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Weather Glossary
Low Pressure and High Pressure Systems
Low pressure occurs where warm air ascends and cools down allowing clouds to form which is the reason
why weather is almost always unsettled when the weather is dominated by Low pressure systems. High
pressure is where cool dry air descends warming as it does so and suppressing cloud formation leading to
dry settled conditions. As you can see air flows between the two systems. Low pressure systems have
weather fronts associated with them while High pressure systems don’t. See also Cyclogenesis and
Explosive Cyclogenisis.
Below you a fax chart showing High pressure and Low pressure systems. Note the cold fronts, warm fronts
and Occluded fronts on the fax chart. Low pressure systems are indicated by L and High pressure by H.
Picture courtesy of the Met Office
Cyclogenesis and Explosive Cyclogenesis (Bomb)
Cyclogenesis refers to the formation of low pressure systems. All Atlantic Depression, Low pressure
systems are a form of Cyclogenesis. These ranging from wave depressions to modification of ex
hurricanes. Cyclolysis refers to when the low pressure weakens soon after the cold air reaches the centre.
Image courtesy of the http://www.britannica.com
Explosive Cyclogenesis refers to the rapid deepening of a Low pressure system where a large pressure
drop of 24 millibars in 24 hours or less. These happen quite often around the UK the best example being
the October 1987 storm once gain.
Atmospheric Pressure or Air Pressure
Atmospheric Pressure is the force per unit of on a surface by the weight of air above it. This is commonly
measured in millibars mb or Inches. So you may see a Low pressure or a High pressure system with the
number 1002 next to it.
This will be the value in millibars mb. The same value would be 29.59 in.