Pressure

In this section you can find information about air pressure related variables, such as the relative topography for example.

Air pressure

The atmosphere is formed by air and surrounds the Earth (see to meteoScool). The air column on each square metre of the Earth's surface weighs about 10 tons, corresponding to a pressure (force per surface) at sea level of about 101,300 N/m², or 101 300 Pa (1013 hPa).
Air pressure decreases with increase in altitude. The air at the bottom of the atmosphere, close to the ground, is compressed by the air above it and therefore more dense, resulting in higher pressure in the lower layers. At the top of the atmosphere, in 12-15 km altitude, air pressure is around 100 hPa. The printed pressure on the main page of our site is the average for the day, and not the actual pressure.
 The only diagram where you can find the temperature and pressure together is in the AIR meteogram, the two are not updated with the measurements.

For meteorological purposes, we provide air pressure information in the following ways:

  1. Standard air pressure: measured at sea level. Air pressure measured at higher altitudes is usually corrected to sea level, to show spatial and temporal differences in pressure. An example of a pressure curve is shown in the meteogram AIR.
  2. Isobars: lines of "same barometric" pressure. These are shown on pressure maps to identify zones of common pressure and spatial differences.
  3. Pressure levels: these are shown on pressure altitude maps (thickness) or AIR diagrams to identify different altitude levels, at which the meteorological information is displayed.

Air pressure has an effect on various phenomena, such as the lower oxygen availability and lower water boiling point at higher altitudes, as well as on convection and cloud formation.

Altitudes are also commonly measured using "altimeters" calibrated with air pressure. These measurements therefore will tend to overestimate altitude, when the air pressure is lower ("depression"), or underestimate altitude when the air pressure is relatively high. Precision altimeters are therefore specially calibrated or use GPS or other reference methods for altitude measurement, and do not use air pressure.

The comparison between the measurements of the pressure stations are possible only if they use the same conventions for the display of pressure (and this is known).
In practice, there are also many mathematical methods to reduce pressure to sea level pressure, which differ by up to 20 hPa depending on the correction height needed. Therefore, measured values from different stations may differ substantially when normalised to sea level, and those differences between stations are often larger than the differences at one station in the course of a day.
Therefore, comparisons of measured data from stations with forecasts are also difficult.
A practical recommendation is to use the pressure values of a system especially to see the changes in the course of the day: this information is the most useful and will help in the interpretation of meteorological phenomena.
In meteoblue.com we show a forecast of air pressure, not observations. These may not provide exact overview for a particular location, but a good view of expected developments, and a better comparison between locations than would be possible by individual stations. These values are available worldwide, with very high resolution in some areas.

Geopotential Height

The geopotential height shows the height of an air parcel in units proportional to its geopotential, which is the potential energy per mass unit. It indicates the potential energy an air parcel of 1 kg contains in a specific height above mean sea level. The unit of the geopotential height is the geopotential meter (gpm) or decameter (gpdm). One geopotential meter is equal to 0.98 J/Kg or 0.98 dynamic meters. The geopotential height is often used to express the altitude of a specific pressure level above sea level (e.g. 500 hPa).

Because air pressure decreases with increase in altitude, a specific pressure (e.g. 500 hPa) can be measured in a specific altitude, where it builds a layer (level) of same pressure. Therefore, geopotential height tells you the altitude where you will find a specific pressure (e.g. 500 hPa). The height varies because air volume depends on temperature (see thickness below). For instance, the 850 hPa level is higher if the air masses are warm.

Thickness

Thickness is a measure of the difference between the height of two pressure levels. It is often expressed in geopotential decametres (gpdm). The 500 hPa-1000 hPa layer is the most common layer that is analysed. It is used to define airmass mean temperatures of the lower troposphere. Further, thickness values can be used to define location of fronts for instance.

Imagine two different pressure levels (e.g. 500hPa and 1000hPa; see geopotential height) in the free atmosphere. Between these two levels there is a specific volume of air whose extent depends on temperature. The higher the temperature of air, the greater the volume. Therefore, if the air between these two levels is warm, the volume is bigger and as a result, the difference in altitude between the pressure levels is greater. On the other side, if the air layer in between cools, volume decreases. Thus, thickness simply describes the difference in altitude between two layers in the atmosphere.