Manual La Crosse WS-2800-IT (page 25 of 40) (English)

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RELATIVE (AIR) PRESSURE

WHAT IS RELATIVE PRESSURE?

Relative (also referred to as atmospheric) pressure is defined as the force per unit area exerted against a surface

by the weight of air above that surface at any given point in the Earth’s atmosphere.

Basically, even the atmosphere has weight. For most purposes, we can get air pressure by measuring the

hydrostatic pressure caused by the weight of air above a certain point.

Low pressure areas have less atmospheric mass above their location, whereas high pressure areas have more

atmospheric mass above their location. As elevation increases there is less overlying atmospheric mass, so that

pressure decreases with increasing elevation.

AIR PRESSURE AND WEATHER FORECASTING

Air pressure is commonly used for weather prediction, as a region of high air pressure indicates fair weather, while

low pressure tells us that rain or storms are more likely. When used in combination with wind observations,

reasonably accurate short term forecasts can be made.

Each geographic area has its own ‘normal’ range of air pressure values. You can obtain this information from your

local weather reporting station (e.g. the airport). With that information, you can calibrate the weather display to

the reporting station (see Operations Manual for accessing the SET menu). When calibrated, the relative increase

and decrease in air pressure values will correlate with your local media reports. If you do so, remember that any

hardware reset will require the weather display to be re-calibrated.

Simultaneous relative/air pressure readings from across a network of weather stations allow maps of air pressure

areas to be produced, which were the first form of the modern weather map when created in the 19th century.

Isobars (lines of equal pressure), when drawn on such a map, gives a contour map showing areas of high and low

pressure.

Localized areas of high atmospheric pressure act as a barrier to approaching weather systems, diverting their

course. Low atmospheric pressure, on the other hand, represents the path of least resistance for a weather

system, making it more likely that low pressure will be associated with increased storm activities.

Typically with personal weather forecasting, we focus on the change in air pressure over time. The larger the

change in pressure, especially if more than 3.5 hPa (0.10 inHg), the larger the change in weather can be expected.

If the pressure drop is rapid, it indicates that a low pressure system is approaching, and there is a greater chance of

rain. Rapid pressure rises are associated with improving weather conditions, such as clearing skies.

RELATIVE PRESSURE ALARM

You can configure an alarm to sound when the relative pressure goes above or below a certain point. For more

details on how to set an alarm, see the section on Setting Alarms.

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La Crosse WS-2800-IT User Manual - English - 31 pages

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La Crosse WS-2800-IT Manual

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