]> The top of the mesosphere and the base of the thermosphere. The mesopause is usually located at heights of 85?95 km, and is the site of the coldest temperatures in the atmosphere. Temperatures as low as 100 K (- 173?C) have been measured at the mesopause by rockets. See atmospheric shell. 0 0 Air Mass A widespread body of air, the properties of which can be identified as 1) having been established while that air was situated over a particular region of the earth's surface (airmass source region), and 2) undergoing specific modifications while in transit away from the source region. A forecast of weather conditions for a period extending beyond three or more days from the day of issuance.