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In geography, aspect generally refers to the direction to which a mountain slope faces. For example, a slope 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 term can also be used to describe the shape or alignment of a coastline. Here, the aspect is the direction which the coastline is facing towards the sea. For example, as northwest-trading coastline with sea to the northeast (as in most of Queensland) has a northeasterly aspect.

Description: Slope effect, a vegetational result of aspect, in evidence in the coastal sage scrub community of southern California's Santa Monica Mtns. The slope on the left side is north-facing, thus moister and dominated by Ceanothus sp.. The south-facing slope on the right side is much drier (receiving more direct sun), and is more sparsely vegetated with the more drought tolerant Artemisia californica and Yucca whipplei.

The aspect of a slope can produce 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. In Australia, for example, remnants of rainforest are almost always found on east-facing slopes which are protected from dry westerly wind.

Similarly, in the northern hemisphere a south-facing slope will be more open to sunlight and warm winds and will therefore generally be warmer than a north-facing slope. This can be seen in the Swiss Alps, where farming is much more extensive on south-facing than on north-facing slopes. In the Himalayas, this effect can be seen to an extreme degree, with south-facing slopes being warm, wet and forested, and north-facing slopes cold, dry but much more heavily glaciated.

For a farmer, thus, choosing as aspect that will shelter from hot, dry winds or from cold can be critical to successful growth of crops. Most farming agencies will state in manuals whether a crop prefers a poleward or equatorward aspect (if this is important). Glaciology is also influenced by aspect: unless precipitation patterns dictate otherwise (as in Iceland where glaciers accumulate on the much wetter southwestern side) glaciers always accumulate downwards on the poleward side of mountains.

In some locales there are patterns of soil differences related to differences in aspect. Strong slopes with equatorward aspects tend to have soil organic matter levels and seasonal influences similar to level slopes at lower elevation whereas poleward aspects have soil development similarities to level soils at higher elevations. Soils with a prevailing windward aspect will typically be shallower, and often with more developed subsoil characteristics, than adjacent soils on the leeward where decelerating winds tend to deposit more air-borne particulate material. Outside of the tropics, soils with an aspect directed toward an early afternoon solar position will typically have the lowest soil moisture content and lowest soil organic matter content relative to other available aspects in a locale. Aspect similarly influence seasonal soil biological processes that are temperature dependent. Particulate laden winds often blow from a prevailing direction near solar early afternoon, the effects combine in pattern common to both hemispheres.

These are usually of importance only in the tropics, but there they produce many unexpected climatic effects:

• The dryness of the Dahomey Gap, due to the rain-bearing winds moving parallel to the coast.

• The summer dryness of the Coromandel Coast due to the southerly monsoon flowing parallel to the coast. Its wetness during the northeast monsoon is similarly explained.

• The anomalous late autumn rainy seasons of central Vietnam and the coastal zone of northeastern Brazil for the same reason as above.

• The unusual dryness of Port Moresby compared to the rest of New Guinea is due to the fact that the National Capital District lies parallel to the trade winds which have a drying effect. In Gulf Province and Lae, which receives their full force, rainfall during southern winter is exceedingly heavy.

• The relative dryness of the Queensland coast has the same cause as with Port Moresby.

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