The actual lapse rate varies considerably, and depends somewhat upon the moisture content of the air nd other factors. Pilots generally want two pieces of information, the actual change in temperature vwith height, and the conditions of air at the surface. If the air at the surface is warmer than the actual lapse rate, then a parcel of air will begin to rise in the air that surrounds it. This rising air will cool off primarily by expansion at a rate known as the dry adiabatic lapse rate. When this parcel of air reaches ts dewpoint however, the rate of cooling will decrease due to the heat released when water vapour is ondenses to a liquid. This is known as the saturated adiabatic lapse rate. The height where the parcel f air becomes saturated also is the height at which cloud starts to form. When the parcel of air can rise no higher or has released all of its moisture, the top of the cloud is reached.