Hot desert, and the tropical biomes compared

Overview

Rainfall of a hot desert, three main strategies desert plants use to cope with drought and what a climate graph for tropical forest would look like.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Where are the hot deserts found?

   Model answer

   In two belts centred on roughly 20-30 degrees north and south, beneath the descending limbs of the Hadley cells. In the northern hemisphere these include the Sahara, the Arabian and the Thar; in the southern hemisphere the Kalahari and Namib, the Atacama and the Australian deserts. They are best developed on the western sides of continents, where cold offshore currents reinforce the stability, and in continental interiors far from any moisture source, and they are largely absent from eastern coasts at the same latitude where onshore winds off warm seas bring rain.

2. 2

   Describe the rainfall of a hot desert.

   Model answer

   Annual totals are below 250 mm and in the driest cores below 50 mm, with some places recording no rain for several years. Just as important as the total is the unreliability: rainfall is highly erratic, and when it comes it often arrives as a single intense convectional downpour that delivers a large share of the annual total in a few hours, producing flash floods. There is no meaningful seasonal pattern to depend on, and potential evapotranspiration greatly exceeds precipitation in every month.

3. 3

   Why is the daily temperature range in a hot desert so extreme?

   Model answer

   Because there is almost no cloud and very little water vapour in the air. By day, cloudless skies allow the maximum insolation to reach a surface with little vegetation or moisture to absorb the energy as latent heat, so ground and air temperatures soar, commonly above 40 degrees Celsius. By night, the same absence of cloud and water vapour means very little of the outgoing long-wave radiation is trapped or re-radiated back to the surface, so heat escapes rapidly to space and temperatures can fall to near or below freezing. A daily range of 20 to 30 degrees, and sometimes closer to 40, is normal, and in most hot deserts it exceeds the annual range of mean monthly temperatures.

4. 4

   Why is relative humidity so low in hot deserts?

   Model answer

   There is very little moisture to evaporate at the surface, since there is no standing water and only sparse vegetation to transpire, and the descending, adiabatically warmed air of the subtropical high has a very high capacity to hold water vapour. A large capacity combined with a small actual amount of vapour gives a very low relative humidity, often under 20 per cent in the afternoon, which in turn is why condensation and cloud rarely form.

5. 5

   What are the three main strategies desert plants use to cope with drought? Give an example of each.

   Model answer

   Succulence: storing water in swollen tissues and reducing water loss with a thick waxy cuticle, few stomata that open only at night, and leaves reduced to spines, as in cacti. Drought-avoidance by depth: sending a very long tap root down to reach the water table, as in phreatophytes such as mesquite and acacia, or spreading a very wide shallow root net to capture the first drops of an occasional shower. Drought-escape: living as an ephemeral, in which a seed lies dormant for years and then germinates, flowers and sets seed within a few weeks of a rain event, so the plant is only alive when water is available.

6. 6

   Beyond storing water, how else do desert plants reduce their losses?

   Model answer

   Leaves are small, thick, waxy, hairy or absent altogether, cutting the surface area available for transpiration and reflecting solar radiation. Stomata are few, sunken into pits and typically open at night rather than in the heat of the day. Some species carry out their gas exchange at night and store carbon dioxide chemically until daylight. Many are spaced widely apart, and some release chemicals that inhibit the germination of neighbours, so that each individual has sole use of the moisture beneath a large area of ground; this is why desert vegetation looks so regularly spaced.

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