Barometers

Instrument that measures atmospheric pressure

Two types Mercury barometer Anenoid barometer (digital)

How Do Barometers Work?

Balances weight of mercury with atmospheric pressure

Atmospheric pressure is “weight of air” so level of mercury changes until it matches

Forecasting Weather

Changes in atmospheric pressure are a commonly used way to forecast changes in weather

Sudden drop in atmospheric pressure often forecasts an incoming storm

Guidelines

Decreasing barometric pressure indicates storms, rain and windy weather.

Rising barometric pressure indicates good, dry, and colder weather.

Slow, regular and moderate falls in pressure suggest a low pressure area is passing in a nearby region. Marked changes in the weather where you are located are unlikely.

Guidelines

Small rapid decreases in pressure indicate a nearby change in weather. They are usually followed by brief spells of wind and showers.

A quick drop in pressure over a short time indicates a storm is likely in 5 to 6 hours.

Large, slow and sustained decreasing pressure forecasts a long period of poor weather. The weather will be more pronounced if the pressure started rising before it began to drop.

Guidelines

A rapid rise in pressure, during fair weather and average, or above average pressure, indicates a low pressure cell is approaching. The pressure will soon decrease forecasting poorer weather.

Quickly rising pressure, when the pressure is low, indicates a short period of fair weather is likely.

A large, slow and sustained rise in pressure forecasts a long period of good weather is on its way.

Defining “Rapid” Change

Term Pressure Tendency Over 3 Hours

Steady Less than 0.1 mb

Rising/Falling Slowly 0.1 to 1.5 mb

Rising/Falling 1.6 to 3.5 mb

Rising/Falling Quickly 3.6 to 6.0 mb

Rising/Falling Very Rapidly More than 6.0 mb

Issues with Barometers

Mercury barometers contain substantial quantities of mercury

Not recommended for home use because of health threat it poses to children and pets

Alternative to Barometer

A liquid barometer designed for home use is the weatherglass

Weatherglasses are usually glass vessels shaped like long-spout pitchers or watering cans that react to changes in atmospheric pressure using water as the sensing liquid.

How the Weatherglass Works

Indicates changing pressure through changes in the water level in the barometer's spout.

High/Rising level indicates lowering of air pressure

Low/Falling level indicates higher air pressure.

Downside to the Weatherglass

Most units do not allow reading of pressure changes in standard units

No indicators to judge smaller pressure trends over short time periods

Minimal aid to the serious weather forecaster.

Sources

http://www.islandnet.com/~see/weather/eyes/barometer2.htm

http://www.islandnet.com/~see/weather/eyes/barometer3.htm

http://weather.about.com/od/weatherinstruments/a/barometers.htm