By J. Vicedo, et. al.,

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The meteorologist is able to make such comparisons on a special graph of temperature, humidity and pressure values called a tephigram. The diagrams which follow in this chapter are simplified versions of this graph. 5. The first step to an understanding of stability/instability is to understand the adiabatic process. Chapter 2 Page 1 © G LONGHURST 1999 All Rights Reserved Worldwide Stability Adiabatic Change (Transformation) 6. If a parcel or particle of air moves vertically within the atmosphere, the pressure exerted on the parcel by the surrounding air will decrease as the parcel rises and increase as the parcel descends.

Note that in warmer air the pressures would occur at higher levels and in colder air at lower levels than those in ISA. Analysis of the pressures and corresponding heights shows that at lower levels the pressure reduces with height most rapidly. For example a 200hPa change in pressure occurs over a height range of 8000ft (from 10-18,000ft) whereas the same pressure change occurs between 18,000 and 30,000ft, and again between 30,000 and 53,000ft. 62. One hecto-pascal of pressure equates to an average height change of approximately 27ft (8m) near mean sea level, 50ft (15m) at about 18,000ft (5500m) and 100ft at 40,000ft.

Air described as ‘humid’ has a high RH. 90. From the preceding paragraph it can be seen that air at low level with a temperature of +15°C and a water vapour content (WVC) of 4 gms/kg would have a relative humidity of approximately 36% since: RH (%) Chapter 1 Page 41 = actual WVC ------------------------------------------------------------------------------------------------------ × 100 maximum WVC for the given temperature = 4 gm/kg × 100 ----------------------------------11 gm/kg = 36% © G LONGHURST 1999 All Rights Reserved Worldwide The Atmosphere 91.