AIR PRESSURE

Keeping an Atmosphere Atmosphere is kept by the world’s gravity –Low mass (small) worlds= low gravity =almost no atm. –High mass (large) worlds = high gravity = thick atm. Gravity and pressure –Air pressure depends on how much gas there is i.e. The atmospheric thickness.

Gravity and Atmospheric Pressure The stronger the gravity, the more gas is held by the world and the greater the weight of atm. on a point

Earth’s Atmosphere About 10 km thick Consists mostly of molecular nitrogen (N 2 ) and oxygen (O 2 )

The air is made up of molecules.

Gravity pulls the air molecules toward the earth, giving them weight. The weight of the air molecules all around us is called the air pressure.

High altitudes = lower pressure Low altitudes = higher pressure

Atmospheric Pressure Gas pressure depends on both density and temperature. Adding air molecules increases the pressure in a balloon. Heating the air also increases the pressure.

Air pressure is equal in all directions. Pressure = force per unit area

As elevation goes up Barometric pressure goes down. This is an inverse relationship.

to measure air pressure. A Barometer is used

In 1643, Evangelista Torricelli invented the barometer

Torricelli’s barometer used a glass column suspended in a bowl of mercury. The pressure of the air molecules pushed the mercury up into the glass tube. The weight of the mercury in the tube was equal to the weight of the air pressing down on the mercury in the dish.

As atmospheric pressure increases… The mercury in the tube rises.

The Mercury Barometer Good:Bad: Simple to construct Simple to construct Highly accurate Highly accurate Glass tube is fragile Glass tube is fragile Mercury is very toxic! Mercury is very toxic!

The Aneroid Barometer No fragile tubes!No fragile tubes! No toxic chemicals!No toxic chemicals! No batteries!No batteries! Never needs winding!Never needs winding!

An aneroid barometer uses a cell which has had most of the air removed. As the air pressure around the cell increases, it presses on the cell, which causes the needle to move. Television weather forecasters usually give barometric pressure in inches of mercury. However, meteorologists measure atmospheric pressure in millibars. MILLIBARS

Two types of barometric pressure measurements: Station pressure is the actual pressure at the recording location. It is affected by the local altitude. Sea level pressure is referenced to sea level, so it has the same altitude anywhere in the world. Station pressure on a mountain top will be lower than station pressure in a valley. Scientists need a fixed point of reference in order to compare barometer readings in different locations. That is why barometer readings are sometimes adjusted for elevation above sea level at the station location.

Most aneroid barometers have a needle which can be set to remember the previous reading.

Changing Pressure A rising barometer = increasing air pressure. This usually means: Rising barometer readings indicate that a high pressure system is approaching. Higher atmospheric pressure is usually associated with fair weather and clearing skies.

Changing Pressure A falling barometer = decreasing air pressure. This usually means: Falling barometer readings usually indicate the approach of an area of low pressure. Low pressure readings are usually associated with storm systems. Tornadoes and hurricanes can produce very low barometric readings.

Air Movement and Flow

Fluids (air and water) flow from areas of high pressure to areas of low pressure. Change in pressure across a horizontal distance is a pressure gradient. Greater the difference in pressure and the shorter the distance between them, the steeper the pressure gradient and the stronger the wind. Movement of air across a pressure gradient parallel to Earth’s surface is called a wind and winds are named for the direction from which they come. 6-1

The Atmosphere in Motion Atmospheric pressure is a measure of the force pressing down on the Earth’s surface from the overlying air. Pressure is often measured in different units including: –atmospheres (1 atmosphere is the average atmospheric pressure at sea level), –millibars (1 atmosphere = millibars), –pounds per square inch or psi (1 atmosphere = 14.7 pounds per square inch), –mm or inches of mercury (1 atmosphere = 760 mm or inches of mercury) –torrs (1 torr = the pressure exerted by 1 cm of mercury). Low air density results in rising air and low surface pressure. High air density results in descending air and high surface pressure.

Isobars in millibars, the closer the isobar the stronger the winds High Pressure Low Pressure Rain

DO NOW: What’s in a letter? Take a moment to think about what each letter stands for when used on a weather map

High PressureLow Pressure Type of phenomenon Weather system Determined by… Changes in air pressure Moving inward on isobars… Pressure IncreasesPressure Decreases Density of air HigherLower Representation on a map H (typically blue)L (typically red) Motion of air Clockwise, air sinksCounterclockwise, air rises Also known as… AnticycloneCyclone Motion of air causes a zone of… DivergenceConvergence Stability of atmosphere StableUnstable Compare/Contrast Chart High and Low Pressure

High Pressure

LOW PRESSURE

Low vs. High

Heating and Cooling of Air