The Tropical Rainforest Ecosystem » Tropical Rainforests: Location and Climate

Where are tropical rainforests?

A tropical rainforest is a dense, tall forest that grows in a hot, wet climate. Rainforests are found in a belt around the Earth, between the Tropics. These are two lines of latitude: the Tropic of Cancer at about 23.5 degrees north and the Tropic of Capricorn at about 23.5 degrees south.

Most rainforest is much closer to the Equator than that. It is mostly found within about 10 degrees of the Equator, the line of latitude at 0 degrees.

The main areas are:

• The Amazon basin in South America, the largest rainforest, mostly in Brazil.
• The Congo basin in central Africa.
• South-east Asia, for example Indonesia and Malaysia.
• Central America, a smaller area to the north of South America.
• Parts of West Africa, along the coast of the Gulf of Guinea.
• Northern Australia, in a small area in the north-east.

When you describe a distribution in the exam, name the continents, use latitude, and say what the pattern looks like: a belt either side of the Equator, broken up by oceans and other climates.

Key terms:

• Tropical rainforest: a dense, tall forest found in the hot, wet climate near the Equator.
• Tropics: the area between the Tropic of Cancer and the Tropic of Capricorn.
• Equator: the line of latitude at 0 degrees, halfway between the poles.
• Equatorial climate: the hot, wet climate with no seasons found close to the Equator.

What is the equatorial climate like?

Tropical rainforests grow where the climate is much the same all year. The equatorial climate has four main features.

Why is it so hot? Overhead sun and latitude

The reason is latitude, which is how far a place is north or south of the Equator. Near the Equator the sun is high in the sky all year, and it is overhead at midday on two days a year.

• The sun's rays hit the ground almost straight on, so the energy is concentrated on a small area.
• The rays pass through less atmosphere, so less heat is lost on the way.
• The sun is high all year, so the strength of the sun hardly changes. That is why the temperature range is so small and there are no distinct seasons.

Further from the Equator the sun is lower in the sky. The same energy is spread over a larger area, so temperatures fall as latitude increases.

Why is it so wet? Low pressure and convectional rainfall

The strong heating also explains the rain. The hot ground warms the air above it. Warm air is lighter, so it rises. Rising air creates an area of low pressure at the surface. Low pressure brings cloud and rain.

Convectional rainfall is rain caused by heated air rising. It happens in the equatorial region almost every day. Write it as numbered steps:

1. The overhead sun heats the ground and the forest in the morning.
2. Water evaporates from the wet ground, rivers and leaves, so the air becomes warm and full of water vapour.
3. The warm, moist air rises, leaving an area of low pressure at the surface.
4. As it rises, the air cools.
5. The water vapour condenses into tiny water droplets and forms tall, dark clouds.
6. The droplets join together and become heavy, then fall as heavy rain, often with thunder and lightning, usually in the afternoon.

The next morning the sun heats the ground again and the cycle repeats. This is why rainfall is so reliable and so high.

Key terms:

• Latitude: how far a place is north or south of the Equator, measured in degrees.
• Overhead sun: when the sun is directly above a place at midday, so its rays hit the ground at the steepest angle.
• Low pressure: an area where air is rising, often bringing cloud and rain.
• Convectional rainfall: rain caused when the ground is heated and warm, moist air rises, cools and condenses.

Reading an equatorial climate table

Here is a climate table for a made-up place, Station R, in the equatorial region.

Month	J	F	M	A	M	J	J	A	S	O	N	D
Temperature (°C)	27	27	27	27	27	26	26	26	27	27	27	27
Rainfall (mm)	280	290	310	300	250	200	170	150	180	200	230	270

Fig. 1.1: Made-up climate data for Station R.

• Temperature: it is 26 to 27°C in every month, so the range is only 1°C. This is high and almost constant.
• Rainfall: every month has at least 150 mm, and the total is 2,830 mm, which is over 2,000 mm. The wettest month is March (310 mm) and the driest is August (150 mm).
• Seasons: nothing marks out a winter or a dry season, so there are no distinct seasons.

On a climate graph, temperature is shown as a line and rainfall as bars. An equatorial graph shows an almost flat line near the top and tall bars in every month.

Exam-style question

Study Fig. 1.1, the climate data for Station R, a made-up place in the equatorial region.

(a) State the month with the highest rainfall. [1 mark]

(b) Describe the temperature at Station R. [2 marks]

(c) State two areas of the world where tropical rainforests are found. [2 marks]

(d) Explain why equatorial regions have high temperatures and heavy rainfall throughout the year. [6 marks]