Pressure levels
Air pressure Isobaric levels Pressure surfaces
A pressure surface connects points with the same air pressure. Weather charts use these levels to compare temperature, wind and moisture through the atmosphere.
Floors that change their height
An 850 hPa chart shows conditions wherever the pressure is 850 hectopascals. That surface does not have the same height everywhere. Pressure decreases upwards, but the rate depends partly on the temperature of the air column.
Roughly how high are these levels?
On small screens, scroll sideways to see all columns.
| Pressure level | Approximate height | Useful for |
|---|---|---|
| 850 hPa | 1.5 km |
Air-mass temperature and moisture |
| 700 hPa | 3 km |
Mid-level moisture and flow |
| 500 hPa | 5–6 km |
Large-scale troughs and ridges |
| 300 hPa | 9 km |
Strong upper winds and the jet stream |
These are guides. Geopotential height gives the actual level. Over high mountains, lower pressure surfaces may even lie below the ground, so their values cannot be interpreted as conditions in the free atmosphere there.
Minus five aloft does not mean frost below
A temperature of −5 °C at 850 hPa is not a forecast for the walk to school. Sunshine, the ground, wind and the layers in between affect surface temperature. The upper-air chart describes the air mass; the two-metre temperature describes conditions near the ground.