Definition
A constant-pressure surface in the atmosphere where the air pressure is 400 millibars (40 kilopascals). This level typically sits near 24,000 feet MSL, though its exact altitude varies with temperature and weather systems. Upper-air weather charts depict winds, temperatures, and heights along this surface to show conditions in the upper troposphere.
Plain English
An imaginary layer high in the atmosphere defined by a specific air pressure rather than a fixed altitude. Forecasters use it to map winds and weather features at high cruising levels, usually around 24,000 feet.
Context Anchor
Seen in Significant Weather Prognostic Chart explanations, especially when describing the layer of weather from the surface up to about 24,000 feet.
Derivation
‘mb’ stands for millibar, a unit of pressure. The number 400 refers to the pressure value itself, so the ‘400 mb level’ literally means ‘the layer of atmosphere where the pressure equals 400 millibars.’
Why Pilots Care
Pilots consult the 400 mb level to anticipate strong upper winds, clear air turbulence, and temperature that affect fuel burn and ride quality at typical jet cruising altitudes.
Grounding Statement
Picture a wavy, invisible sheet draped through the sky at the altitude where pressure happens to be 400 mb -- it dips lower in cold air masses and bulges higher in warm ones.
Intuition Check
Do not read “400 mb level” as one exact altitude everywhere. It means a pressure level, usually near 24,000 feet, and its actual height can move up or down with the atmosphere.
Example Sentence 1
Before the high-altitude leg, the crew checked the forecast winds at the 400 mb level to estimate their groundspeed at FL240.
Example Sentence 2
Checking the 400 mb chart showed a jet stream core directly along our route.