 Air Pressure – the weight of the gases in the atmosphere pushing on the surface of the Earth.

 At sea level, standard air pressure is 14.7 lb/in 2 or mb or in of Hg

Standard Units of Pressure at Sea Level

 Atmospheric pressure is a measurement of the force due to air molecules at a given altitude.  The pressure of the atmosphere decreases as altitude increases.

This plastic bottle was sealed at 14,000 feet on top of Mauna Kea, and was crushed by the increase in atmospheric pressure (at 9,000 feet and 1,000 feet) as it was brought down towards sea level.

The greatest change in air pressure occurs in the lower atmosphere.

 Remember, air pressure is the weight of the atmosphere pushing down from above.  The higher you go in the atmosphere the smaller the column of air that is pushing down from above.

Air pressure is measured with a Barometer There are Two Types of Barometers: Aneroid Barometer Mercury Barometer

 In the following activity you will investigate how air pressure changes with altitude. You will also determine the air pressure at various altitudes above sea level.

Record & Average Sea-Level Pressures

 Air pressure can be analyzed on a weather map using Isobars – lines that connect points of equal air pressure.  Each isobar differs in pressure by 4 millibars.

Why Does Air Pressure Change?  So far we know the air pressure decreases with altitude because there are less air molecules pushing down from above.  It also changes with temperature & humidity.

A look at High an Low pressure

Two types of Pressure Systems--

 Cool temperatures  Rising barometer  Fair weather  Air moves out from center - clockwise  Warm temperatures  Falling barometer  Poor weather  Air moves toward center- counterclockwise

 Pressure Gradient – a change in pressure between two places. This creates a force that makes wind move from areas of high pressure to areas of low pressure Strong WindsLight Winds

Differential heating of land and water –  Differences in the heating of land and water creates different pressure systems.  Land heats up and cools off faster than water.  Land temperatures change quickly while sea temperatures change slowly.

Sea Breeze

Sea/land breeze animation Land Breeze

Valley Breeze Mountain Breeze

Why are the winds (arrows) drawn in a curved path and not in a straight line?

 Caused by the rotation of the Earth.  Wind and water are deflected to the right in the Northern Hemisphere  to the left in the Southern Hemisphere [Coriolis Effect animation]

[Tornado video]Tornado video [Hurricane Katrina]Hurricane Katrina

Anemometer- measures wind speed Wind Vane – measures wind direction

Global Pressure Belts are caused by the uneven heating of the Earth which results in High and Low Pressure areas

0º 30º N 60º N 90º N 30º S 60º S 90º S Subpolar Low Equatorial Low (Doldrums or ITCZ) Subtropical High (Horse Latitudes) Polar High Subtropical High (Horse Latitudes) Subpolar Low Polar High

 Air always moves from high to low Pressure.  Winds are named for the direction they are coming from.

0 º 30 º 60 º 90º N 30 º 60 º 90º S Northeast Trade Winds Southeast Trade Winds Prevailing Southwesterlies Prevailing Northwesterlies Polar Southeasteries Polar Northeasterlies