Flight Training:Instability for Idiots

Winds and temperatures aloft (FDs). By checking the temperatures aloft, you can develop a crude idea of the atmosphere's vertical temperature profile. A quick shortcut for determining standard temperatures (in Celsius) is to double the altitude, subtract 15, and change the resultant number into a negative figure (use positive numbers for lower altitudes during warmer months). For example, let's say you want to know the standard temperature for 10,000 feet. Two times 10 is 20. Subtract 15 and you come up with five degrees Celsius. Change to a negative number and you come up with a standard temperature of minus five degrees Celsius. (The actual standard temperature for that altitude is minus 4.812 degrees Celsius, so minus five degrees is close enough.) With colder-than-standard temperatures aloft and a source of moisture and lifting, you have a strong hint that the air is ripe for fireworks.


http://flighttraining.aopa.org/magazine/1998/May/199805_Instability_for_Idiots.html