Why do meteorologists use weather maps

The dew point - an all-rounder in weather forecasting

Water vapor plays a crucial role in meteorology. Despite its camouflaged appearance as an invisible and odorless gas, the gaseous state of aggregation of water is an omnipresent part of the troposphere. The troposphere is the lowest layer of the earth's atmosphere, which, depending on the temperature, reaches a thickness of around eight kilometers at the poles and up to around 17 kilometers at the equator. Almost all weather-relevant processes such as cloud formation and precipitation processes take place there.

Today's topic of the day should focus in particular on the water vapor content in air layers close to the ground. In the weather forecast, the so-called "dew point" has proven to be most effective in this regard. It defines the temperature to which an unsaturated quantity of air must be cooled over a level, chemically pure water surface in order to reach saturation. In the saturation state, the relative humidity is 100 percent, so the dew point and temperature are then the same. In the event of oversaturation, the air is no longer able to absorb additional moisture, which would make the excess water vapor noticeable in the form of haze and fog. Since, in accordance with the World Meteorological Organization (WMO), the standard temperature is measured at a height of two meters, the humidity measurement provides the corresponding dew point at the same level. The difference is referred to as the so-called "spread". For example, meteorologists like to hear briefly: "Oh, Berlin already has ten over minus ten." This means that in Berlin the air temperature is currently ten degrees in two meters with a simultaneous dew point of minus ten degrees (spread = 20). In this case, the relative humidity would only be around 23%, so the air is very dry.

The dew point is now used in daily practice with a wide variety of forecast parameters.

1.) Fog

Since - as already mentioned in the section above - fog represents an oversaturation of the air, the use of the dew point is essential for the fog forecast. If, for example, loosening and weak winds are to be expected in the night and the spread was already low in the evening hours, the probability of fog is increased. Or, in another case, if a moist, warm air mass with high dew points sweeps over cold waters where the water temperature is below the dew point, the air in the layers close to the surface is rapidly cooled, so that oversaturation and thus fog formation sets in. With south or south-west locations, this natural spectacle of the lake fog can often be marveled at in spring over the North and Baltic Seas.

2.) Minimum temperature

In locations without a change in air mass, the dew point in the evening generally provides a good guide value for the lowest temperature to be expected. If it is very close to the measured temperature (spread close to zero), a significant cooling in the night hours is hardly to be expected. If, in the opposite case, the difference is very large, a rapid temperature decrease usually sets in with sunset.

3.) Snowfall

Statements related to questions like: "Does snow fall and if so, does it stay where it is?" can be estimated using the dew point. If the mean value of temperature and dew point (corresponds approximately to the so-called "wet temperature") is less than two degrees, the occurrence of snowfall is usually probable; at zero degrees or less, the snow remains at suitable ground temperatures. This rule of thumb could also be observed very impressively during the wet snowfall yesterday, Wednesday, in the Lausitz and the Erzgebirge foothills. Due to the cold prehistory of the past few days, the air there was still very dry (low dew points) when it started to rain, which meant that snow fell at the beginning. This initially remained on the frozen ground before the wet temperature (or the dew point) rose to over zero degrees in the evening hours. As a result, the precipitation turned into rain and the fallen snow melted away again.

4.) cloud base

Knowledge of the Henning rule of thumb is particularly important in aviation meteorology. It says that the spread multiplied by 125 is approximately the lower limit of cumulus clouds in meters.

As you can see, the often inconspicuous dew point can be useful in a wide variety of ways when making your own forecast and looking through the weather maps. Perhaps more and more of yours in the future?

Dipl.-Met. Robert Hausen

German Weather Service Forecast and Advice Center Offenbach, December 1st, 2016

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