Sahel

The Sahel is a region in West Africa. 

It is a strip of dry land about 5500 kilometres long and 450 kilometres wide. The Sahel covers parts of (from west to east) the Gambia, Senegal, southern Mauritania, central Mali, Burkina Faso, southern Algeria and Niger, northern Nigeria and Cameroon, central Chad, southern Sudan, and northern South Sudan. It lies at the southern edge of the Sahara Desert and is located between the dry desert land to the north and the forest areas to the south.

Climate
The Sahel has a semi-arid climate. The temperature is high throughout the year. 
There is little rainfall in the Sahel (between 100-150 mm and 600mm). It is concentrated during summer months and can be unreliable. It may be very dry in some years, especially if a large area of low pressure, which brings rain, is not carried North over the Sahel by strong winds.

Desertification
The Sahel is facing a big problem, Desertification. The distribution of rainfall in the Sahel region is uneven. The rainfall in the region has been below average since 1970. The Sahel is becoming drier on the whole. The problem of drought affects most of the countries. As a result, crops and animals die. 

Soil erosion
There is no vegetation to protect the soil, which is then removed by flash floods and strong wind. This leads to soil erosion. 

Deforestation
The population in the Sahel is growing. To meet the ever-increasing demand for firewood, a lot of trees have been cut down. More land has become barren. This is called deforestation. 

Overcultivation
Because people farm intensively in order to grow more crops, the soil becomes dry and infertile. If this process continues for a long time, no crops can be grown. This is called overcultivation. 

Overgrazing
People keep many animals for food. However, the grass cannot support so many grazing animals. This is called overgrazing. 

Deforestation

Deforestation is clearing Earth's forests on a massive scale, often resulting in damage to the quality of the land. The habitats of the animals are lost and many animals die. At the moment every 90 minutes the tropical rainforest in the Amazon will be 2.2 square kilometres smaller. That is about 270 football pitches.

Deforestation also results in global warming (climate change).

Forests still cover about 30 percent of the world’s land area, but areas of the size of Panama (± 75,000 sq. km) are lost each and every year.

The world’s rain forests could completely vanish in a hundred years at the current rate of deforestation.

Forests are cut down for many reasons, but most of them are related to money or to people’s need to provide for their families. The biggest driver of deforestation is agriculture. Farmers cut forests to provide more room for planting crops or grazing livestock. 

Often many small farmers will each clear a few acres to feed their families by cutting down trees and burning them in a process known as “slash and burn” agriculture.

Forests are often planted to protect against natural disasters. When forests are lost, very often the soil they protected is also lost. This loss of soil is often called erosion. We give out carbon dioxide when we breathe and plants and trees take it in by photosynthesis. 

Without forests rain falls scarcely on hot areas.

Erosion

Erosion is the process of natural forces moving rocks and soil. Rocks and soil can be referred to as earth materials.

The natural forces that make erosion happen are:

water	water	Happens when water moves the pieces of rock or soil downhill and carries small pieces of material with it.
	waves	Carry away small pieces of material. A wave can wash up onto the surface of rock or soil and then carry away pieces of material as it flows back into the ocean or lake.
Wind		Occurs when wind moves pieces of earth materials. Wind erosion is one of the weakest kinds of erosion. Small pieces of earth material can be rolled along the ground surface by wind. Very small pieces can be picked up and carried by the wind. Occasionally, wind can carry small pieces of earth materials over large distances.
Ice		Usually happens when a glacier moves downhill. As the ice of the glacier moves downhill, it pushes and pulls earth materials along with it. Ice erosion is one of the strongest kinds of erosion because glaciers can move very large rocks.
Gravity		Is the simplest kind of erosion. Gravity simply pulls loose earth materials downhill. Landslides are dramatic examples of gravity erosion).

Erosion is a geological process. Geology is the study of the structure of the earth and the processes that change the earth. Erosion occurs only at the earth's surface.

Most of the energy that makes erosion happen is provided by the sun. The sun's energy causes the movement of water and ice in the water cycle and the movement of air to create wind. If the sun did not provide energy for erosion, only gravity erosion would still occur.

Erosion can cause problems that affect humans. Erosion can also cause problems for humans by removing rocks or soil that support buildings.

Difference between low, middle and high altitudes

The next graphic shows the difference the low, middle and high altitudes.

Longitude

Longitude is a way to say where a place is on the Earth. 

It is measured starting from an imaginary north-south line called the Prime Meridian. (A meridian is an imaginary line drawn from the North Pole to the South Pole.) Longitude says how far east or west of the Prime Meridian any place is.

Longitude is measured using degrees, the same way an angle is. The Prime Meridian is 0° (zero degrees), and the farthest away is +180° eastward and -180° westward. 

Latitude

Latitude is a measure of the distance you are located from the equator.  

It is commonly shown as an imaginary horizontal line that goes across the earth on maps and is used along with longitude as a reference point to determine location. 

The definition of latitude is the angular distance north or south from the earth’s equator measured through 90 degrees. 

Lines of latitude form circles around the earth, with 0 degrees latitude being at the equator and 90° latitude representing the poles. 

For example:

• Amsterdam, The Netherlands, is located at approximately 52 degrees North latitude, Madrid, Spain, is located at approximately 40 degrees North latitude,
• São Paulo, Brazil, is located at approximately 23 degrees South latitude and
• Cape Town, South Africa, is located at approximately 33 degrees South latitude.

Albedo

The amount of sun's rays that is reflected by an earthly body is called albedo. The word albedo is derived from albus, a Latin word for white.

Albedo is a measure of how much light that hits a surface is reflected without being absorbed.  Something that appears white reflects most of the light that hits it and has a high albedo, while something that looks dark absorbs most of the light that hits it, indicating a low albedo.

It is the reflection of a certain area of earth, like snow and mountains, on the surface. 
For example, the albedo of snow covered mountains hurts the eyes. 

Material	Albedo values (% reflected)
Fresh snow	80% - 95%
Light roof	35% - 50%
Grass	25% - 30%
Bricks and stone	20% - 40%
Concrete dry	17% – 27%
Water bodies	10% - 60%
Forest	10% - 20%
Crops and grassland	10% - 25%
Dark roof	8% - 18%
Asphalt	5% - 10%
Earth’s albedo (average)	31%