Temperature, Pressure, Temperature, Pressure, Density Density

and Vertical Motionand Vertical Motion

Adapted from Scott Denning’s presentation for CSU CMMAP course

Summer 2007By Jim Barnaby Summer 2008

Temperature, Pressure, Temperature, Pressure, Density Density

and Vertical Motionand Vertical Motion

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

Present Atmospheric Present Atmospheric CompositionComposition

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

What is Air Temperature?What is Air Temperature?

• Temperature is a measure of the kinetic (motion) energy of air molecules– So…temperature is a measure of air molecule speed

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

PressurePressure• Pressure is defined as a force applied per unit area

• Molecules bumping into an object also create a force on that object, or on one another

• Air 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

Air Pressure – (atmospheric pressure) is the force of air pressing down on earth’s 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

Density 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)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 1

Sample 2

Pressure and DensityPressure and Density

• Gravity holds most of the air close to the ground

• The weight of the overlying air is the pressure at any point

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?

Density is the Key to Density is the Key to Buoyancy!Buoyancy!

Air DensityAir Density

Density of the air (air pressure) is affected by three factors.

• Temperature• Water vapor• Elevation

Air Density is Affected by: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?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 convergence– buoyant motion in which the air rises because it is less dense

than its surroundings - stability is especially important here

Vertical Motion and TemperatureVertical Motion and Temperature

Rising air Rising air expands, using expands, using energy to push energy to push outward against its outward against its environment, environment, adiabaticallyadiabatically cooling the aircooling the air

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

High Pressure Creates Fair High Pressure Creates Fair WeatherWeather

Air 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 Low Pressure Creates Poor WeatherWeather

Air 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.