Look For The Following Key Ideas In Chapter 6 • The interaction of ocean and atmosphere moderates surface temperatures, shapes Earth's weather and climate, and creates most of the sea's waves and currents. • Different amounts of solar energy are absorbed at different latitudes. The tropics are warmer than the polar regions because of this difference. • Uneven solar heating causes convection currents to form in the atmosphere. The direction of air flow in these currents is influenced by the rotation of Earth. • To observers on the surface, Earth's rotation causes moving air (or any moving mass) in the Northern Hemisphere to curve to the right of its initial path, and in the Southern Hemisphere to the left. The apparent curvature of path is known as the Coriolis effect. • Each hemisphere has three large atmospheric circulation cells: a Hadley cell, a Ferrel cell, and a polar cell. Air circulation within each cell is powered by uneven solar heating and influenced by the Coriolis effect.
Key Ideas Continued… • The atmosphere responds to uneven solar heating by flowing in three great circulating cells over each hemisphere. The flow of air within these cells is influenced by Earth’s rotation (Coriolis effect). • Large storms are spinning areas of unstable air that develop between or within air masses. Extratropical cyclones originate at the boundary between air masses. • Tropical cyclones, the most powerful of Earth's atmospheric storms, occur within a single humid air mass.
Earth’s Atmosphere Is Composed Mainly of Nitrogen, Oxygen, and Water Vapor What are some properties of the atmosphere? • The lower atmosphere is a fairly homogeneous mixture of gases. • Water vapor occupies up to 4% of the volume of the atmosphere. • The density of air is influenced by temperature and water content. (right) Ascending air cools as it expands. Cooler air can hold less water, so water vapor condenses into clouds. Descending air warms as it compresses and the clouds evaporate.
The Solar Heating of Earth Varies with Latitude How solar energy input varies with latitude. Equal amounts of sunlight are spread over a greater surface area near the poles than in the tropics. Ice near the poles reflects much of the energy that reaches the surface there.
The Solar Heating of Earth Varies with Latitude Earth as a whole is in thermal equilibrium, but different latitudes are not. (top left) The average annual incoming solar radiation (red line) absorbed by Earth is shown along with the average annual infrared radiation (blue line) emitted by Earth. What factors govern the global circulation of air? • Uneven solar heating • The Coriolis effect (bottom left) The ocean does not boil away near the equator or freeze solid near the poles because heat is transferred by winds and ocean currents from equatorial to polar regions.
The Solar Heating of Earth Also Varies with the Seasons The seasons are caused by variations in the amount of incoming solar energy as Earth makes its annual rotation around the sun on an axis tilted by 23 ½°. During the Northern Hemisphere winter, the Southern Hemisphere is tilted toward the sun and the Northern Hemisphere receives less light and heat. During the Northern Hemisphere summer, the situation is reversed.
Winter (Northern Hemisphere tilts away from sun) 23½° To Polaris Spring (sun aims directly at equator) Summer (Northern Hemisphere tilts toward sun) Fall (sun aims directly at equator) Stepped Art Fig. 7-5, p. 151
The Solar Heating of Earth Also Varies with the Seasons A convection current forms in a room when air flows from a hot radiator to a cold window and back. Air warms, expands, becomes less dense, and rises over the radiator. Air cools, contracts, becomes more dense, and falls near the cold glass window.
Earth’s Uneven Solar Heating Results in Large-Scale Atmospheric Circulation A hypothetical model of Earth’s air circulation if uneven solar heating were the only factor to be considered.
The Coriolis Effect Deflects the Path of Moving Objects The Coriolis effect is the observed deflection of a moving object, caused by the moving frame of reference on the spinning Earth. How does this apply to the atmosphere? As air warms, expands, and rises at the equator, it moves toward the pole, but instead of traveling in a straight path, the air is deflected eastward. In the Northern Hemisphere air turns to the right. In the Southern Hemisphere air turns to the left.
The Coriolis Effect Deflects the Path of Moving Objects (above-left) Sketch of the thought experiment in the text, showing that Buffalo travels a shorter path on the rotating Earth each day then does Quito. (above-right) A continuation of the thought experiment. A look at Earth from above the North Pole shows that Buffalo and Quito move at different velocities.
The Coriolis Effect Deflects the Path of Moving Objects The final step in the experiment. As observed from space, cannonball 1 (shot northward) and cannonball 2 (shot southward) move as we might expect; that is, they travel straight away from the cannons and fall to Earth. Observed from the ground, however, cannonball 1 veers slightly east and cannonball 2 veers slightly west of their intended targets. The effect depends on the observer’s frame of reference.
The Coriolis Effect Influences the Movement of Air in Atmospheric Circulation Cells Global air circulation as described in the six-cell circulation model. Air moves in cells, influenced by the Coriolis effect.
Six Atmospheric Circulation Cells Exist in Each Hemisphere A large circuit of air is called an atmospheric circulation cell. Three cells exist in each hemisphere. Hadley cells are tropical cells found on each side of the equator. Ferrel cells are found at the mid-latitudes. Polar cells are found near the poles. What are some of the wind patterns found between and within cells? Doldrums are calm equatorial areas where two Hadley cells converge Horse latitudes are areas between Hadley and Ferrel cells. Trade winds are surface winds of Hadley cells. Westerlies are surface winds of Ferrel cells.
Atmospheric Circulation Generates Large-Scale Surface Wind Patterns Winds over the Pacific Ocean on 20 and 21 September 1996. Wind speed increases as colors change from blue-purple to yellow-orange, with the strongest winds at 20 meters per second (45 miles per hour). Wind direction is shown by the small white arrows. Although instantaneous views such as this one depart substantially from wind flow predicted in the six-cell model, the average wind flow over many years looks remarkable like what we would expect from the model.
Monsoons, Sea Breezes and Land Breezes Monsoons are patterns of wind circulation that change with the season. Areas with monsoons generally have dry winters and wet summers. Sea breeze is cool air from over the water moving toward land. Sea breezes occur after sunrise. Land breezes occur after sunset when air warmed by the land blows toward the water.
Monsoons Are Wind Patterns That Change with the Seasons A monsoon is a pattern of wind circulation that changes with the season. (The word monsoon is derived from mausim, the Arabic word for season). Locations where monsoons occur typically have wet summers and dry winters. (left) Monsoon patterns. During the monsoon circulations of January and July, surface winds are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
Sea Breezes and Land Breezes Arise from Uneven Surface Heating The flow of air in coastal regions during stable weather conditions. (Top) In the afternoon, the land is warmer than the ocean surface, and the warm air rising from the land is replaced by an onshore sea breeze. (Bottom) At night, as the land cools, the air over the ocean is now warmer than the air over the land. The ocean air rises. Air flows offshore to replace it, generating an offshore flow (a land breeze).
Storms Are Variations in Large-Scale Atmospheric Circulation Storms are regional atmospheric disturbances. Storms have high winds and most have precipitation. Tropical cyclones occur in tropical regions. These storms can cause millions of dollars worth of damage and endanger life. Extratropical cyclones occur in Ferrel cells, and are winter weather disturbances. These storms can also cause extensive damage. Both types of storms are cyclones, orrotating masses of low-pressure air.
Storms Are Variations in Large-Scale Atmospheric Circulation The genesis and early development of an extratropical cyclone in the Northern Hemisphere Figure a: The stages of the development of a tropical cyclone in the Northern Hemisphere Figure b: How precipitation develops in an extratropical cyclone.
Tropical Cyclones Form in One Air Mass The internal structure of a mature tropical cyclone, or hurricane. (The vertical dimension is exaggerated in this model of a hurricane.)
Tropical Cyclones Form in One Air Mass Tropical cyclones can develop in zones of high humidity and warm air over a sea surface exceeding 26°C (79°F), the areas showed in red on the map.
Tropical Cyclones Form in One Air Mass The dynamics of a tropical cyclone, showing the influence of the Coriolis effect. Note that the storm turns the “wrong” way (that is, counterclockwise) in the Northern Hemisphere, but for the “right” reasons.
N Equator Air starts moving toward a zone of low pressure and veers off course to right Core of tropical cyclone rotating to the left, counter-clockwise L Air starts moving toward a zone of low pressure and veers off course to right Stepped Art Fig. 7-22, p. 163
Tropical Cyclones Form in One Air Mass The breeding grounds of tropical cyclones are shown as orange shaded areas. The storms follow curving paths: First they move westward with the trade winds. Then they either die over land or turn eastward until they lose power over the cooler ocean of mid-latitudes. Cyclones are not spawned over the South Atlantic or the southeast Pacific because their waters are too chilly; nor are they born in the still air - the doldrums - within a few degrees of the equator.
Chapter 6 Summary The water, gases, and energy at the Earth’s surface are shared between the atmosphere and the ocean. The two bodies are in continuous contact, and conditions in one are certain to influence conditions in the other. The interaction of ocean and atmosphere moderates surface temperatures, shapes the Earth’s weather and climate, and creates most of the sea’s waves and currents. The atmosphere responds to uneven solar heating by flowing in three great circulating cells over each hemisphere. This circulation of air is responsible for about two-thirds of the heat transfer from tropical to polar regions. The flow of air within these cells is influenced by the rotation of the Earth. To observers on the surface, the Earth’s rotation causes moving air (or any moving mass) in the Northern Hemisphere to curve to the right of its initial path, and in the Southern Hemisphere to the left. The apparent curvature of path is known as the Coriolis effect. Uneven flow of air within cells is one cause of the atmospheric changes we call weather. Large storms are spinning areas of unstable air occurring between or within air masses. Extratropical cyclones originate at the boundary between air masses; tropical cyclones, the most powerful of Earth’s atmospheric storms, occur within a single humid air mass. The immense energy of tropical cyclones is derived from water’s latent heat of evaporation.