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For a clear understanding of the various types of atmospheric disturbances, the study of air masses provides an essential background. Air masses that move along as part of the large scale motion systems of the general circulation are largely responsible for bringing about changes in the day-to-day weather, particularly so in the middle latitudes.

The movements of air and daily temperature changes are the two important factors that bring about a change in atmospheric stability. It may be pointed out that the steep lapse rate renders the air more unstable, while a smaller lapse rate promotes stability.

Stability or instability prevailing in the atmosphere largely controls the daily weather. If there is stability in the atmosphere, it would resist vertical motions. Thus, stable air does not favour cloud formation.

While discussing the stability and instability conditions in dry air, it was pointed out that a rising air parcel cools at the dry-adiabatic lapse rate, which is always the same (10″C for every kilometer of ascent).

Our previous discussion of instability conditions was related to a parcel of air ascending or descending through a layer of air that was at rest along the vertical.

The term ‘conditional instability’ refers to the state of a column of air when its vertical distribution of temperature is such that the layer is stable for dry air but unstable for saturated air.

Atmospheric stability is defined as that condition in the atmosphere in which vertical motions are absent or definitely restricted; and, conversely, instability is defined as the state wherein vertical movement is prevalent.”

When in an ascending air mass the process of consideration starts after the temperature has come down to saturation point, the latent heat of condensation is released leading to warming of the rising air.

Like solid objects the concept of equilibrium is equally applicable to the atmosphere as well. But the following characteristics render the discussion of atmospheric equilibrium complicated:

Adiabatic processes are those where no heat is added to or subtracted from an ascending or descending air parcel. The changes in temperature that are brought about in such air parcels are referred to as adiabatic temperature changes.

A correct understanding of the concept of atmospheric equilibrium is the most important pre­requisite to appreciate different atmospheric processes associated with various kinds of weather phenomena.

The atmosphere has got a layered structure. Layers of the atmosphere differ from one another in respect of temperature, density and humidity of the air. Vertical motions are produced due to these different physical properties of the atmospheric layers.

The beginning of artificial rain-making goes back to 1946, just after World II, when V.J. Schaefer and E. Langmuir, two American scientists, discovered by their experiments in the General Electric Laboratory, New York that certain types of clouds may be modified to produce precipitation.

From the very dawn of civilization it has been the endeavour of man to get the required amount of precipitation at the proper time. According to Crowe, as a mystical art rain-making appears to be almost as old as man himself, but the methods employed for many centuries fell more in the realm of public […]

The reliability or dependability of precipitation is closely related to its variability. Petterssen has defined variability thus: “the variability is defined as the ratio of the mean of deviations from the mean (averaged without regard to sign) to the mean itself.

Even though the seasonal distribution of precipitation is an important attribute of the climate of a region, its diurnal variation is no less important a climatic feature.