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Slide 1: 

Introduction to the Atmosphere: The Cause of Wind and Rain The Sun: The Source

Slide 2: 

Major Features of Earth and Atmosphere Our perspective of the Earth and atmosphere has broadened from a narrow, local view of incomprehensible mystery to a comprehensive global picture. On December 8, 1972, the Apollo 17 astronauts photographed the Earth from space on their way to the Moon (Next slide). The photo contains a wealth of information about our planet and its atmosphere listed in the Key (Slide after next). The continent of Africa (1), largely surrounded by deep blue ocean, holds center stage. The golden regions are Africa’s great deserts, the Kalihari (2) and Namib in the southwest, and the Sahara (3) stretching across the width of northern Africa, and continuing in the Arabian peninsula (4). These deserts are largely confined between 20° and 30° latitude, in subtropical belts that contain most of the world’s great deserts. The broad, green band in the center of Africa houses the Congo Rainforest (5), which straddles the equator like all tropical rainforests. Much further south, bathed in the December sun, lies the white, ice-covered continent of Antarctica (6), outlined by a giant seam that distinguishes it from the surrounding sea ice and from clouds. The North Pole, out of sight at the top and cast into December darkness, is also covered with ice and snow. White clouds cover the globe in a variety of intricate patterns. Several large, comma-shaped cloud swirls occupy the ocean between Antarctica and Africa, and one covers Africa’s southern tip (7). They mark the low pressure areas called extratropical cyclones, that bring the middle and higher latitudes most of their stormy weather and supply Antarctica with its snow. The band of cloud spots and blotches over Africa’s green central band constitute the tropical rainbelt (8) and include many towering thunderstorms. Few of these clouds venture over Africa’s deserts. Even the northern fringe of green land just south of the Sahara (9) is clear, for December is the dry season there since the sun and the tropical rainbelt is centered south of the Equator where the sunglint (10) brightens the water between Madagascar and Africa. The cloud line on the east Coast of Madagascar (11) shows that the trade winds strike the coast there and produce clouds and rain as they ascend the mountains. A small pinwheel cloud system resembling a spiral galaxy appears at the upper right (12), near the southern tip of India. This system is a tropical cyclone or hurricane and the dots are thunderstorms that are analogs to stars in galaxies. Its majesty from space contrasts with its destructiveness on Earth.

Slide 3: 

Apollo 17 View of Earth 08 Dec 1972 This famous photo shows Earth’s main climate zones and weather features Subtropical Deserts Tropical Rainforest Antarctic Ice Cap Tropical Cloud Belt Frontal Cyclones Tropical Cyclone

Slide 4: 

1 2 3 4 5 6 7 8 9 11 10 6 9 12 Features Described in Text 1. Africa 2. Kalihari Desert 3. Sahara Desert 4. Arabian Peninsula 5. Congo Rainforest 6. Antarctica 7. Comma Cloud 8. Tropical Rainbelt 9. Sahel 10. Sunglint 11. Orographic Cloud 12. Tropical Cyclone

Slide 5: 

Definitions Trade Winds Tropical winds that blow toward the equator from the east. Intertropical Convergence Zone (ITCZ) The zone along which the Trade Winds converge and rise. The ITCZ is marked by abundant convective clouds and showery rain. Cyclone A storm marked by low atmospheric pressure and characterized by counterclockwise (in the Northern Hemisphere), inward spiraling winds. Cyclones are generally regions of cloudiness and precipitation. Tropical Cyclone A cyclone that forms over warm tropical waters some distance from the equator, marked by a central clear eye and spiral cloud bands. It has the appearance of a spiral galaxy. Extratropical Cyclone A cyclone that forms outside the tropics as a result of the meeting of polar and tropical air masses. Marked by fronts. (Example - Blizzard) Front A boundary line separating tropical and polar air masses

Slide 6: 

Sunlight varies over the Earth producing variations of temperature, density and pressure Sunshine and Temperatures on Earth Width of Sunbeam striking a specified area of ground

Slide 7: 

Temperature Differences Drive the Winds When air is heated it expands.Warm air tends to rise because it is light. When air is cooled it contracts and becomes denser. Because cold air is dense it tends to sink. The Thermoscope Expansion and contraction of air caused by temperature changes is illustrated by the thermoscope to the right. The thermoscope is essentially an air thermometer. When air is cooled the air in the bulb contracts and the colored water is drawn up into the tube. When air is heated it expands, forcing water out of the tube (and some air if it is heated enough).

Driving the Winds : 

Driving the Winds Temperature, density and pressure differences drive the winds. If the Earth did not rotate and its surface were uniform, one circulation cell with warm, light, rising air at the equator and cold, dense, sinking air at the poles would fill each hemisphere.

Winds on the Rotating Earth : 

Winds on the Rotating Earth Earth’s rotation breaks winds in each hemisphere into three cells or zones. Tropical E-W Trade Winds Mid Latitude W-E Winds Polar E-W Winds Cloudy, rising air prevails near equator and 60° lat Dry, sinking air prevails near 30 ° and 90 ° lat.

Slide 10: 

Effect of Rotation on Winds in Tropics Coriolis Force: Earth’s rotation deflects moving air to its right (left) in the North (South) Hemisphere. Result: In each tropical circulation cell, Trade Winds are deflected to the West as they approach the equator. Note that rain occurs where air rises and deserts are located where air persistently sinks.

Slide 11: 

Clouds, Rain, and Rising Air Rising air produces clouds and rain. But why?

Slide 12: 

As air cools its water vapor capacity decreases. When capacity falls below vapor content (indicated by the mixing ratio) the excess vapor condenses to form liquid water or ice. All clouds and rain are formed by the process of cooling air.

Slide 13: 

As air rises pressure decreases. Air then expands and cools. The fastest way to cool air and form clouds is to make the air rise. Thus, most clouds and almost all precipitation are produced by rising air.