Structure of the Atmosphere / Density Altitude
To understand the weather it helps to know a little about the air. Air is simply a gas comprised primarily
of nitrogen and oxygen and a variable amount of water vapor. Air has mass and weight. At sea level
where the air is most dense it exerts a pressure of 14.7 pounds per square inch.
Air density is affected by altitude, temperature, humidity and pressure. Air density alters wing
performance. The higher the altitude, temperature or humidity the less dense the air will be and the faster
you will have to move to generate lift.
For every 1000ft of altitude you gain, the air density drops approximately 4%.
For every 5ºF increase in temperature, the air density will drop 1%.
The amount of water vapor a mass of air can hold depends on its temperature. As air heats up it is able to
hold more water. Low relative humidity means that the air at its current temperature could hold a lot more
water. When the temperature of an air mass drops, its relative humidity increases and when relative
humidity reaches 100%, water condenses and a cloud is formed.
Humid air is lighter then dry air and therefore has a tendency to rise just as when air is heated.
Solar Heating and Circulation
Solar heating and the resulting high and low pressure systems causes all atmospheric circulation. The sun
heats the ground, which heats the air above it. Different surfaces heat up at different rates. Some examples
of terrain that heat up slowly are water, snow, green grass, and forests. Examples of terrain that heat up
more quickly are asphalt, dry fields, dark soil and dark rocks.
As air warms up it expands, becomes less dense, and tends to rise. When warm air rises, cooler air moves
in from the surrounding areas to replace it.