lecture 03

Lecture 3Tropical 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 Lecture 3 Tropical Climate

Factors Determining the Distribution of Energy:: 

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. Duration of solar energy is determined by the length of day and night. Factors Determining the Distribution of Energy:

Daylength: 

Daylength

Length of Day in Various Northern Latitudes*: 

Length of Day in Various Northern Latitudes*

Energy of the Atmosphere: 

Radiant energy from the sun provides 99.97% of total energy of the atmosphere. Sun’s temperature = 12,000°F (6,000°K) 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. Energy of the Atmosphere

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. Illustrating the “greenhouse effect” of earth’s atmosphere

Slide8: 

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).

Slide9: 

When the sun is overhead at the equator (March 21 and September 23) the amount of atmospheric ray penetration varies with location: 1 atmosphere at equator 1.56 atmospheres at 40° N&S 45 atmospheres at poles

Slide10: 

Oblique rays deliver less energy because: Their energy is spread over a large surface. They pass through a thicker layer of absorbing atmosphere. Note: Distance of earth to sun is a 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.

Effect of Altitude: 

Temperature declines 2.6°F (2.0°C) for every 1000 ft. This is because atmospheric thermal energy is obtained from the earth’s surface and only indirectly from the sun. Air at lower altitudes has more water vapor and dust and is a more efficient absorber of terrestrial radiation. Effect of Altitude

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 Circulation of air is similar to circulation of water in a pan of water heated by a Bunsen burner. Air Circulation

Circulation of the Atmosphere: Winds: 

Winds refer to the movement of the atmosphere felt on the earth’s surface. Wind tend 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 the come from. Winds coming from the east and moving to the west are known as Easterlies. Circulation of the Atmosphere: Winds

Idealized Representation of Earth’s Surface Winds: 

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 10–12° 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 Pressure Zones in the Tropics and Subtropics

High Pressure Zone: 

A band about 30° north and south. This area is known as the Horse Latitudes, characterized by calms and unstable, unsteady winds. In the days of sailing ships, horses got sick at this point and were often thrown overboard. High Pressure Zone

Tropical Winds: 

Doldrums In 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 not air movement. A balloonist would think the opposite. Tropical Winds

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 flow 10–15 miles per hours, fairly steady 10 to 12° to 25° N&S. NE north of the equator; SE south of the equator. Trade Winds

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 different seasons. Monsoons represent a great break in general circulation of the atmosphere. Monsoon Winds

Pressure Zones and Air Circulation: 

Idealized Actual due to land masses Pressure Zones and Air Circulation

Temperature: 

Isotherm = lines of equal temperature. Coldest isotherm is used to delineate climate 18°C or 64.4°F = coldest month (mean temperature) Any location with coldest month below 18°C 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 Temperature

Total Annual Solar Radiation: 

Total Annual Solar Radiation

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, condensing into rain. Precipitation 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). Note the ITC moves seasonally as it follows the sun. High pressure Dry (winter dry) Low pressure (ITC wet) High pressure Dry (winter dry)

Slide30: 

Hypothetical arrangement of tropical wet (Af), 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.

Precipitation Map: 

Precipitation Map

Arid Areas of Earth: 

Arid Areas of Earth

There are Other Patterns of Rainfall: 

Monsoon (Malabar coast of India, SW Coast of Burma and Thailand): In monsoon area, total rainfall may surpass equatorial rainfall. In India 50–60 inches in 2 months of July & August. There are Other Patterns of Rainfall

Slide34: 

Orographic Precipitation: This is due to mountain lifting of trades. Always get high precipitation where mountain ranges are transverse to trade winds.

Patterns of Temperature and Precipitationin Tropical Climates: 

Af Tropical Rainy Am Tropical Monsoon Aw Tropical Savanna Patterns of Temperature and Precipitation in Tropical Climates