How the atmosphere effects flying

 There are many types of pressures in the atmosphere. We as pilots are really only concerned with one though. That is atmospheric pressure. Atmospheric pressure is one of the basic factors of weather changes. It helps to lift our aircraft. And it runs some of our primary flight instruments. Under standard conditions at sea level the average pressure exerted on our bodies is around 14.70 pounds per square inch. But its thickness is limited. That is why at 18,000 msl that number is halved. Due to the fact that the pressure of the atmosphere changes with time and location a standard reference was developed. The standard temperature and pressure at sea level is 15 degrees C and 29.92 inches or mercury. This is what we use a reference point to set our instruments. As you climb in the atmosphere the standard lapse rate or change is around 2 degrees C per thousand feet as you go up to 36,000 feet msl. Above 36,000 feet msl the temperature is constant up to 80,000 feet msl. The standard lapse rate for pressure is 1" per thousand feet up you go till you reach 10,000 feet msl.

 Pressure altitude is the height above a standard datum plane, which is a theoretical level where the weight of the atmosphere is 29.92 inches of mercury. Where pressure altitude comes into play with flying is in airplane performance, and the assigning of altitudes above 18,000 feet msl. When we are up flying, we will use the local pressure to set our altimeter which tells us our altitude that we are flying at. That rule applies to everyone flying below 18,000 feet msl. Once you cross over 18,000 feet msl everyone turns there altimeter to 29.92 which is the standard pressure. They do this to make sure that everyone is on the same page when they are assigned altitudes. See every time we set a different pressure into the altimeter it will change the indicated altitude. So to make sure that everyone is seeing the same altitude between both controller and pilots, above 18,000 everyone uses 29.92 regardless of the local pressure.

 Density altitude is the altitude in the standard conditions described before, that you would be at based off the current air density. This number is very important because it will determine the performance of your airplane. A higher density altitude means that your airplanes performance will be decreased for three main reasons. First off, the engine takes in less air at higher density altitudes and therefore puts out less power. Secondly, thrust is decreased because a propellor is less efficent in thin. Thirdly, lift is reduced because thin air exerts less force on the airfoils. Density altitude is pressure altitude corrected for nonstandard temperature.

 There are many things that effect the density of the air. Things such as altitude, temperature, and humidity. Things that may results in a high density altitude are high elevations, low atmospheric pressures, high temperatures, high humidity, or some combination of the those. Lower elevations, high pressure, low temperature, and low humidity will lead to a lower density altitude.