Isotherm zero degree

The isothermal zero degree (also called isothermal level of congelation and noted 0 °C or Iso 0° ) represents the minimal Altitude to which the Température reaches the value of 0 Degree Celsius in a free atmosphere (in the absence of reverberation of the solar rays by snow, etc) in an area of terrestrial surface and a given time interval. This measurement is taken over one relatively short period of time (of about a day, sometimes less).

The isothermal 0°C is a fictitious border between two masses of air: above the isotherm 0°C, the air is with a negative temperature, below the isotherm, the temperature is positive. The profile of this border and its variations are studied in Météorologie for forecasts and recommendations various. It does not appear on the general charts weather, but figure for example on the bulletins of forecasts in mountain.

“Isothermal” is a female term, although the use often devotes the male kind by abuse. In a context where the degrees Fahrenheit prevail, the isotherm 0°C notes Iso 32F .

Measure, modeling

To locate the structure of Temperature of the atmosphere, there exist various means:

the Radiosondage exploration, by releasing Sounding-balloon S, is the oldest means and more running. Many countries proceed to the catch of these data twice a day, in various distant points of several hundreds of Kilomètre S one from the other:

$\ mbox {Iso 0} \ simeq altitude + (T_ {altitude} \ cdot 165)$

This method could not give access to more that one order of magnitude, and is not usable in the event of variations of the fast weather conditions (a few hours). To also take account of the probability that an inversion of temperature affects the zone (see hereafter).

Zero multiples

Broad scale known as synotic

In the zones of hot faces, the soft air is forced to go up along the face by the cold air of the sub-bases. It will be established a stratification of the temperatures which could be isothermal or to even heat from surface until a certain height then will decrease according to a normal heat gradient. Under certain conditions where the circulation of the winds channels a mass of air on the ground of temperature lower than the freezing point, the temperature of the raised air will be able to be above 0°C and to remain it on a certain thickness before falling down under zero. This leads to situations where Précipitation S frosting develop: Glaze, Grésil, Snow in grains and rolled Snow.

Small scales known as méso-scale

In mountain, several isotherms 0°C can coexist. It can for example happen that three masses of air are superimposed, forming three distinct layers. By Subsidence, a cold mass of air fall and fills up the valley, while hot air is placed above. A third layer of air, cold, overhangs the two first. It is then hotter above the valley, but less than in high-altitude.

A sounding-balloon released since the valley would cross successively these three layers of air in balance. In the first, the temperature would fall for example with altitude, at a rate of 0,65°C all the 100m (the atmospheric gradient standardized of the ICAO), and forms a ceiling which contributes to the stability of the phenomenon. This configuration of the masses of air, partitioned and superimposed, is maintained then until a depression comes to upset thermal balances or which the temperatures equalize (the evening and the night, for example).

See too

Isothermal Meteorology

of temperature
Variation in temperature
Limit rain/snow
Face (meteorology)