Lapse Rate and Earth's Atmosphere

Atmosphere varies with increase in altitude. Temperature, pressure and density are the important parameters that vary with altitude. A static atmospheric model known as “International Standard Atmosphere” is referred for determining the parameters at a particular altitude. 

Video Credits: GATE Aerospace Engineering

Therefore temperature variation is of special interest as it varies at a constant rate at lower altitudes. Lapse Rate is a rate at which temperature varies with altitude (temperature gradient). Lapse rate is denoted as ‘a’.Earth’s atmosphere is divided into five layers namely troposphere, stratosphere, mesosphere, thermosphere, and... exosphere. Aeroplanes normally fly within 25 kilometres of the Earth’s atmosphere consisting two layers of the atmosphere, the troposphere and stratosphere. 

These layers are divided on the basis of lapse rate as gradient layer and constant temperature layer. According to ISA the temperature at mean sea level is taken to be 288.16 Kelvin (15 ° C), according to ISA.

0 – 11 Km is known as the gradient layer as the temperature decreases at a constant rate with altitude, and 11 – 25 Km is known as constant temperature layer as the temperature remains constant with increase in altitude. At gradient layer the temperature decreases at a rate of 6.5 ° C per kilometre. The lapse rate and variation of temperature with increase in altitude can be known from the following table,

The temperature at thermosphere ranges between 200 to 500 ° C. In exosphere the temperature varies from 0 to over 1700° C