This term shows us the analogy to snow: at temperatures just below the melting point, much more changes take place than at temperatures far below. Snow that is -20°C cold, or - as is often the case in our winter snow cover - only -6°C, is mega-hot in this sense. This is why everything in a snowpack is constantly transforming or recrystallizing.

How it transforms depends on the temperature gradient, i.e. the temperature difference between different snow crystals. If the temperature difference is large, the water vapor migrates from the warm part to the cold part and freezes there in such a way that it forms crystals, which we later know as weak layers. If the temperature difference is small or the crystals are at the same temperature, they become smaller and smaller and roundish - the snow becomes firmer and settles faster. If the temperature is so high that it even melts and then freezes again, we find different crystal shapes. Our groom, the air temperature, constantly influences this process: during the formation of the snow in the cloud and later in the existing snow cover and on its surface, which is in constant heat exchange with the air temperature.

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Rotten snow due to high humidity and high air temperature. — Faulty snow due to high humidity and high air temperature
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Lukas Ruetz

The partnership

However, the air temperature alone is not decisive for the snow. It has already married up with air humidity. Humidity influences snow in the same way - only indirectly: through its influence on the snow temperature by means of irradiation and radiation, evaporation and sublimation.

With regard to radiation, the whole thing is quickly explained: the higher the humidity, the more the heat radiation from the snow surface is hindered. The water particles in the air reflect the heat radiation back to the ground and the snow surface becomes warmer or cools down less. A cloud screen prevents the radiation more or less completely, high humidity greatly hinders the radiation.

The same principle applies to sublimation and evaporation: the higher the humidity, the less the snow surface can cool down. Evaporation of liquid water on a moist snow surface and sublimation of snow on the snow surface cools the snow cover - the phase transition from liquid to gaseous or from solid to gaseous requires energy, i.e. heat.

To work better with this and to better estimate the influence of humidity, there are three measures: the relative humidity in percent, the dew point in degrees Celsius and the wet bulb temperature in degrees Celsius. As cold air can absorb less moisture in absolute terms (absolute means in grams per cubic meter) than warmer air, there is a temperature mark to which an air parcel must be cooled in order to reach the point where the moisture begins to condense, i.e. the relative humidity reaches 100%, while the absolute humidity remains the same (where should more moisture come from?). At this point, the air packet can no longer carry the moisture that it was able to carry in the warmer area, so the moisture must condense out. This temperature is the dew point. As the dew point rises while the air temperature remains the same, the relative humidity increases. The wet bulb temperature describes something similar to the dew point, but is not normally shown on weather station graphs and is therefore less relevant for us. Roughly speaking, it is usually located exactly between the air temperature and the dew point. The dew point and wet bulb temperature cannot be higher than the air temperature, as the humidity cannot be greater than 100%.

If the wet bulb temperature and dew point are below 0°C, snow only sublimates. Even if the air temperature is above 0°C! Due to the low humidity, the vapor pressure of the air is so low that the molecules of the snow crystals on the surface are "ejected" directly from the solid to the gaseous state into the air. If the wet-bulb temperature rises above 0°C but the dew point remains below, snow partially melts. This means that part of it changes from the solid to the liquid phase and the surface becomes moist. The vapor pressure of the air rises with increasing humidity and the molecules of the snow crystals no longer have the opportunity to all turn directly into vapor due to the higher pressure, but must remain on the snow surface and thus become liquid water. Some of them still pass directly into the gaseous phase. If the relative humidity rises even further and the dew point also rises above 0°C, snow melts exclusively: in other words, it no longer sublimates and only changes from a solid to a liquid state. The snow cover melts, it thaws. Only a small proportion of the water that has now formed evaporates. This means that all of the heat that was previously extracted by pure sublimation or sublimation and evaporation is lost. In thawing weather, the snow cover warms up much faster, moistens much faster and evaporates much faster. All due to air temperature AND humidity.

Dew point and relative humidity. — Dew point and relative humidity
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Lukas Ruetz

In practice: In dry air, fresh snow stays fluffy for longer than in moist air. In dry air, a snow cover forms more quickly and softens more slowly (or not at all!). In dry air, the snow cover soaks more slowly. This is all due to the effects on the snow temperature caused by surface cooling. In extremely dry air, a load-bearing snow cover forms at air temperatures of around +6°, +7°, or even +8°C. In very humid air, only up to +1° or +2°C, and with a cloud screen there is no harsh cover at all up to 0°C. In other words: with high humidity, a lower air temperature is sufficient to cause the snow temperature to rise. With lower humidity, the air temperature can rise higher to warm the snow.

Both partners go hand in hand through the life of the snow. Although the air temperature has a dominant influence on the outside, in the background the threads are pulled in the same way by the air humidity. Particularly in borderline situations (= in the area of the melting point and just above), the air humidity decides where the snow will go. Behind a strong man is a strong woman or: Dad wears the pants, mom picks them out.

Summary: Marriage & marriage contract

The influence of humidity can therefore be regarded as having the same practical relevance as air temperature. Both have an effect on the snow temperature and therefore the shape and properties of the snow. We only perceive the influence of air temperature more strongly because it has a direct effect on the snow through heat exchange. Air humidity, on the other hand, has an indirect effect on the snow temperature via radiation/radiation and evaporation/sublimation. Air temperature and humidity maintain an equal marriage in the overall picture, which we must recognize as such!

Note: The influence of humidity on snow should be given the same attention as that of air temperature.