Temperature, Pressure, Density and Vertical Motion Adapted from Scott Denning’s presentation for CSU CMMAP course Summer 2007 By Jim Barnaby Summer 2008 Temperature, Pressure, Density and Vertical Motion Temperature, Pressure and Density Buoyancy and Static Stability (aka vertical motion) Present Atmospheric Composition Present day atmosphere consists of nitrogen, oxygen, water vapor, carbon dioxide, argon and trace gases 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 Pressure • 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 psi At Fort Collins ~ 12.1 psi On Longs Peak ~ 8.6 psi Air 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. Density 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) • Same number of molecules and mass Sample 1 • Sample 1 takes up more space Sample 2 • Sample 2 takes up less space • Sample 2 is more dense than sample 1 Pressure and Density • Gravity holds most of the air close to the ground • The 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 Density Density of the air (air pressure) is affected by three factors. • Temperature • Water vapor • Elevation 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 convergence – buoyant motion in which the air rises because it is less dense than its surroundings - stability is especially important here Vertical Motion and Temperature Rising 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 Weather 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 Weather 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.