Lecture 3
Tropical Climate

The geometrical relationship between the earth and sun is responsible for the earth's climates.
Climate of the earth is based on:

Temperature (solar radiation)
Winds and pressure
Daylength
Altitude

Factors determining the distribution of energy:

  1. The intensity of solar radiation is a function of the angle at which sunlight reaches a portion of the earth's surface.
    The angle is due to the curvature of the earth.

  2. Duration of solar energy is determined by the length of day and night.

Daylength

Latitude
 Shortest day
 Longest day
0 12:07 12:07
10 11:32 12:42
20 10:56 13:20
30 10:14 14:04
40 9:20 14:00
50 8:05 16:21
60 5:54 18:49
70 0:00 24:00
80 0:00 24:00
90 0:00 24:00

Length of Day in Various Northern Latitudes

Month 0º 10º 20º 30º 40º 50º 60º 70º 80º 90º
January 12:07 11:35 11:02 10:24 9:37 8:30 6:38 0:00 0:00 0:00
February 12:07 11:49 11:21 11:10 10:42 10:07 9:11 7:20 0:00 0:00
March 12:07 12:04 12:00 11:57 11:53 11:48 11:41 11:28 10:52 0:00
April 12:07 12:21 12:36 12:53 13:14 13:44 14:31 16:06 24:00 24:00
May 12:07 12:34 13:04 13:38 14:22 15:22 17:04 22:13 24:00 24:00
June 12:07 12:42 13:20 14:04 15:00 16:21 18:49 24:00 24:00 24:00
July 12:07 12:40 13:16 13:56 14:49 15:38 17:31 24:00 24:00 24:00
August 12:07 12:28 12:50 13:16 13:48 14:33 15:46 18:26 24:00 24:00
September 12:07 12:12 12:17 12:23 12:31 12:42 13:00 13:34 15:16 24:00
October 12:07 11:55 11:42 11:28 11:10 10:47 10:11 9:03 5:10 0:00
November 12:07 11:40 11:12 10:40 10:01 9:06 7:37 3:06 0:00 0:00
December 12:07 11:32 10:56 10:14 9:20 8:05 5:54 0:00 0:00 0:00
(In hours and minutes on the 15th of each month)
fig. 1

Energy of the Atmosphere

Radiant energy from the sun provides 99.97% of total energy of the atmosphere.
Sun's temperature = 12,000ºF (6,000ºK).
Earth intercepts 1/5 billionth of this energy.
Solar energy is the engine which drives the earth's atmosphere and oceanic circulation.
Radiant energy travels through space as electric magnetic waves traveling at 186,000 miles per second.

Illustrating the "greenhouse effect" of earth's atmosphere.
The glass in the roof and sides of the greenhouse, like the atmosphere, is relatively transparent to short-wave solar energy, but relatively opaque to long-wave earth radiation.

 fig. 2

When the sun is overhead at the equator (March 21 and September 23) the amount of atmosphere that rays penetrate varies with location:

1 atmosphere at equator
1.56 atmospheres at 40ºN&S
45 atmospheres at poles

Oblique rays deliver less energy because:

  1. Their energy is spread over a large surface

  2. They pass through a thicker layer of absorbing atmosphere

Note: Distance of earth to sun is as trivial factor in the amount of energy received, but does change in orbit.
The earth is actually closer to the sun in December than in June.

 fig. 3

Oblique solar rays deliver less energy at the earth's surface than vertical rays, both because their energy is spread over a larger surface (top), and because they pass through a thicker layer of reflecting and absorbing atmosphere (bottom).

Effect of Altitude

Temperature declines 2.6ºF (2.0ºC) for every 1000 ft.
This is because atmospheric thermal energy is obtained from the earth's surface and only indirectly from the sun.
Lower air has more water vapor and dust and is a more efficient absorber of terrestrial radiation.

Air Circulation

Energy of air comes from reradiation of the earth's surface.

Warm air is light & rises = low pressure = associated with "hot & rainy" conditions.

Cool air is heavy and sinks = high pressure = dry (cooler) conditions

 fig. 4

Circulation of air is similar to circulation of water in a pan of water heated by a candle

fig. 5

Circulation of the Atmosphere: Winds

Winds refer to the movement of the atmosphere felt on the earth's surface.

Wind tends to move from high pressure to low pressure but the actual movement of winds is very complicated.

Winds are named for the direction from which they come from.

Winds coming from the east and moving to the west are known as Easterlies.

 fig. 6

Idealized Representation of Earth's Surface Winds.

Pressure Zones in the Tropics and Subtropics

Low Pressure Zone
Inter-tropical Convergence zone (ITC)

  • Also known as Equatorial Trough

  • 10-12º band straddling the equator

  • Moves with the sun

  • This is an area of low pressure because of the intensity of solar radiation which heats the air

    		•  fig. 7

    High Pressure Zone

  • A band about 30º north and south.

  • This area is known as the Horse Latitudes, characterized by calm and unstable, unsteady winds.

  • In the days of sailing ships, horses got sick at this point and were often thrown overboard.

    		•  fig. 8
    fig. 9

    Tropical Winds

    Doldrums

  • In the ITC winds are weak.

  • It refers to the lack of progress of sailors in this area of the ocean due to calms, squalls, and light, baffling winds.

  • However, there is a massive upward movement of air, but this is not apparent on the surface.
    • A sailor would say there is no air movement
    A balloonist would think the opposite.

    Trade Winds

  • Dependable winds moving from high pressure zone of horse latitudes to the edge of the ITC.

  • They veer to the west because of the rotation of the earth, thus are easterly winds.

  • The trade winds dominate the tropics.

  • Winds blow 10-15 miles per hour, fairly steady 10 to 12ºN & 25ºS.

  • NE north of the equator; SE south of the equator.

    • Monsoon winds

  • Monsoon is an Arabic word meaning season.

  • Monsoon winds reverse themselves seasonally.

  • Best developed in Western parts of oceans or eastern parts of continents, particularly Asia.

  • Monsoon is based on differential thermal heating and cooling of land areas creating zones of high and low pressure over land in differnet seasons.

  • Monsoon represents a great break in the general circulation of the atmosphere.
    		•  fig. 10

    Pressure Zones and Air Circulation

    fig. 11

    Idealized (left) and actual due to land masses (right)

    Temperature

    Isotherm = lines of equal temperature.
    Coldest isotherm is used to delineate climate.
    18ºC or 64.4ºF = coldest month (mean temperature)
    Any location with coldest month below 18ºC isotherm is not considered tropical.
    However, these areas generally are within the tropics.
    Temperature based on total annual solar radiation.
    This is affected by:

    Cloud cover
    Daylength.

    Total Annual Solar Radiation

    Latitude Thermal days
    0 365.2
    10 360.2
    20 345.2
    30 321.0
    40 288.5
    50 249.7
    60 207.8
    70 173.0
    80 156.6
    90 151.6
    Thermal days = avg. total daily solar energy at equator

    Precipitation

    The ITC is slightly to the north of the equator and moves with the second path of the sun.

    Thus, in equatorial areas, rainfall is high and steady; the hot air rises and cools, which condenses into rain.

    Note the ITC moves seasonally as it follows the sun.

    High pressure Dry (winter dry)
    Low pressure (ITC wet)
    High pressure Dry (winter dry)

    A month with less than 2.4 inches (60 mm) is considered a dry month in the tropics.

    In the tropics, winter is typically dry (there are exceptions, Hawaii for example).

    		 fig. 12
    fig. 13
    fig. 14
    Hypothetical arrangement of tropical wet (Ar), tropical wet-and-dry (Aw), and dry (B) climates with respect to latitude, ITC, and wind and pressure belts at the time of the times of the extreme seasons. fig. 15

    Precipitation Map

    fig. 16

    Arid Areas of Earth

    fig. 17

    There are other patterns of Rainfall.

    Monsoon (Malabar coast of India, SW Coast of Burma and Thailand)

    In monsoon areas, total rainfall may surpass equatorial rainfall.

    In India 50–60 inches in 2 months of July & August.

    		 fig. 10

    Orographic Precipitation.

    This is due to mountain lifting of trades.

    Always get high precipitation where mountain ranges are transverse to trade winds.

    		 fig. 18
    Rainfall contrasts on windward and leeward slopes.

    Patterns of Temperature and Precipitation in Tropical Climates

    fig. 19 fig. 20 fig. 21

    Close window