Exposition (géographie)

In physical geography and physical geology, aspect (also known as exposure) is the compass direction or azimuth that a terrain surface faces. For example, a slope landform on the eastern edge of the Rockies toward the Great Plains is described as having an easterly aspect. A slope which falls down to a deep valley on its western side and a shallower one on its eastern side has a westerly aspect or is a west-facing slope. The direction a slope faces can affect the physical and biotic features of the slope, known as a slope effect. The term aspect can also be used to describe a related distinct concept: the horizontal alignment of a coastline. Here, the aspect is the direction which the coastline is facing towards the sea. For example, a coastline with sea to the northeast (as in most of Queensland) has a northeasterly aspect. Aspect is complemented by grade to characterize the surface gradient. Aspect can have a strong influence on temperature. This is because of the angle of the sun in the northern and southern hemispheres which is less than 90 degrees or directly overhead. In the northern hemisphere, the north side of slopes is often shaded, while the southern side receives more solar radiation for a given surface area insolation because the slope is tilted toward the sun and isn't shaded directly by the earth itself. The further north or south you are and closer to winter solstice the more pronounced the effects of aspect of this are, and on steeper slopes the effect is greater, with no energy received on slopes with an angle greater than 22.5° at 40° north on December 22 (winter solstice). The aspect of a slope can make very significant influences on its local climate (microclimate). For example, because the sun's rays are in the west at the hottest time of day in the afternoon, in most cases a west-facing slope will be warmer than a sheltered east-facing slope (unless large-scale rainfall influences dictate otherwise). This can have major effects on altitudinal and polar limits of tree growth and also on the distribution of vegetation that requires large quantities of moisture.