Characteristics of the Atmosphere
Before commencing any flight operations it is necessary to have some basic knowledge regarding pressure, temperature and density and the way these three could interact with eachother during your flight. As pressure builds up density increases as well and could affect the performance of your aircraft dramatically.
Pressure
The air is made up of numerous of molecules all pressing down on eachother. Therefore, atmospheric pressure may be defined as the weight of a single column of air. As we already know, atmospheric pressure decreases steadily as altitude increases with the most dramatic decrease in the lower levels of the atmosphere (troposphere) (see "Atmosphere Build-up for more information"). From this we can conclude that pressure does not decrease linair but more or less like a parabolic function. For example, the pressure above the tropopause is only about 25 to 30% of that prevailing at sealevel and above this layer almost reaches vacuum. As some of you already know, 10.000ft is about the highest altitude at which a person could breath without having the need for addtional oxygen. In case of a decompression a pilot always tend to reach this altitude as quick as possible. Above this altitude, the need for supplemental oxygen is required.
Temperature
Temperature is hotness (or coldness) and is a measure for the thermal energy that a volume of fluid posseses. When talking in terms of air, temperature indicates the molecular activity, which is affected by the distance between molecules, and is directly related to pressure. Therefore, temperature greatly affects aircraft performance. The ISA states that the temperature decreases with a rate of 1,98°C every 1000ft up to the tropopause (36.090ft) to a minimum of -56,6°C.
Density
Density is defined as a mass per unit volume and is indicated by the Greek letter rho (ρ). A common way to express density is hectopascal (hPa) or millibars (mb) and mainly depends on your location. The United States use millibars while most of Europe uses the hectopascal as standard density unit. Speaking of the ISA, density is assumed to be 1013.25 hpa or 29.92 mb at MSL. From here density decreases with approximately 75% till the tropopause is reached and greatly affects airspeed and the way it is measured. Air density is primarily affected by temperature and pressure although the presence of water vapour, or humidity, can have a tremendous effect. Air containing water vapour is less dense than dry air at the same temperature thus affecting aircraft performance.

There is a fundamental relationship between temperature(K), pressure and density and is expressed as follows:

Density relation p = pressure in hpa
T = temperature (K)
ρ = density

The Kelvin temperature scale is based on value of zero which is equivalent to -273,15°C. When converting from Kelvin to degrees celcius substract 15 from the total Kelvin value to gain the correct celcius value. As we can see the Kelvin scale does not use a degree symbol nor negative values.

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