Arid climates

Case study for Population and the Environment: Characteristics and distribution of two major climatic types to exemplify relationships between climate and human activities and numbers.

How much rainfall do arid climates get per year?

Less than 250mm per year

Distribution of hot deserts

Occur 30 degrees north and south of the equator, maximum air temperatures are above 40 degrees

Distribution of temperate deserts

Occur at higher latitudes, cooler than hot deserts but still as arid

Temperature ranges and rainfall distribution

Rain is rare and unreliable, it falls in infrequent heavy storms. Temperature ranges can be huge, nights are usually much colder than days

Deserts are found 30 degrees north and south of the equator

1. Air moves in circular patterns between the equator and 30 degrees north and south of it. These are Hadley cells.

2. In a Hadley cell air rises at the equator, the air cools as it rises and moisture condenses and falls as rain, leaving the air dry

3. Dry air descends around 30 degrees north and south of the equator

4. In area where the air descends a zone of high pressure it created

5. Winds blow outwards from high pressure areas so no moisture can be brought in by the wind

6. The area has low precipitation so desert margins are found there

Deserts are found in the middle of continents

1. Central parts of continents are usually more arid than coastal areas

2. Moist wind from the sea moves inland and the moisture held is dropped as precipitation

3. So when the wind reaches the centre of a large continent it’s carrying little moisture, so rainfall is low

Deserts occur next to mountain ranges

1. Tall mountain ranges force winds upwards

2. As air rises it cools and is less able to hold water

3. Any moisture held is dropped as precipitation over the mountains, so the wind that moves inland has very little moisture, so little rain falls

4. This is called the rain shadow effect

Deserts form near cold ocean currents

1. In some places cold ocean currents run along the coast

2. Wind is cooled as it travels over the cold water and its ability to hold moisture is reduced

3. Moisture that’s stored in the atmosphere is released as precipitation over the ocean before reaching land

4. So when the wind reaches the land there’s very little moisture left, so little rain falls

Why is the dry climate a challenge for human occupation of arid regions?

• Agriculture is impossible in most areas because rain falls infrequently, usually in the form of heavy conventional rainstorms, which can cause soil erosion and flash floods

• Water can be drawn from oases to irrigate crops nearby. Crops such as cereals can be grown in the shade of taller food plants. These methods have allowed some people to settle around oases.

• Other native people are nomadic- they move from place to place, which allows people to keep grazing livestock despite the sparse vegetation

Modern solutions to the challenges of arid climates

• Groundwater for irrigation can be more easily accessed with motorised pumps. Modern irrigation techniques include drip irrigation, where water slowly drips onto crops, minimising evaporation

• Economic development is often hindered by climate, but major cities can develop if there is enough money to invest in obtaining water. Las Vegas in the Mojave Desert relies on water from Lake Mead, created by a dam on the Colorado River, for 90% of its water supply.

Temperature variations in hot deserts

• Mean annual temperature is usually between 20 and 30 degrees

• Large seasonal variations- up to 50 degrees in summer and below 0 degrees in winter

• Large daily variations- up to 50 degrees in the day and below 0 degrees at night. This is due to the dry desert air, which can’t block sunlight during the day or trap heat at night.

Temperature in desert margins

• Mean annual temperature is between 10 and 20 degrees, temperature variations are usually between 10 and 35 degrees

Soil in hot deserts

1. Desert soils are infertile because they don’t contain much organic matter- few plants grow

2. Soils are sandy or stony

3. Soils are dry due to low rainfall and high temperatures

Soil in desert margins

1. More fertile than soil in deserts

2. Soil contains more water

3. Soil is less sandy and stony because there’s more weathering