Earlier discussion of air masses and fronts will serve as a background for a clear understanding of atmospheric disturbances. Remember that the term atmospheric disturbance is more general than storm.
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The atmospheric disturbances include such variations in the secondary circulation of the atmosphere that cannot be placed in the category of storms. The thermal and dynamic effects of general circulation, as represented by the zones and wind belts, give rise to secondary circulations which produce the most noticeable changes in weather.
Even though the term ‘secondary circulation’ includes such atmospheric phenomena as monsoon circulations, air masses, fronts and upper level vortices, it is particularly applied to the travelling disturbances which originate in the middle and high latitudes as well as within the tropics.
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Extra tropical cyclones, anticyclones and tropical cyclones offer some of the best examples of such disturbances. As a matter of fact, the general circulation pattern may be regarded as a background upon which atmospheric disturbances of varying magnitudes are superimposed.
Besides the major features of secondary circulation, the day-to-day weather of a locality is also controlled by such local phenomena as are included in the category of third-order circulations.
They include gravity winds, fall winds, valley and mountain breezes, land and sea breezes, thunderstorms, and tornadoes etc. The effects of these local disturbances on weather are sometimes quite dramatic.
The most interesting aspect of the second-and third-order circulations is that sometimes they mask the general circulation. The general circulation is like the flow of a stream, and various types of atmospheric disturbances may be likened to the eddies and cross-currents which characterize the same.
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If we look at the daily weather maps published regularly, various irregularities of flow will appear on them. The synoptic charts published in the countries of middle latitudes in particular are always full of such irregularities.
It is to be pointed out that weather maps depicting the weather conditions of a particular day at a particular locality fail to show the seasonal wind-flow or isobaric patterns. On the contrary, annual charts or maps of pressure and winds cannot show the travelling atmospheric disturbances or any local weather phenomenon on them.