Monday, 21 February 2022 08:20

Valley Climate Grade 12 Geography Notes

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1.4 Local climate (valley and city climate)

Valley climate occurs on a local scale and lasts for a few hours. In this section we will focus on:

  • Anabatic and katabatic winds
  • Aspect

Anabatic means ‘moving upward’ and katabatic means ‘going downhill’.
1.4.1 Anabatic and katabatic winds
The structure of a valley and the heating and cooling that occurs during a day cause anabatic and katabatic winds to occur. This is shown in Figures 1.4.1A and 1.4.1B.
In the exam, you may be asked to draw, label or describe how anabatic or katabatic winds form in valleys. Learn to redraw and label Figures 1.4.1A and 1.4.1B below.
22Effect of anabatic winds on settlements

  • Anabatic winds take pollution out of the valley.

Effects of katabatic winds on settlements

  • Katabatic winds trap pollution in the valley.
  • Katabatic winds bring cold temperatures to the valley.

Katabatic winds lead to the development of the thermal belt (zone of warmer temperature above the valley floor) and a frost pocket (an area of very cold temperatures at the bottom of a valley where frost occurs) in a valley at night. This is shown in Figure 1.4.1C.
Effects of warm thermal belt on settlement and farming

  • People will build their houses halfway up the slope of a valley to be in the warmer thermal belt (point A on Figure 1.4.1C).
  • Crops which need warm, frost-free conditions will be planted in the thermal belt, for example sugar cane (point A on Figure 1.4.1C).

Effects of frost pockets on settlement and farming

Related Items

  • Crops which can withstand cold conditions (such as frost) can be planted at the bottom of the valley, for example potatoes (point B on Figure 1.4.1C).
  • Pollution is trapped in the cold air below the temperature inversion at night, as shown in Figure 1.4.1D.

1.4.2 Aspect
Aspect refers to the direction in which a slope faces. This determines whether the Sun’s rays will hit the side of the valley directly or indirectly (obliquely). We will focus on how aspect influences the temperatures of north- and south-facing slopes in the southern hemisphere.
In the exam, you may be asked to draw, label or describe how the thermal belt and a frost pocket form in valleys. Learn to redraw and label Figure 1.4.1C.
Let us look at how the Sun’s rays affect slope temperatures in the southern hemisphere:

  • North-facing slopes receive the direct rays of the Sun, making them warmer.
  • South-facing slopes receive the indirect rays of the Sun, making them cooler.

Figure 1.4.2A shows how aspect influences the temperatures of north- and south-facing slopes in the southern hemisphere.
24Activity 2
Figure 1.4.2B (left) illustrates valley climates. Study the diagram and answer the questions that follow.

  1. Name the valley winds depicted in A and B. (2 × 2 = 4)
  2. State ONE advantage of the wind labelled A. (1 × 2 = 2)
  3. Name the layer labelled C. (1 × 2 = 2)
  4. Explain how the wind labelled B influences:
    1. Farming in the valley (2 × 2 = 4)
    2. Industry in the valley (2 × 2 = 4)

Answers to activity 2

    1. Anabatic wind/upslope wind
    2. Katabatic wind/downslope wind/gravity wind(4)
  2. It can carry pollution out of the valley. (2)
  3. Inversion layer/thermal belt (2) 
    1. Winds bring cold air into the valley.
      These winds cause a frost pocket to form.
      The frost can kill the crops. (4)
    2. Cold conditions make working conditions in the industry difficult.
      Pollution produced by the industry will be trapped in the valley. (4)

In the exam, you may be asked to draw, label or describe how aspect affects the temperature on a north- or south-facing slope in the southern hemisphere. 
Answers to activity 2 continued
1.4.3 City climates
Urban areas (cities) experience a different climate compared to the surrounding rural areas. This results in the formation of a heat island over the city. In this section, we will focus on the causes of a heat island. An urban heat island is when the city has warmer temperatures than the surrounding rural areas.
In the exam, you may be asked how the temperatures change as you move towards the centre of the city. Figure 1.4.3A shows how the temperatures increase the closer you get to the city centre (also called the Central Business District, or CBD). Note also the lower temperatures over the park.
26In the exam, you may be asked to say what causes a city to be warmer.
Table 1.2 below lists the factors that cause higher temperatures in the city.

Factors that cause heat island  Explanation 
Artificial (human-made) surfaces  Surfaces like tar absorb more heat. 
Surface area (the sides of the buildings add to the surface area)  With a greater area, more heat is absorbed. 
Pollution   More factories and cars release more pollution, which traps the heat. 
Artificial heat sources (not from the sun, human-made sources)  Factories, cars and air conditioners release heat into the air.

Table 1.2: Factors that cause a heat island
Sustainable ways to reduce the urban heat island effects

  • Promote greenbelts (plan and have more parks or recreational areas with trees and plants).
  • Plant more trees in the city.
  • Increase vegetation cycles by planting rooftop gardens.
  • The new buildings should not be built with material like glass or any reflecting material.
  • The height of the buildings should have a limit.
  • Have laws that force the factories to reduce the amount of pollution they produce.
  • Improve and encourage people to use public transport.
  • Promote lift clubs to work or to school.