Understanding Air Pressure: Significance of Horizontal and Vertical Variations

1. Chapter 5 Air Pressure

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. Air Pressure Measurement • Barometer Aneroid   Mercury

5. 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

6. 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

7. 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

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

10. 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 • Highest: 1083.8mb (32.01in Hg) in Siberia

11. 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

12. 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

13. 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

14. 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

16. 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!!

17. 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

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

19. 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

20. 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

21. 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

22. 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

23. Basic Understandings (Ch. Review) • Air pressure is the weight of a column of air acting on a given surface area • At any point in the atmosphere the air pressure is equal in all directions • A barometer measures air pressure – there are two types: mercurial and aneroid

24. Basic Understandings (Ch. Review) • Pressure and Density decrease with altitude • 50% of atmosphere is below 3.5 miles • 99% of atmosphere is below 20 miles • Barometer readings are adjusted to sea level to remove the influence of elevation

25. Basic Understandings (Ch. Review) • Cold air masses are denser and exert higher pressure than warm air masses • Dry air masses are denser than humid air masses • Temperature has a greater influence on density and pressure than humidity • Variables of state: temperature, pressure, and density

26. 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