Limit rain/snow

In the field of the Meteorology, the limit rain/snow is used to represent two concepts: the Altitude (or the interval of altitudes) to which the Pluie transforms itself gradually into Neige and the geographical place where precipitations change rain with snow in a Tempête. In both cases, this limit is related on the altitude of the Isotherme zero degree and to the Température of the mass of Air above this Isotherme.

Vertical field

The temperature variation in the atmosphere is influenced by the type of mass of air. The importance of the rate of the Précipitation S which fall there also has an influence on the Altitude of the snow line, because the more the flake S fall intensely, the more the vertical distance under the isotherm 0°C can be large. Indeed, to melt, these flakes must absorb energy and thus cool the layer locally where they pass.

In general it is admitted that it can snow 300 meters below the isotherm 0°C, that is to say at a temperature in free atmosphere ranging between 1,5 and 2°C - knowing that the standard decrease of the temperature is on average of 0.65°C every 100 meters. This rate is however an average from which reality is often very distant, in particular in the case of inversions of temperatures observed in the zones of hot Face, in those of loss of heat de per night radiation or layer of snow close to the ground and finally in those between the plain and the Mountain in winter.

The limit rain/vertical snow varies daily according to the mass of air. In the moderated areas, it is in general at its maximum altitude in summer (3 to 6 km) and minimal in winter (surface with 1 km). It is allowed that as one moves towards the south, the average temperature increases 1°C every 150 kilometers, from where athwartship variations. The annual average has tended to go up for a few decades with the total warming of the atmosphere. Even if the limit snows/rain tends to rise, all the mountainous solid masses are not touched identically for example, being given their own altitude and them particular Latitude.

This vertical limit between snow and the rain is very important for:

aviation because one observes important icing there.
the public and road transport because it will make the difference between falls of Neige or Pluie and even of formation of Verglas. These various situations calling a reaction very different for maintenance from the roads and control to be followed.
In mountain where the type of vegetation varies according to this limit.

Nival cover

In the case of the mountains, the limit rain/snow will also indicate altitude where snow remains after a snowfall at a given season. The latter is then influenced not only by the snowfall but by the temperature of the ground and the cover of former snow. It exists more or less simple observations of grounds there making it possible to describe the phenomenon:

as soon as the Neige holds on the ground or on any surfaces;
fine of the last drops of rain in the snowflakes (criterion classically selected);
as many drops of rain of the snowflakes;
first traces of flakes in the rain…

Horizontal field

The atmosphere is in three dimensions and one notes an average temperature slope between the Ecuador and the Pôles as one mentioned previously. The weather systems nourissent of this difference in the atmospheric Circulation to give rise to the depressions. In these last, the structure of temperature is not only vertical but also horizontal. According to the season and the type of system, the Isotherme zero degree will be found at a variable altitude at the various places of the Earth affected by the depression.

When this isotherm approaches the ground sufficiently, the rain changes into snow. When the trajectory of such a system is followed, one can note the geographical limit between the rain and snow. In general, that occurs in cold season in the average and high latitudes since in the Tropiques, the isotherm never goes down rather low. Once again, this limit is crucial in all the fields of human activity.

See too

Isothermal Meteorology

zero degree
Precipitation S

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