Instability has diurnal and seasonal fluctuations. Because the atmosphere is heated
from the ground upward during daylight, a sunny day frequently produces surface-
based convection. Convection represents an unstable condition and the top of the
convection depends on the characteristics of the environmental air. At night, the
cooling atmosphere usually becomes much more stable. Therefore, surface-based
instability cycles with diurnal heating. Seasonally, locations can have significant
variations in surface heating and upper air conditions so that the atmospheric pro-
files are more prone to instability in some seasons than others. In most climates,
winters have less instability than summers. Also, instability can markedly increase
inmonsoonregions as the flow regime shifts to vapor-rich maritime tropical air
arriving over a continent.
Inversions are conditions in which the temperature of the environmental air
increases with altitude as compared to the average decline typical of the tropo-
sphere. The air parcel in an inversion cannot rise spontaneously. So, inversions
are associated with stable air. Low-level inversions are common overnight via
radiational cooling during regionally quiet weather and higher inversion layers
relate to the descent of air from aloft, especially associated with the subtropical
highs. Whatever their root causes, inversions act as lids on weather, preventing sig-
nificant upward mixing. Stagnant air and high pollution levels are associated with
locations that have frequent inversions.
Atmospheric Stability 23