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Measuring Air Pressure

Measuring Air Pressure. Atmospheric pressure. Two common units in the US Inches of Mercury - refers to the height of a column of mercury measured in hundredths of inches. This is what you will usually hear from the NOAA Weather Radio or from your favorite weather or news source.

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Measuring Air Pressure

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  1. Measuring Air Pressure

  2. Atmospheric pressure • Two common units in the US • Inches of Mercury - refers to the height of a column of mercury measured in hundredths of inches. • This is what you will usually hear from the NOAA Weather Radio or from your favorite weather or news source. • At sea level, standard air pressure in inches of mercury is 29.92

  3. Atmospheric pressure • Millibars • Range from about 100 to 1050 • At sea level, standard air pressure in millibars is 1013.2 • Weather maps showing the pressure at the surface are drawn using millibars

  4. Although the changes are usually too slow to observe directly, air pressure is almost always changing • Caused by changes in air density, which is related to temperature • Warm is less dense than cooler air, because the gas molecules have greater velocity and are further apart than in cooler air

  5. So, while the average altitude of the 500 millibar level is around 18,000 feet (5,600 meters) the actual elevation will be higher in warm air than in cold air

  6. Barometer • Weather instrument that measures atmospheric pressure • (also known as air pressure or barometric pressure) - the weight of the air in the atmosphere

  7. Two types of Barometers • There are two main types of barometers – the most widely available and reliable Mercury Barometers, or the newer digital friendly Aneroid Barometer

  8. Mercury Barometer • Devised by Evangelista Torricelli in 1643 • Glass tube about 3 feet high with one end open and the other end sealed • The tube is filled with mercury and the glass tube sits upside down in a container, called the reservoir, which also contains mercury • The mercury level in the glass tube falls, creating a vacuum at the top.

  9. The barometer balances the weight of mercury in the glass tube against the atmospheric pressure just like a set of scales Weight of mercury is less than the atmospheric pressure, the mercury level in the glass tube rises Weight of mercury is more than the atmospheric pressure, the mercury level falls

  10. Atmospheric pressure is basically the weight of air in the atmosphere above the reservoir The level of mercury continues to change until the weight of mercury in the glass tube is exactly equal to the weight of air above the reservoir

  11. Low Pressure • Areas of low pressure • Air is rising away from the surface of the earth more quickly than it can be replaced by air flowing in from surrounding areas • This reduces the weight of air above the reservoir so the mercury level drops to a lower level

  12. High Pressure • Areas of high pressure • Air is sinking toward the surface of the earth more quickly than it can flow out to surrounding areas • There is more air above the reservoir, so the weight of air is higher and the mercury rises to a higher level to balance things out

  13. Digital Barometers • Modern weather instrument • Uses electrical charges to measure air pressure • This enables them to take multiple accurate recordings of the pressure and produce more accurate weather forecasts

  14. Aneroid Barometers • Uses a small, flexible metal box called an aneroid cell • Small changes in external air pressure cause the cell to expand or contract • Many models include a manually set needle which is used to mark the current measurement so a change can be seen

  15. Forecasting the Weather • Changes in atmospheric pressure are one of the most commonly used ways to forecast changes in the weather, because weather patterns are carried around in regions of high and low pressure

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

  17. Large, slow and sustained decreasing pressure forecasts a long period of poor weather • A large, slow and sustained rise in pressure forecasts a long period of good weather is on its way • Quickly rising pressure, when the pressure is low, indicates a short period of fair weather is likely. • A quick drop in pressure over a short time indicates a storm is likely in 5 to 6 hours

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