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Barometric pressure

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Coldicote | 13:37 Sun 03rd Jan 2010 | Science
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I was given an unusual and most absorbing Christmas present - a weather station, about 27cms wide x 12 x 3, plus an outdoor sensor. Amongst other information it gives temperature and humidity outdoors and indoors, barometric pressure, sunrise and sunset times, phases of the moon and not least a weather forecast that seems to be proving remarkably accurate. One thing I find difficulty 'getting my mind around' is WHY when barometric pressure rises does it mean fine weather? Conversely when the atmospheric pressure is low WHY doesn't that mean calm settled weather? It seems almost the opposite to what one might expect. Can any scientists out there explain please?


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There are a number of reasons why low pressure areas can be associated with inclement or unsettled weather.

Low pressure areas generally contain air that is in the process of rising from the earth’s surface. If the air contains enough moisture this moisture condenses (as the temperature drops with increased altitude) and the condensation falls as rain, sleet or snow (depending on the temperature at lower levels).

Strong winds are also a product of low pressure areas as they try to “fill” by equalising the pressure.

Low pressure areas also often form the boundaries between warm and cooler air. These temperature differences give rise to disturbances along them and these form “weather fronts” which often contain rain.

By contrast High Pressure areas are usually areas of sinking air. This usually (though not always) prevents clouds from forming and leads to dry, settled conditions, hot in the summer but cold in the winter (especially at night). Areas of high pressure tend to be more stable and little or no wind is produced whilst they are around.

The above applies to temperate zones such as northern Europe. Different conditions exist in polar and tropical zones. For example, low pressure in the tropics often produces settled weather.
Strangely, water vapour weighs less than air itself. Thus, when there is water vapour present in air, the total weight of the air is reduced, and we have a low pressure area. Conversely, when the lower weight water vapour is absent, the air weighs more in total, and we have a high pressure area.
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Very many thanks for these informative answers.

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