1. Air Pressure - #1 element of weather prediction.
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 Barograph
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

8. Air Pressure Units
Millibars, inches of Mercury, Pascals

9. 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: mb (32.01in Hg) in Siberia

10. 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

11. 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 mb

12. 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

13. 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

14. 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!!

15. 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

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

17. 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

18. 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

19. 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

20. 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

21. 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