The importance of sun angle

Just as sun angle makes the difference overall between temperatures at different latitudes of the earth, it makes a significant difference on a local scale too. If a slope is angled towards the sun when the sun is low in the sky, it gets more of a full beam and so the surface temperature of soil or leaves (and the air just above) will be warmer. On a slope that is in the "wrong" direction relative to the sun, much of the day is spent in shadow or being sunlit at an angle, so it will be colder than if it had been on the flat.

Figure 4.8.

Distribution of temperatures on a sunny summer's day on a hill in Massachusetts, USA. The more southerly-facing slope has warmer temperatures than the opposite slope facing northwards. Elevation from 340 m to 540 m by 40 m. Temperature from 19.0oC to 22.5°C by 0.5oC. After Bonan.

On the equator, the sun travels a path right overhead and does not shine more on either a southern or a northern slope: in fact, it shines slightly more on east and west-facing slopes which catch additional energy from the sun around sunrise and sunset. At higher latitudes in the southern hemisphere, the sun tends to be in the northern half of the sky, so a north-facing slope will be warmer. In the northern hemisphere, south-facing slopes are warmest because the sun stays mostly in the southern half of the sky. For example, one study during a summer's day on a hill in Massachusetts found that the maximum temperature reached during the day was 3.5°C warmer on the south-facing slope than on the north-facing slope (Figure 4.8). In fact, in the mid-latitudes the sun does wander slightly into the "other" half of the sky during the early and late parts of the day during the summer; but always more energy is received from the south in the northern hemisphere, and from the north in the southern hemisphere.

Such local slope angle effects can make a difference to the ecology. A study on flowering times in a wooded valley in Indiana found that several species of wild-flowers bloomed about a week earlier on a south-facing slope than a north-facing one. This is because plants often need to be exposed to a certain amount of heat during the season before they will flower; on the warmer sunlit slope this required "heat sum'' was reached sooner.

The differences with aspect tend to be most striking for types of plants which are right at the edge of their ranges, and barely able to survive in the local climate. Sometimes, they are warmer-climate plants that are at the poleward edge of their range. For example, on sand dunes on the coast of eastern England there grows a type of wild lettuce known as prickly lettuce (Lactuca virosa) which is at the northern edge of its distribution range in Europe. In England it will grow only on the south-facing slopes of dunes, gathering just enough energy for itself to grow and set seed. On coastlines farther south in Europe (e.g., most of France) prickly lettuce grows on both the north and south sides of dunes because the microclimate is warm enough even on the north sides, given the generally warmer air temperatures. Similarly, the stemless thistle (Onopordum acaulon) only grows on the south side of hills at the northern edge of its range in Yorkshire, northern England. In southern England, there is enough warmth for it to grow on both the northern and southern sides of hills.

As well as temperature, the severity of aridity differs between north and south-facing slopes. The stronger the beam of sunlight, the droughtier the conditions as more water is evaporated. In semi-arid areas of southern Europe, many "north European'' plant species requiring cool damp climates only survive on north-facing slopes. I remember once walking on the steep northward-facing slope of a hill in Provence in the south of France. It was covered in beech (Fagus sylvatica) forest and in the shade and dampness of the understory I could for all the world have been in my rainy native land of England—a strangely comforting form of deja vu. Yet, when I topped the brow of the hill to the southern side, in the space of a few meters I was back into hot, dry air, surrounded by typical open Mediterranean scrub. The influence of sun angle had made all the difference between survival of deciduous forest, and its replacement by oily brush that burns every few years.

The difference in moisture availability with aspect can even be noticeable on a more miniature scale on tree trunks; the northern side of a tree trunk in northwestern Europe tends to have a lot more mosses growing on it than the drier, hotter south-facing side.

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