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High and Low pressure areas


hoch tief EN
Source: Physik, Uni - München

Low-pressure areas

In fact of the insolation the earth`s surface is warming up and acts like a heating to the surrounding air. The air expands while heatening and because of the expansion has a lower concentration than the cold air. As a consequense of this the warm air raises up and warms also the regions above. Simultaneously the air takes off from the "heating" - the ground- and so is cooling down. If this process occurs in a greater area, there will arise a great packet, warmer, expanded air, which is now higher stratified (towering) than the surrounding air. If this heigh difference grows, the warmer air starts to flow laterally over the adjoining cooler air packets. In fact of this the air is cooling down further, getting heavier and finally sinking to the ground, when it hits the less cool regions.
Based on this process, a circulation arises: The air raises up under high pressure, is spreading out laterally and is flowing down to the areas with lower pressure.The remaining air is cooling down under certain conditions and so is decreasing. Because of the drain it has lost some of its mass so the pressure is sinking. 
By warming the air can absorb also more water. This air is moved up by the increase of the water . If the air is cooling down, humidity will pass out again:clouds are formed in the altitude. An evidence for the lateral runoff of warmer air masses is the construction of cirrostratus clouds, which compress more and more. Cirrostratus clouds are formed by the continuing flow of more warm and humid air. If the flow of warm air (advection) goes on, altostratus clouds are builded, followed by nimbostratus clouds. These nimbostratus clouds are usually connected to enduring drizzel, the so called "steady rain". Because of the rain the cloud ceiling is breaking off the more and more until finally the sun reaches the ground and starts to warm up the air again. 

High-pressure areas

In an anticyclone aerial masses drop extensively. At the same time the air warms itself up, so that no condensation and consequently no cloud formation can take place. In ground nearness the air flows out of the anticyclone in the direction of depression out - it diverges. Hence, there is no formation of fronts in the hight. During the subsidence of the aerial masses an inversion forms. At that the clouds resolve.


A anticyclone is builded quiet slowly. The forces of circulation in the subtropic areas lead to stable anticyclones.


Because there are differences with the origin or development, one divides anticyclones into three categories:

A cold high originates if air cools off, for example, in winter about a cool land mass (e.g., Central Asian high). Then the air has a bigger density and exerts a higher pressure on the base. In the middle widths it can also originate in the form of flat wedges in the back of cyclones as a ridge of high pressure.

A dynamic high is generated by Rossby-waves (Polarfrontjetstream). The dynamic Azores high exerts, on this occasion, big influence on the weather of Central Europe.

A hight high is an anticyclone which appears by big heights and is shown, hence, in hight weather charts. It is always connected with a ground low-pressure area, because with the warming of surfaces the vertical pressure gradient is lowered and is reflected the relative atmospheric pressure reduction on the ground with increasing height in a pressure relatively higher to the horizontal surroundings. Hence, one can derive the other way round also from a ground high (also thermal high) a height low-pressure area.


Thermic Low: ground low, height low


A ground low developes if the density of the ground-near air is decreasing while the air is warming up. The warm air is breaking away from ground and rises up ( thermic), what leads to a pressure decrease at the ground level ( in higher regions the air pressure is rising because of the inflowing ground-near warm air). The ground-near pressure decrease leads to a spacious inflow of external air ( winds).

A height low developes if cold air decreases from great heights what reduces the air pressure in the higher regions of the air. However, on the ground the air pressure is decreasing. On the ground instead the air pressure is increasing. The height low is based on a heigh of 5km and is identified because of its low temepratures in comparison to its surroundings.On the height weather card (e.g. 500 hPa-topopgraphy) you can see the height low through some closed isohypses, on ground weather cards it is nearly not noticeable.

A thermic low-pressure area developes by a difference in the air pressure which is realized by a warming (insolation) oder a cooling. Whether it is a ground low or a height low depends on the concerning air regions.



 Dynamic low (cyclone) 

A dynamic low developes if the air flow converges in lower regions of the atmosphere and diverges in the height while the air is kept up in the centre of the low.


meteogram
meteogram:
Getaria (43.30°N / -2.20°E)

Isobars

If we choose a location on the card which have the same air pressure in a certain perimetre and if we connect this with a line, there will be a circle. These circles are called isobares.
This process is repeated again and again with a different air pressure without that the circles touch or cross each other. An international standart for the gap between the circles is 5hPa on weather cards (hPa = unit for the measurement of the air pressure). The closer the lines of the circle are locatetd parallely, the higher is the pressure gradient.

 
In the heart of a cyclone, the smallest circle of a low, the pressure is the lowest. The more you depart from the heart, the higher is the air pressure.

In the high-pressure area it is the other way round. The highest pressure is in the heart of the height and the more you depart from it the more the pressure is sinking.
 


Hochdruck
Hochdruckgebiet

meteorologie tiefdruck 250.jpg 1588122126 01
Tiefdruckgebiet

Worldmap for relative barometric pressure / topography


Rel.Topographie Luftdruck
Rel. Luftdruck/Temperatur, klicken um die aktuelle Karte zu sehen

weather happenings

By warming the air can absorb more humidty and can rise up because of its lower density and so the humidity is lifted to higher regions. There, the air is cooling down and so losing its water holding capacity. After a rapid increase the air reaches a great height over a warm area, it has kept more water than it could keep beeing cool. It flows exensively to the sides while exuding humidity: so, cirrus are formed in great height, which compress more and more - until finally cirrustratus clouds are built. This clouds are replaced laterally by the further inflowing air. Because of the increase of warm air ( advection), altostratus clouds are built, followed by nimbostratus clouds, which are usually connected with steady rain. The cloud ceiling is breaking up more and more because of the rain until finally the sun reaches the ground and warms the air.




Questions for training

In the following, you can query and test your know-how - and apply it. If you are in doubt, read the text and images above again.
Try it out ;-). We -the meteoblue Team  - wish you some fun! 

Take a look at the card on the right handsite. It`s an extract of a pressure area in 850 mbar ( about 1.5km) height.Can you tell by reference to the pressure gradient, wether it is a low-pressure area or a high-pressure area?

Follow now how the pressure area is coming closer to Dublin on the animated card. Pay attention to the pressure card in the higher regions (height at 500 mb, about 5km). Do you find here the same allocation of the pressure area?
These cards are located in europe- so on the northern hemisphere. Try to understand by reference to the animated cloud- and precipitation cards, in which direction the wind of the pressure area is blowing (also because of this you can recognize wether it is a low-pressure area or a high-pressure area).

Attention: On the southern hemisphere, the winds of the pressure areas blow the way round.


Kartentief-Bild
Click to enlarge

Animated maps - Click to enlarge and play:

Tiefdruckgebiet-Druckkarte
Temperature at 850 hPa


Tiefdruck-Wolkenkarte-Animation-small
Cloud cover

Tiefdruck-Animation Lustdruck Hoch
Height at 500 hPa

Tiefdruck Wolken Animation-klein
Precipitation



Below you can see 4 different figures. 2 of them are high-pressure areas and 2 are low-pressure areas. Note, which belongs to the low and which to the height. ( tip: Pay attentionn to the pressure of the isobares).

Bildschirmfoto 2010-07-12 um 11.36.10
Bild 1: Anklicken zum Vergrößern

Bildschirmfoto 2010-07-12 um 11.38.12
Bild 2: Anklicken zum Vergrößern

Bildschirmfoto 2010-07-12 um 11.36.36
Bild 3: Anklicken zum Vergrößern

Bildschirmfoto 2010-07-12 um 11.44.24
Bild 4: Anklicken zum Vergrößern