In this blog post I explain a number of terms that come up in many mountain weather reports and are easy to mix up. Namely the freezing level, the snowfall line or boundary and the snow line. While these are related, they are certainly not the same. Meteorologists know this very well, of course, but often there is not enough time in the weather forecast to explain this sufficiently. In this blog I elaborate on it in detail.
Freezing level: in a free atmosphere
The first concept is the frost boundary. This is the altitude above sea level where the temperature drops below 0°C and thus freezes. This is an important concept: whether or not moisture freezes and thaws has a huge impact on the type of precipitation that falls, but also on air saturation. For example, the freezing level can be at an altitude of 1000 meters, but it can also be relatively moist or very dry. The latter is important for the snowfall boundary, but I’ll get to that later.
The freezing level as calculated in weather models is usually the freezing level in the free atmosphere. This means: disconnected from the so-called boundary layer. This boundary layer is the layer in which we experience the weather, the lower hundred(s) meters of the atmosphere. This is highly subject to local influences and, for example, sunshine.
A small example to illustrate the above: on a fresh winter morning it cools down considerably. Cars freeze, water turns to ice and the fields are white with dew. The freezing level is 0 meters: after all, it is freezing. As soon as the sun rises, the temperature rises rapidly. The rising sun creates heat from the ground and generates turbulence. The warming air rises. Under these circumstances, there is usually a (morning) inversion, where the boundary layer cooled strongly. The temperature now rises with altitude.
At 3000 meters altitude, however, this so-called daily routine hardly occurs: day or night is equally cold, with some minor differences. These differences become greater as more mountains end up in this air layer: they influence the air layer, so that a daily temperature swing can be detected in the mountains at 3000m altitude – although this strongly depends on the wind strength. After all, a stronger wind causes more mixing with the surrounding air layer: the free atmosphere.
The snowfall boundary: tough to predict!
All precipitation originates as snow. There are a few exceptions: fog can be so thick that it simply rains off as drizzle. And drizzle itself doesn’t always start as snow. But all other precipitation starts as snow – including hail.
This snow is formed high in the clouds. Moisture rises from the warm bottom. Rising air cools, and colder air can hold less moisture: it condenses into clouds. High in the clouds it freezes and the droplets transform into snowflakes. When these are heavier than the rising air below, or are pushed to the side, they begin to fall. As the snowflakes descend, depending on the circumstances, they will thaw. Little snowflakes get bigger and bigger, until they are water droplets: rain. When these water droplets are thrown up again by the rising airflow of the storm, hail is formed. In winter these ascending air currents are not as strong and hail is a rarer phenomenon than in summer. Once thawed, the rain does not turn back to snow.
Melting snow is a difficult process: a lot of heat is extracted from the atmosphere when it snows, which allows a package of air to cool and melt more slowly. Also, whether or not the snow melts depends on the humidity. The air in the clouds themselves is completely saturated: a humidity of 100%. Otherwise there wouldn’t be a cloud. But the layers of air beneath it are not necessarily saturated. With a dry layer of air below the shower, the so-called wet bulb temperature is significantly below zero: the snow does not melt, but evaporates. The snow that does not evaporate remains dry.
If the entire air column cools down due to heavy snowfall, precipitation cooling occurs. This leads to isothermal: the entire air column cools to freezing point and thus does not get warmer or colder with changes in altitude as it normally does.
But even if the snow falls through a warmer layer of air, it won’t melt away right away. This depends on the intensity of the precipitation and the temperature (and humidity) of course. The melting of snow can take several hundred meters. It is therefore perfectly possible (which you often see in practice) that the freezing level is 600 meters and wet snow or sleet falls on the ground.
Snow line: accumulation
The snow line is a result of temperature and previous accumulation of snow: snow that has remained on the ground without thawing. The moment the snowfall boundary is lower than the bottom of the valley (or the point where you are), the snowline is at the bottom of the valley. The snowfall line is usually above the snowline: the lower hundreds of meters of snowfall usually exist out of wet snow and it won’t accumulate. Accumulation will only occur when the freezing level is sufficiently low. And simply as long as the snow doesn’t thaw away, you’re on the snow line. In the high mountains it rises sharply with the season, but intensive showers can just leave behind tens of centimeters of snow, even in summer.