vortexknology, The Atmosphere

The atmosphere is 21% oxygen, 78% Nitrogen and 1% other elements.

It has a weight of 14.7 lbs./sq/in. and on a standard day (for pilots and aircraft performance requirements and testing) the standard day temperature and pressure are 59°F (15°C) and  29.92 in/Hg (1013.2mb) respectively.

What layers of the atmosphere do we fly in?

TROPOSPHERE - This is the layer in which we live and the layer with the most weather. The suns radiation is absorbed by the earth and released as heat. The heated air (temperature) cools as it rises through a  thickness from 4-12 miles. The thinnest of this layer is at the poles and thickest at the equator. Depending on the route an aircraft takes, it flies in both the Troposphere and the next layer...the Stratosphere. 

Prior to entering the stratosphere,  the temperature levels off and the layer between the troposphere and stratosphere is called the tropopause. 

TROPOPAUSE - This is where you will find the jet stream and the steering current for weather and what pilots use to increase ground speed by flying in it from a west to east direction.

I was on a flight to Germany from Dover AFB, De on the C-5 Galaxy. We entered a very powerful jet stream that gave us a ground speed of almost 800kts and a flight time of 6.5 hrs.  However, the next day it took 12 hrs to get home to Dover.

The very winds that pushed (tailwind) us so fast were now bucking (headwind) us on the way home.

STRATUSPHERE: This layer starts from 4-12 miles above the earths surface to about 32 miles. This is generally a very stable layer because the temperature increases with altitude.  

Atmospheric Conditions

Atmospheric conditions cannot be discussed without including Temperature, Humidity and Altitude.  These are three variables that behave independently and if aircraft performance is calculated incorrectly can mean the difference between life and death.

No matter your location on the earths surface, the higher you go the less air pressure there is. If the pressure is less, the air density is less. In fact, above 18,000ft. MSL (Mean Seal Level), all aircraft altimeter settings are the same...29.92 in/Hg. Does that sound familiar?

As altitude increases, the temperature decreases (exception: temperature inversion) at an approximate rate of 3.5°F (2.0°C)/1000ft. up to about 36,000 ft.

When a rising air parcel reaches a certain temperature, the moisture within that air parcel can no longer be absorbed. Think of it as a sponge. A dry sponge can absorb a lot more water than a damp or wet sponge.  The same is true of the atmosphere.  The higher the air parcel climbs the colder the temperature gets thus we have visible moisture....clouds.