The New Neotropical Companion

hot and humid, with the temperature averaging

somewhat higher during dry season than in rainy

season. Realize that in the tropics, daily air temperature

fluctuation actually exceeds average annual seasonal

fluctuation, and humidity remains high, at about 88%

in rainy season and 77% in dry season.

Trade Winds, Doldrums, and the

Amazonian Heat Engine

As mentioned in chapter 1, the tropics are warm and

generally wet because the sun’s radiation falls most

directly and most constantly upon the equator, thus

disproportionally warming Earth in the tropical zone.

At the equator, day length is 12 hours throughout the

year. As one travels either north or south from the

equator, Earth’s axial tilt results in the sun’s rays falling

much more obliquely and for shorter periods of time

(both of which make for colder air temperatures) for

part of the year (called winter) and in the well- known

cycles of day length associated with the changing

seasons within temperate and polar regions.

Equatorial heat quickly builds up, and the air rises,

carrying the warmth. Heat strongly facilitates the

evaporation of water, so water vapor rises as well. The

warm, moist air is cooled as it rises, some eventually

condensing as liquid water, which then falls as

precipitation, accounting for the rainy aspect of tropical

climates. The normal flow of warm, moisture- laden air

is from equatorial to more northerly and southerly

latitudes. Again, as air rises, it cools and becomes

increasingly dense. Moisture condenses, falling as

rain, creating a backward flow of drier air toward the

equator, where the cycle will, of course, be repeated. At

the equator are two giant convective air masses, one

from the north and one from the south, called Hadley

cells. These, along with major ocean currents, form the

Intertropical Convergence, or ITC, the major climatic

heat engine on the planet.

In the Amazon Basin, precipitation ranges between

150 and 300 cm (59– 118 in) annually, averaging

around 200 cm (approx. 80 in) in central Amazonia.

What is most interesting about this pattern is that

approximately half of the total precipitation is brought

by incoming easterly trade winds off the Atlantic

Ocean, while the other half is the result of internal

ecological evapotranspiration from the vast forest that

covers the basin. (Evapotranspiration is the evaporation

of water taken up by tree roots and transferred through

the vascular system of trees to leaves, where the water

is evaporated into the air as an adaptation that aids in

keeping the tree tolerably cool.) Up to 75% of the rain

falling within a central Amazonian rain forest may

come directly from evapotranspiration. This represents

a tight recycling of water and, in essence, shows that

Amazonia helps produce its own climate, a rather

remarkable reality. This vast precipitation and water-

recycling system is essentially in equilibrium, though

increasing large- scale deforestation would significantly

disrupt it.

Tropical areas generally fall within the belts of the

trade winds (so- named because they proved favorable

JFMAMJJASNOD

0

5

10

15

Rainfall

Toronto, Canada

Rain (in.)

Daily maximum

temperature

Daily minimum

temperature

Temperature ( ̊F)

20

25

50

75

100

25

0

J F M A M J J A S 0 N D

50

75

100

5

10

15

20

Rainfall

Belém, Brazil

Rain (in.)

Daily maximum temperature

Daily minimum temperature

Temperature ( ̊F)

Figure 2– 3. The two graphs show the sharp contrast between a typical temperate climate, such as is found in Toronto, Canada,

and conditions in the Torrid Zone, such as you would find in Belém, Brazil. Reprinted with permission from Kricher, John.

Tropical Ecology. Princeton, NJ: Princeton University Press, 2011.

chapter 2 why it is hot, humid, and rainy in the tropics 31