Meteorologists measure the atmospheric pressure because it is helpful
when analysing and predicting winds and weather. The pressure is
measured using a barometer, in units of millibars (mb), hectopascals
(hPa), or inches of mercury. A correction is then applied to calculate
the equivalent sea-level pressure.
On a weather chart, lines joining places with equal
sea-level pressures are called isobars. Charts showing isobars
are useful because they identify features such as anticyclones
(areas of high pressure), depressions (areas of low pressure),
troughs and ridges which are associated with particular kinds
of weather.
The analysis
chart shows the weather situation for 0000 GMT (midnight) last
night.
The forecast
chart shows the expected situation at 0000 GMT tonight.
The composite
charts combine the actual chart and the satellite image for 0000
GMT.
All three charts are updated once a day.
The interpretation
pages describe weather systems and fronts in more detail.
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