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Barometers measure atmospheric pressure, that is to say the weight of the air column above our heads. High pressures (anticyclones) usually mean fine weather while low pressures (depressions) are associated with bad weather. But this is a little too simple and barometer readings should be interpreted in respect of sudden or slow pressure changes. Seasons should also be taken into account as well as types of wind, humidity and high altitude phenomena.
Sudden fall of pressure:
This may mean that rain is on the way.
Regular fall of pressure:
If the weather is fine, there should be no change.
Steady rise of pressure:
A slow and regular rise of pressure means that fine weather is on the way.
Sudden rise of pressure:
A sudden rise in barometric pressure after a time of low pressure is an indication that the fine weather will not last for long.