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The freezing level or freezing level height (FLH) represents the altitude in which the temperature in a free atmosphere is at 0 °C , i.e. the freezing point of water. FLH is important for weather in mountainous regions and aviation and over time an indicator of climate variability and climate change.
Any given measure is valid for only a short period of time, often less than a day as variations in wind, sunlight, air masses and other factors may change the level.
The freezing level height (FLH) represents the altitude, at which the air temperature is at 0 °C, the freezing point of water. It indicates the altitude at which rain transitions to snow.[1] It is also called 0 °C (zero-degree) isotherm, where an isotherm represents the line on a weather map with the same temperature.[1]
Above the freezing altitude, the temperature of the air is below the freezing point of water and ice may form. Below it, the temperature is above freezing.
The freezing level height is studied in meteorology and used for a variety of forecasts and predictions, especially in climate science,[1] serving as an indicator of climate variability and climate change. [2]
The freezing level height is an important cornerstone of alpine climate. Whilst not given on general weather forecasts, it is used in on bulletins giving weather forecasts for mountainous areas. The freezing height level and icing forecasts are of interest to aviation.[3]
Freezing level height changes correlate with changes in snow cover, evolution of glaciers and changes in permafrost.[4]
The 700 hPa pressure level or about 3000 m above sea level has historically been assumed as a rough estimate of the freezing level height.[citation needed]
The freezing level height is determined by measuring the temperature in the free atmosphere i.e. allowing reflection of the sun by snow, icing conditions, etc at different altitudes. [citation needed]
There are several different methods to examine the temperature and calculating the freezing level height:[citation needed]
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 a radius of up to two kilometers.[citation needed]
The freezing level height varies by season and is much lower in the winter than in the summer.
The the 0 °C isotherm can be very stable over a large area. It can vary under two major conditions locally and globally:
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