|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:
||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.