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.

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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.

Continue reading here: Bumps and hollows in the landscape have their own microclimate

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  • cristina pisani
    Is the sun angle effecting plant growth?
    5 months ago
    Which side of the mountIn will be the warmest in southern hermisphere amd why?
    8 months ago
  • onni
    Which side of a mountain will be warmest in the southern hemisphere?
    9 months ago
  • Orla
    What type of microclimate and vegetation on low sun angle slope?
    1 year ago
  • andwise
    Which aspect gets more flowers?
    2 years ago
  • ulrich
    What is the air temperature in southfacing slope in Northern Hemisphere?
    2 years ago
  • scarlett
    How does the changing angle of the sun affect vegetation throughout the world?
    2 years ago
  • ren
    What is sun aspect affect plants?
    2 years ago
  • Jessamine
    Can half a hemisphere be facing the sun?
    2 years ago
  • Sven
    What is significant about sunlight from the south?
    2 years ago
  • Elanor
    Why do we plant cucumbers north facing in the southern hermisphere?
    2 years ago
  • matti
    What is the importance of ground angle with the sun?
    2 years ago
  • Sophia
    How does the changing angle of the sun effect the vegetation throughout the world?
    2 years ago
  • jemima roper
    Is the sun south facing in the northern hemisphere?
    2 years ago
  • dawit semere
    Why the South facing slopes in the Southern hemisphere are colder than the northfacing slopes?
    2 years ago
    Why the South facting slopes in the Southern hemisphere are colder than the north facing slopes?
    2 years ago
  • Pirjo
    Why do we experience more vegetation in north facing slope?
    3 years ago
  • Ulrich K
    Why Vegetation does not good well on SouthFacing slope?
    3 years ago
  • bellisima sandheaver
    What kind of plants found in the north facing slope in the southern hemisphere?
    3 years ago
  • milen
    Why is the slope aspect facing the sun have more vegetation?
    3 years ago
  • Carmela
    What is the ground sun agnle on a south facing slop?
    3 years ago
  • Ansegar
    When is the southern hemisphere angled towards the sum?
    3 years ago
  • senait
    Why are the south facing slopes in the southern hemisphere colder than the north facing slopes?
    3 years ago
  • Aydin
    Why the northen slop soil is always the warmest soil?
    4 years ago
  • rowan bracegirdle
    Why will the ground on the northern slope have warmer temperature?
    4 years ago
  • Clara
    Why the northrn slope is always the warmest?
    4 years ago
  • crystal
    Why bedrooms face northwards due to climatic conditions?
    4 years ago
  • kidane
    Why flowers do not grow on south facing slope?
    4 years ago
  • gerontius
    Why are the southfacing slopes in the southern hemisphere colder than the northfacing slopes?
    4 years ago
  • shane ritchie
    Why will the soil in northern slope have warmer temperatures?
    5 years ago