The atmospheric agencies are by far the most important of the destructive or denuding agents, because no part of the land surface is altogether exempt from their activity. Their work is described by the general term weathering, and is shown at once by the different appearance of freshly quarried stone from that which has been long exposed in the face of a cliff, or even in ancient buildings. While such agents as rivers and the sea do work that is much more apparent and striking than that of the atmosphere, yet they are more locally confined, and even in their operations the atmosphere renders important aid. Though no part of the land surface is entirely free from the destructive activity of the atmosphere, the rapidity and intensity of this activity vary much in different places. There are, in the first place, the great differences of climate to be considered, differences in the amount and distribution of the rainfall, of temperature, and of the winds. In the second place, the resistance offered by the various kinds of rocks to the disintegrating processes differs very greatly, in accordance with the differences of hardness and chemical composition.

Again, the presence or absence of a covering of protective vegetation has an important influence upon the amount and character of the destruction effected.

The outcome of all these varying factors is to produce very irregular land surfaces. While the tendency of the atmospheric agencies is gradually to wear down the land to the level of the sea, yet in that process some parts are cut away much more rapidly than others; and hence the first effect of denudation is an increasingly irregular surface. The overlying screen of soil conceals many of these irregularities, especially the minor ones; and were that screen removed, the rock surface would be seen to be much more irregular and rugged than the actual surface of the ground.

So long as the land surface is varied by hill and valley, it is said to possess relief; and when it has all been planed down to a flat or gently sloping surface, raised but slightly above the level of the sea, it is said to have reached the base-level of erosion, or to be base-levelled.

The atmospheric agents may be conveniently divided into (1) rain, (2) frost, (3) changes of temperature, (4) wind.