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 (N2) and oxygen (O2)

The air is made up of molecules.

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

High altitudes = lower pressureHigh altitudes = lower pressure

Low altitudes = higher pressureLow altitudes = higher pressure

Atmospheric Pressure

Gas pressure Gas pressure depends on both depends on both density and density and temperature.temperature.

Adding air Adding air molecules molecules increases the increases the pressure in a pressure in a balloon.balloon.

Heating the air Heating the air also increases also increases the pressure.the pressure.

Air pressure is Air pressure is equal in all equal in all directions.directions.

Pressure = force per unit areaPressure = force per unit area

As As elevation elevation goes upgoes up

Barometric Barometric pressure pressure

goes goes down.down.

This is an inverse relationship.This is an inverse relationship.

to to measure measure air air pressure.pressure.

A A BarometerBarometer

is is usedused

In 1643, Evangelista Torricelli In 1643, Evangelista Torricelli invented the barometerinvented the barometer

Torricelli’s barometer Torricelli’s barometer used a glass column used a glass column suspended in a bowl of suspended in a bowl of mercury. The pressure mercury. The pressure of the air molecules of the air molecules pushed the mercury up pushed the mercury up into the glass tube.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 As atmospheric atmospheric pressure pressure increases…increases…

The mercury in The mercury in the tube rises.the tube rises.

The Mercury BarometerThe Mercury Barometer

Good:Good: Bad:Bad:

•Simple to constructSimple to construct

•Highly accurateHighly accurate

•Glass tube is fragileGlass tube is fragile

•Mercury is Mercury is very very toxictoxic!!

The The AneroidAneroid BarometerBarometer

•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 An aneroid barometer uses a cell which has uses a cell which has had most of the air had most of the air removed. removed.

As the air pressure As the air pressure around the cell around the cell increases, it presses increases, it presses on the cell, which on the cell, which causes the needle to causes the needle to move.move.

Television weather forecasters usually give barometric Television weather forecasters usually give barometric pressure in inches of mercury. However, meteorologists pressure in inches of mercury. However, meteorologists measure atmospheric pressure in measure atmospheric pressure in millibars.millibars.

MILLIBARS

Two types of barometric pressure measurements:Two types of barometric pressure measurements:

Station pressure Station pressure is the actual is the actual pressure at the recording pressure at the recording location. It is affected by the location. It is affected by the local altitude.local altitude.

Sea level pressureSea level pressure is is referenced to sea referenced to sea level, so it has the level, so it has the same altitude same altitude anywhere in the anywhere in the world.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 Most aneroid barometers have a barometers have a needle which can be needle which can be set to remember the set to remember the previous reading.previous reading.

Changing PressureChanging Pressure

A A rising barometerrising barometer = increasing air pressure. = increasing air pressure.

This usually means: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 PressureChanging Pressure

A A falling barometerfalling barometer = decreasing air pressure. = decreasing air pressure.

This usually means: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

Isobars in millibars, the closer the isobar the stronger the winds

High Pressure

Low Pressure

Rain

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 = 1013.25 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 29.92

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.

Heating and Cooling of Air

The Gas Law• Ideal Gas follows kinetic molecular theory, made up of large

number of molecules that are in rapid random motion following perfect elastic collitions losing no momentum

• How the Kinetic Molecular Theory Explains the Gas Laws• The pressure of a gas results from collisions between the gas

particles and the walls of the container. • Each time a gas particle hits the wall, it exerts a force on the

wall. • An increase in the number of gas particles in the container

increases the frequency of collisions with the walls and therefore the pressure of the gas.

• Avogadro's Hypothesis• As the number of gas particles increases, the frequency of

collisions with the walls of the container must increase. • This, in turn, leads to an increase in the pressure of the gas. • Flexible containers, such as a balloon, will expand until the

pressure of the gas inside the balloon once again balances the pressure of the gas outside.

• Thus, the volume of the gas is proportional to the number of gas particles.

The Gas Laws• Charles Law

– The volume of a gas increased with the temperature – The volume of a given amount of dry ideal gas is directly

proportional to the Kelvin Temperature provided the amount of gas and the pressure remain fixed.

– When we plot the Volume of a gas against the Kelvin temperature it forms a straight line.

– V1 / T1 = V2 / T2

• Boyle’s Law– the product of the pressure and volume are observed to be

nearly constant. – The product of pressure and volume is exactly a constant for

an ideal gas. – p * V = constant

WATER VAPOR

(9.8oC)