Atmospheric Pressure in Desert Environments

What is Atmospheric Pressure?

Atmospheric pressure is the force per unit area exerted on a surface by the weight of air above that surface in the atmosphere of Earth (or that of another planet).

Equation = Force/Area

The Scarcity of Rainfall in the Sahara

Dryness rather than heat is what makes the Sahara a desert climate. Divergent air masses within the high-pressure system dry the air before fronts can form. Even the subtropical zones near the west see relatively low amounts of precipitation. The Sahara Desert averages under 3 inches of rainfall yearly. Some regions see no rainfall for many years, especially in the central areas of Algeria.

Nevertheless, the Sahara becomes by far the hottest part of Africa during the summer months. The temperature has reached over 55c on a number of occasions.

Case Study: Sahara

But that's not it.

We will focus on the Southern Sahara, which has an interesting climate, being that bit more tropical than most other parts. This includes nations such as Niger and northern Nigeria.

Average temperatures are lower here than in the north, particularly on higher ground. In fact, the highest parts of the southern Sahara Desert get snow during winter! In the western section (e.g. Morocco), the cold current from the Canary Islands creates humidity and fog, but reduces precipitation.

Sudan and Egypt, and their tropical deserts

On the banks of the Nile, and in Sudan, the more luscious desert environment has been taken advantage of, and a number of plants have been grown there which could not have been grown anywhere else in the deserts of these countries.

Explaining the Hadley Cell...

Which brings us to the desert....

Some of the cool air does not fall.

• At about 30 degree latitudes in both hemispheres (north and south), the air descends.
• As it does, it warms meaning the air expands
• To the north and south of these desert latitudes, the air once again ascends, producing moisture for the land; finally, over the poles, the air descends again.

2. From the tops of these storms, the air flows towards higher latitudes, where it sinks to produce high-pressure regions over the subtropical oceans and the world's hot deserts, such as the Sahara dessert in North Africa.

That's why the sky is so clear and the rain very seldom in desert environments.

Explaining the Hadley Cell...

1. When tradewinds cross oceans (e.g. over the Atlantic to Africa; the Pacific to Chile), they reach the tropics where they rise and form thunderstorms and rain.

This is called the Inter-Tropical-Convergence Zone (ITCZ).

Here on the equator line....

This warm, moist air then rises, and eventually cools.

• The reason why it rains so much here, is because some of the water droplets fall as rain
• The reason why this area is so humid, is because the cooler air loses its moisture. This moisture falls to the surface.

How does this relate to Geography and the earth?

The Earth's atmosphere moves in generally, predictable patterns that are largely driven by the sun's rays and the earth's rotation.

• At the Equator, the sun's rays are perpendicular with the Earth's surface and so solar heating is intense.

Atmospheric Pressure in Desert Environments

by Joe, Ellis and Rhys