Figure 12-3.  Circulation pattern in a static environment.  1,000 feet of altitude gain, and the pressure decreases at a rate  of about one inch per 1,000 feet of altitude gain.  At the top of the troposphere is a boundary known as the  tropopause, which traps moisture and the associated weather  in the troposphere. The altitude of the tropopause varies with  latitude and with the season of the year; therefore, it takes  on an elliptical shape as opposed to round. Location of the  tropopause is important because it is commonly associated with  the location of the jet stream and possible clear air turbulence.  Above the tropopause are three more atmospheric levels. The  first is the stratosphere, which extends from the tropopause to  a height of about 160,000 feet (50 km). Little weather exists  in this layer and the air remains stable, although certain types  of clouds occasionally extend in it. Above the stratosphere  are the mesosphere and thermosphere, which have little  influence over weather.  As noted earlier, the atmosphere is in constant motion.  Certain factors combine to set the atmosphere in motion, but a  major factor is the uneven heating of the Earth’s surface. This  heating upsets the equilibrium of the atmosphere, creating  changes in air movement and atmospheric pressure. The  movement of air around the surface of the Earth is called  atmospheric circulation.  The unequal heating of the Earth’s surface not only modifies  air density and creates circulation patterns; it also causes  changes in air pressure or the force exerted by the weight  of air molecules. Although air molecules are invisible, they  still have weight and take up space.  Imagine a sealed column of air that has a footprint of one  square inch and is 350 miles high. It would take 14.7 pounds  of effort to lift that column. This represents the air’s weight;  if the column is shortened, the pressure exerted at the bottom  (and its weight) would be less.  Heating of the Earth’s surface is accomplished by several  processes, but in the simple convection-only model used for  this discussion, the Earth is warmed by energy radiating from  the sun. The process causes a circular motion that results  when warm air rises and is replaced by cooler air.  Warm air rises because heat causes air molecules to spread  apart. As the air expands, it becomes less dense and lighter  than the surrounding air. As air cools, the molecules pack  together more closely, becoming denser and heavier than  warm air. As a result, cool, heavy air tends to sink and replace  warmer, rising air.  The weight of the shortened column of air at 18,000 feet is  approximately 7.4 pounds; almost 50 percent that at sea level.  For instance, if a bathroom scale (calibrated for sea level)  were raised to 18,000 feet, the column of air weighing 14.7  pounds at sea level would be 18,000 feet shorter and would  weigh approximately 7.3 pounds (50 percent) less than at  sea level.  [Figure 12-4]  The actual pressure at a given place and time differs with  altitude, temperature, and density of the air. These conditions  also affect aircraft performance, especially with regard to  takeoff, rate of climb, and landings.  Because the Earth has a curved surface that rotates on a tilted  axis while orbiting the sun, the equatorial regions of the Earth  receive a greater amount of heat from the sun than the polar  regions. The amount of solar energy that heats the Earth  depends on the time of year and the latitude of the specific  region. All of these factors affect the length of time and the  angle at which sunlight strikes the surface.  Solar heating causes higher temperatures in equatorial areas,  which causes the air to be less dense and rise. As the warm  air flows toward the poles, it cools, becoming denser and  sinks back toward the surface.  [Figure 12-3]  In general atmospheric circulation theory, areas of low  pressure exist over the equatorial regions and areas of high  pressure exist over the polar regions due to a difference in  temperature. The resulting low pressure allows the high- pressure air at the poles to flow along the planet’s surface  toward the equator. While this pattern of air circulation is  Coriolis Force  12-3  Atmospheric Pressure  Atmospheric Circulation 