Classify the stability of the atmosphere on the basis of the average temperature gradient for the following conditions:
(a) Temperature at ground level is 70°F, temperature at 1500 f t is 80°F.
(b) Temperature at ground level is 70°F, temperature at 2500 f t is 60°F.
(c) Temperature at ground level is 60°F, temperature at 1900 f t is 48°F.