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Understanding Air Pressure for Weather Prediction

Learn about the significance of horizontal and vertical air pressure variations, barometers, air pressure units, and gas laws influencing weather forecasts.

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Understanding Air Pressure for Weather Prediction

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  1. Chapter 5 Air Pressure - #1 element of weather prediction.

  2. Driving Question • What is the significance of horizontal and vertical variations in air pressure?

  3. Air Pressure • Air pressure is a measure of the force that air exerts on a surface • Weight per unit area of the column of air above that location • Weight = mass * acceleration due to gravity • Average air pressure at sea level • 1.0 kg/cm2 • 14.7 lb/in2 • The air pressure at any point is the same in all directions

  4. Mercurial Barometer • More accurate than aneroid • Invented by Torricelli in 1643 • One meter (39 inches) long glass tube sealed on one end • Open end is inverted in pool of mercury • Height of mercury changes as pressure changes • Adjustments are required for temperature and latitude

  5. Aneroid Barometer • Flexible evacuated chamber with a spring inside • As pressure changes allow the chamber to flex • This causes movement in gears which display the pressure

  6. Air Pressure Tendency • The change in air pressure with time • Rising: continuing fair or clearing weather • Falling: approaching inclement weather • Steady: no change • Barograph • An instrument that provides a continuous trace of air pressure with time • Altimeter • An aneroid barometer that is calibrated to measure altitude or elevation

  7. Air Pressure Units • Millibars, inches of Mercury, Pascals

  8. Air Pressure • Meteorologists often express altitude in terms of pressure (850mb map) • Worldwide range in air pressure averages between 970mb and 1040mb • Lowest: 870mb (25.69in Hg) in Typhoon Tip near Guam (Peak Winds of 190 mph) • Highest: 1083.8mb (32.01in Hg) in Siberia

  9. Variations in Air Pressure With Altitude • The maximum air density is at the surface • Number density: the number of gas molecules per unit volume • Decreases with altitude • Thinning of the air with altitude is associated with decline in air pressure • 50% atmosphere’s mass below 3 miles • 99% atmosphere’s mass below 20 miles

  10. The Standard Atmosphere: a model of the real atmosphere averaged across all latitudes for all seasons characterized by • Sea level air temperature of 15oC • Sea level pressure of 1013.25mb

  11. Variations in Air Pressure With Altitude • Denver (mile high city) has an average air pressure 83% less than Boston • Altitude sickness • Lexington is about 900 feet above sea level, so actual pressure is less than “fixed” pressure • In very sparse air (thermosphere) temperature is very high, but heat transfer is low

  12. Horizontal Variations in Air Pressure • On a surface weather map, variations in pressure due to altitude are removed by determining what the pressure would be at that point if that point were at sea level • Easier to observe variations in pressure from one place to another by day and hour

  13. Influence of Temperature and Humidity • Generally, temperature has a greater influence on density and pressure than water vapor • Air pressure drops more rapidly with altitude in cold (more dense) air than warm (less dense) air • Dry air is more dense that humid air!!

  14. Influence of Temperature and Humidity • Cold, dry air masses are more dense and produce higher surface pressures than warm, humid air masses • Change in air pressure is usually accompanied by a change in air mass

  15. Influence of Winds • Divergence (convergence) are caused by winds blowing away (toward) a location

  16. The Gas Law (Equation of State) • Variables of State: temperature, pressure, and density • p = ρRT • Pressure (p) • Density (ρ) • Gas Constant (R) • Temperature (T) • In the atmosphere these 3 variables are constantly changing

  17. The Gas Law • P = ρRT • Pressure is held constant • If T increases then ρ decreases • If ρ increases then T decreases • Temperature and Pressure are inversely proportional

  18. The Gas Law • P = ρRT • Density is held constant • If T increases then P increases • If P increases then T increases • Temperature and Pressure are directly proportional

  19. The Gas Law • P = ρRT • Temperature is held constant • If P increases then ρ increases • If ρ increases then P increases • Density and Pressure are directly proportional

  20. Basic Understandings (Ch. Review) • Important changes in weather often accompany relatively small changes in air pressure at the Earth’s surface. • High or rising pressure indicates fair weather • Low or falling pressure indicates inclement weather

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