Temperature, Pressure, Density and Vertical MotionAdapted from Scott Dennings presentation for CSU CMMAP course Summer 2007By Jim Barnaby Summer 2008

Temperature, Pressure, Density and Vertical MotionTemperature, Pressure and DensityBuoyancy and Static Stability (aka vertical motion)

Present Atmospheric Composition

Present day atmosphere consists of nitrogen, oxygen, water vapor, carbon dioxide, argon and trace gases

Dave: please talk about importance of O2, H2O, CO2, O3, CFCs, and aerosol while this slide is up!

What is Air Temperature?Temperature is a measure of the kinetic (motion) energy of air moleculesSotemperature is a measure of air molecule speed

The sensation of warmth is created by air molecules striking and bouncing off your skin surfaceThe warmer it is, the faster molecules move in a random fashion and the more collisions with your skin per unit time

PressurePressure is defined as a force applied per unit areaMolecules bumping into an object also create a force on that object, or on one anotherAir pressure results from the weight of the entire overlying column of air!

At sea level 14.7 psiAt Fort Collins ~ 12.1 psiOn Longs Peak ~ 8.6 psi

Air PressureAir Pressure (atmospheric pressure) is the force of air pressing down on earths surface. Air pressure equals weight of air directly above that point. Air pressure depends on the density of the air. Denser air exerts more air pressure than less dense air does.

DensityDensity is mass divided by volume

Density is a measure of how closely packed molecules are within a substance (how condensed or crowded they are)ex. Styrofoam vs. steel

Density (mass/volume)Same number of molecules and mass

Sample 1 takes up more space

Sample 2 takes up less space

Sample 2 is more dense than sample 1

Sample 1Sample 2

Pressure and DensityGravity holds most of the air close to the groundThe weight of the overlying air is the pressure at any point

Density is the Key to Buoyancy!Changes in density drive vertical motion in the atmosphere and ocean.

Lower density air rises when it is surrounded by denser air.

-Think of a hollow plastic ball submerged under water. What happens when you release it?

Air DensityDensity of the air (air pressure) is affected by three factors.TemperatureWater vaporElevation

Air Density is Affected by:Temperature density of a fluid (gas or liquid) decreases when the fluid is heated. Less dense air exerts less air pressure. Areas with high temperatures usually have lower air pressures than areas with lower temperatures. Water vapor At a given pressure, moist air is less dense than dry air. A water molecule has less mass than either a nitrogen or oxygen molecule. Therefore, air with a large amount of water vapor exerts less air pressure than drier air. Elevation as elevation increases, air becomes thinner or less dense. Therefore, air pressure decreases with increasing elevation.

Why is stability important?Vertical motions in the atmosphere are a critical part of energy transport and strongly influence the hydrologic cycle

Without vertical motion, there would be no precipitation, no mixing of pollutants away from ground level - weather as we know it would simply not exist!

There are two types of vertical motion:forced motion such as forcing air up over a hill, over colder air, or from horizontal convergencebuoyant motion in which the air rises because it is less dense than its surroundings - stability is especially important here

Vertical Motion and TemperatureRising air expands, using energy to push outward against its environment, adiabatically cooling the air

A parcel of air may be forced to rise or sink, and change temperature relative to environmental air

High Pressure Creates Fair WeatherAir pressure may become high when large air masses come together in the upper atmosphere. These air masses press down on layers of air below. This pressure usually prevents warm, moist air from rising into the upper atmosphere. As a result, clouds do not form; therefore, high pressure usually means fair weather.

Low Pressure Creates Poor WeatherAir pressure may become low when large air masses move apart in the upper atmosphere. This reduces pressure on the layers of warm air below. As a result, the warm air rises. If the warm air is moist, clouds will form in the upper atmosphere, therefore, low pressure may lead to cloudy, rainy weather.

Dave: please talk about importance of O2, H2O, CO2, O3, CFCs, and aerosol while this slide is up!