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Chapter 14

Types of Cloud.

Convection Cloud (continued).

The warmer air will become less dense and begin to rise. Air that is forced to rise in this way will cool at a constant 3 degrees Celsius per 1,000ft. This is called the Dry Adiabatic Lapse Rate (DALR). The air will continue to rise until it has cooled to the same temperature as the surrounding air.  At this point we say that the air has reached equilibrium.

Environmental Lapse Rate (ELR).

As we already know, where uneven heating does not take place, there is still a variation in temperature with height. The air is usually cooler as one moves higher and higher above the earth's surface. The rate at which the temperature drops with height will vary across the world and on a day to day basis. This rate of change is called the Environmental Lapse Rate (ELR).

Now consider the occurence of uneven heating as discussed on the previous page. If the Environmental Lapse Rate outpaces the Dry Adiabatic Lapse Rate, that is to say the ELR is cooling at a faster rate than the DALR, the air will continue to rise. The conditions would be described as unstable.

However, if the ELR is lower than the DALR, the rising air will quickly reach equilibrium. The conditions would be described as stable.

This example shows unstable conditions where the ELR outpaces the DALR.