Zero degree isotherm

Zero degree isotherm

The Zero degree isotherm, also known as the 0° isotherm, also referred to as "zero-line altitude", represents the altitude in which the temperature is at 0 degrees Celsius (the freezing point of water) in a free atmosphere (i.e. reflection of the sun by snow, etc). Any given measure is valid for only a short period of time, often less than a day.

Above the 0 degree isothermic, the temperature of the air is negative. Below it, the temperature is positive. The profile of this frontier, and its variations, are studied in Meteorology, and is used for a variety of forecasts and predictions. Whilst not given on general weather forecasts, it is used on bulletins giving forecasts for mountaineous areas.

Measuring

There are several different methods to examine the structure of the temperature of the atmosphere:
*A radiosonde attached to a weather balloon is the oldest and most common method used. Each area releases two balloons a day in locations hundreds of kilometers apart.
*Measuring devices attached to commercial airliners permits reporting the isotherme, and its height, to aerial traffic.
*Weather satellites are equipped with sensors which scan the atmosphere and measure the infrared radiation it emits.
*Weather radar detects Bright bands, which are radar echoes produced just underneath the isotherme caused by the melting of snow in the layer above 0 Celsius.
*A Wind profiler, an upward pointing radar, can detect the speed of precipitation; which is different for rain, snow, and melting snow.

Depending on the frequency and resolution at which these readings are taken, these methods can report the isotherm with greater or lesser precision. Radiosondes, for example, only report a reading twice daily and provide very rough information. Weather radar can detect a variation every five to ten minutes if there is precipitation, and can scan an area up to two kilometers.

Characteristics

The zero degree isotherm is determined by releasing weather balloons into the atmosphere. This allows a chart to be produced from the data from the balloon.

The isotherm varies globally as weather conditions vary globally. The measure is important for many reasons. It helps to predict the average temperature, the likelihood of snow and avalanches, and it therefore protects skiers and ski resorts.

Variations in the Isotherm

The 0°C Isotherm can be very stable over a large area. It varies under two conditions:
#A change in the mass of air, with a moving hot or cold fronts. This changes the isotherm gradually, over tens of kilometers for a cold front, and hundreds for a hot front.
#Local levels can be changed by wind, reflection of the sun, snow, and humidity level. These can cause it to change rapidly over several kilometers, in both winter and summer. Also, atmospheric subsidence and ascendence can contribute to variations in the level of the 0°C Isotherm.

References

* "The Mountain Manual", Seuil, 2000


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