Chapter 5

Air Pressure - #1 element of weather prediction.

Driving Question

What is the significance of horizontal and vertical variations in air pressure?

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

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

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

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

Air Pressure Units Millibars, inches of Mercury, Pascals

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

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

The Standard Atmosphere: a model of the real atmosphere averaged across all latitudes for all seasons characterized by

1. Sea level air temperature of 15oC

2. Sea level pressure of 1013.25mb

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

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

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!!

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

Influence of Winds Divergence (convergence) are caused by

winds blowing away (toward) a location

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

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

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

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

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