Vertical profile (meteorology)
A vertical profile or temp is a graphic representation of the air temperature , the dew point temperature , the wind speed and the wind direction as a function of the altitude. The data required for this is determined by radiosondes that rise into the atmosphere from weather stations at regular intervals.
Since the air pressure is exponentially related to the altitude via the barometric altitude formula , the altitude scale is also often replaced by a logarithmic pressure scale.
presentation
There are different types of representation, e.g. B. the Stüve diagram or the Skew-T diagram . In order to make orientation easier, various auxiliary lines are drawn into the diagram (see adjacent figure):
- Isobars are lines of constant pressure. They run parallel to the T-axis (horizontal) and each stand for a certain height. The figure shows the pressure in mbar (= hPa) and the height in feet.
- Isotherms are lines of constant temperature. In the Stüve diagram they run parallel to the log-p axis, in the Skew-T diagram they are inclined towards this (hence the name Skew-T from English "skew" = oblique).
- Dry adiabatics : A parcel of air that rises in the atmosphere expands and cools due to the decreasing pressure. If no heat is exchanged with the environment and no water vapor condenses, one speaks of a dry adiabatic ascent. In the diagram, the dry adiabats are the solid, curved lines.
- Moist adiabats : When saturated air rises, moisture condenses and releases condensation heat . This heat benefits the air parcel, which is why the temperature falls less sharply during a wet adiabatic ascent than during a dry adiabatic ascent. The wet adiabats are therefore steeper than the dry adiabats. Here they are drawn with dashed lines.
- Saturation curves: These lines indicate the temperature of a saturated air-water vapor mixture of a given composition as a function of the pressure (and thus the altitude). In this illustration, they are drawn with dashed lines and run roughly parallel to the isotherms.
interpretation
The interpretation of the vertical profile should be explained using the example opposite.
The following variables can be examined with a vertical profile:
humidity
The dew point is a measure of the absolute humidity . If you want to know the absolute humidity at a certain height, you look for the saturation curve that goes through the dew point (green point) at this height.
The relative humidity can be read from the distance between the dew point curve (green points) and the ambient temperature curve (red points). The dew point difference , also known as the “spread”, is a direct measure of the relative humidity. If it is zero - in other words: If the two curves touch, the humidity is 100%. The air is saturated. As a rule, the curves do not intersect, as the excess moisture condenses out in the form of fog , so that no oversaturation occurs.
In this example, the temperature on the floor is 95 ° F = 34 ° C with a dew point of 65 ° F = 18 ° C. The dew point difference is 16 K. This results in a relative humidity of approx. 40%.
Air stratification
The air temperature usually decreases with increasing altitude. If the opposite is the case, one speaks of an inversion . It can be seen very easily in the vertical profile, namely when the temperature curve is clearly inclined to the right. (In the example there is no inversion.)
If the temperature decreases more slowly with increasing altitude than the dry adiabatic temperature gradient, i.e. if the temperature curve is steeper than the auxiliary lines of the dry adiabatic, one speaks of stable stratification, in the opposite case of unstable stratification. (If condensation occurs, the moisture adiabats should be used instead.) In the case of unstable stratification, an air parcel whose temperature is only slightly higher than the ambient temperature could rise without energy being supplied. In the diagram, it would then follow the dry or wet adiabatics until its temperature matches the ambient air through adiabatic expansion. An unstable stratification would therefore facilitate the vertical exchange of air masses. A stable stratification, and especially an inversion, would very quickly slow this ascent.
In the figure you can see this in the area marked pink: The ambient temperature falls between approx. 2000 and 12000 m faster than the wet adiabats.
Cloud formation
Cumulus clouds are formed when moist, rising warm air cools down until saturation is reached and the moisture it contains condenses. This happens exactly under the following condition: Imagine an air parcel on the ground of a certain temperature (here 93 ° F) and follow its dry adiabats on the one hand and on the other hand the saturation curve that goes through the dew point of this air parcel (65 ° F) until both lines meet cut (blue triangle). At this height (black dashed line) and at this temperature (approx. 15 ° C) the moisture begins to condense. This height (approx. 2000 m) represents the lower edge of the clouds - the cloud base.
The air can then continue to rise (see air stratification), but then follows the humidity adiabatic until it again reaches ambient temperature. For the formation of high clouds and thus the formation of thunderstorms, a high level of instability and high absolute humidity in the soil air are prerequisites, as can be seen in this example. It can cumulonimbus clouds form m with a cloud base of 2,000, ranging up to 10 km altitude.
hairdryer
If moist air is deflected upwards by a mountain range, clouds can form. The ascent is therefore initially dry adiabatic until the air reaches its saturation level, and then wet adiabatic. The precipitation removes water from the air. When the air then sinks on the leeward side of the mountains, it is compressed and warmed up, but this time dry adiabatically. The air warms up faster when it sinks than it cooled down when it rises. The temperature is therefore higher after flowing over the mountain than before. This hot, dry wind is noticeable as a foehn on the lee side of the mountains.
Individual evidence
- ↑ Information poster of the German Weather Service ( Memento of the original from September 23, 2015 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 3.3 MB)
- ↑ G. North, T. Erukhimova: Atmospheric Thermodynamics . Cambridge University Press, Cambridge 2009, ISBN 978-0-521-89963-5 ( limited preview in Google Book Search).
- ↑ An online calculator was used for the calculation .
