The “snowfall limit” is the altitude at which the ratio between snowflakes and raindrops is 50:50. The “snow limit” is the elevation at which snow sticks on the ground after falling. The latter is generally 50 to 200 metres higher than the snowfall limit.
The diagnosis and prognosis of the snowfall limit is of general interest to the public. For those living in the Alps or visiting the mountains as a tourist, it is useful to know the height at which snow will fall in the event of bad weather. Skiers also like to know how far down a slope the snow has fallen.
One might assume that the snowfall limit would be closely related to thezero degree level.However, this is only partly true. The snowfall limit is not a sharply-defined line. It is more helpful to imagine a band several hundred metres deep (below the zero degree level) in which snow crystals gradually give way to raindrops. The snowfall limit is therefore always lower than the zero degree level. A good rule of thumb is that snow commonly occurs 200 to 400 metres below the zero degree level.
When there is precipitation in the upper levels of the atmosphere, this is almost always in the form of snowflakes. These then begin to melt once their temperature reaches freezing point as they descend and can absorb additional heat from the surrounding air. However, this very often does not take place at the exact altitude at which the air (ambient) temperature is 0°C.
In fact, the melting process is somewhat more complex, and is dependent on the ambient air humidity in the melting layer. Often, the air surrounding snowflakes is not saturated with water vapour. In rare cases, or sometimes at the onset of precipitation, the humidity may even be as low as 50 percent. On closer investigation, it appears that the snowflake initially evaporates somewhat (known as sublimation), which often takes place above the zero degree level. This causes the surface of the snowflake to cool down, since the evaporation process draws heat energy from the snowflake. Only after falling for a certain distance has the snowflake absorbed enough heat energy for melting. From this point, the snowflake becomes increasingly wet, until it finally becomes a raindrop.
If snowflakes are falling into relatively dry air, it can snow up to several hundred metres below the zero degree level in temperatures well above freezing. The process involving significant evaporation of snow as it descends typically occurs with rising warm fronts. It can be observed clearly from a certain distance, as the undersides of the clouds look as though they have been smudged. In such cases, the snowfall limit can easily be 1,000 m below the zero degree level. If it continues to snow, or if the snow shower becomes heavier, the atmosphere gradually reaches saturation point (100% humidity) as a result of the evaporation process described above. The difference in altitude between the zero degree level and the snowfall limit is then only 200 to 400 m.
There are other special circumstances that can lead to huge fluctuations in the altitude of the snowfall limit, such as an inversion situation, a change in the weather, and the cooling down of precipitation.
Fine weather in autumn or winter can result in a strong inversion situation, in which the temperature initially increases with altitude instead of decreasing. It is entirely possible for it to be milder at 2,500 m.a.s.l. than at 500 m.a.s.l. on the valley floor. If the temperature in the valley is +3°C, the snowfall limit would normally be just slightly higher than the elevation of the location in question at the onset of precipitation. However, because of the warm air in the middle and upper levels of the atmosphere, the snowfall limit is much higher, at 1,500 or even 2,000 m.a.s.l.
The opposite scenario can often occur when the weather changes in winter. The weather forecast reports precipitation with a snowfall limit predicted to rise to 1,500 m.a.s.l., for example. It can be the case, particularly in the inner Alpine valleys, that snow falls as far as the valley floor for quite a while. The reason for this is that the entire air mass inside the valleys is cooled down, which makes it more difficult for the warmer sea air to clear the heavier air masses. This is generally mentioned in the detailed weather forecasts.
In the case of precipitation cooling, heat is removed from the surrounding air as snowflakes melt. This causes the airmass to cool down, thereby bringing down the snowfall limit. In order for this to happen, heavy precipitation (over 2 mm/h) is required, together with as little wind as possible. Moreover, the column of air will thus always contain moisture and will be saturated over a wider vertical range. This prevents the occurrence of the heat-removing evaporation process described above.
The effect of precipitation cooling operates particularly well in narrow Alpine valleys. In weather conditions with little wind, the airmass remains more or less the same, and the heat for melting is drawn from a smaller airmass than in the lowlands. When there is ongoing heavy precipitation, the snowfall limit can sink in such circumstances by several hundred metres, and even up to 1,000 m in extreme cases.