Chapter 4
The tilt of the earth’s axis causes seasonal variation in climate. Because of that tilt, the orientation of the earth’s axis relative to the sun, and thus the incident solar radiation at each latitude, changes as the earth orbits the sun. The position of the solar equator also changes with the seasons.
The earth’s climate tends to be cold and dry toward the poles and hot and wet toward the equator. On a global scale, this pattern originates in the greater intensity of sunlight at the equator than at higher latitudes.
Seasonal variation in temperature increases with distance from the equator, especially in the Northern Hemisphere, where there is less area of ocean to moderate temperature changes (Figure 4.3). At high latitudes in the Northern Hemisphere, mean monthly temperatures vary by an average of 30°C over the year, and extremes vary by more than 50°C annually.
The rising tropical air mass cools as it expands under the lower pressure of the upper atmosphere and radiates heat into space. By the time this air has extended to about 30° north and south of the equator, it has become dense enough to sink back to the earth’s surface and spread out to the north and south, thus completing a cycle within the atmosphere (Figure 4.5). This type of circulation pattern is called Hadley circulation, and the closed cycle of rising and falling air within the tropics is referred to as a Hadley cell.
One Hadley cell forms immediately to the north of the equator and another to the south, like a pair of giant waistbands girdling the earth. The sinking air of the tropical Hadley cells drives less distinct secondary cells, called Ferrel cells, in temperate regions, which circulate in the opposite direction
Differential warming of the earth’s surface creates Hadley circulation. Warm, moist air rises in the tropics, and cool, dry air moves toward the tropics from subtropical latitudes to replace it, forming hadley cells. this circulation pattern drives the secondary Ferrel cells and polar cells at higher latitudes.
The region where surface currents of air from the northern and southern subtropics meet near the equator and begin to rise under the warming influence of the sun is referred to as the intertropical convergence. As the moisture- laden tropical air rises and begins to cool, the moisture condenses to form clouds and precipitation. Thus, the tropics are humid not because there is more water at tropical latitudes than elsewhere, but because water cycles more rapidly through the tropical atmosphere.
Ocean Currents: In large ocean basins, cold surface water circulates toward the tropics along the western coasts of continents, and warm surface water circulates pole ward along the eastern coasts of continents (Figure 4.8). The direction of ocean circulation is another manifestation of the Coriolis effect: ocean currents tend to veer to the right (clockwise) in the Northern Hemisphere and to the left (counterclockwise) in the Southern Hemisphere.
Any upward movement of ocean water is referred to as upwelling. Upwelling occurs wherever surface cur- rents diverge, as in the western tropical Pacific Ocean. As surface currents move apart, they tend to draw water upward from deeper layers. Because deep water tends to be rich in nutrients, upwelling zones are often regions of high biological productivity.
Southern California, at the same latitude, lies to the west of the summer rainfall belt and has a winter- rainfall, summer-drought climate, often referred to as a Mediterranean climate. Named for the Mediterranean region of Europe, which has the same seasonal pat- tern of temperature and rainfall, Mediterranean climates are also found in western South Africa, Chile, and Western Australia— all regions lying along the western sides of continents at about the same latitude north or south of the equator.
The upper layer of warm water above the thermocline is called the epilimnion, and the deeper layer of cold water below it is called the hypolimnion.
An El Niño event appears to be triggered by a reversal of these pressure areas (the so-called Southern Oscillation) and of the winds that flow between them. As a result, the westward-flowing equatorial currents stop or even reverse, upwelling off the coast of South America weakens or ceases, and warm water— the El Niño current— piles up along the coast of South America.
The warm tropical waters that dominate the eastern Pacific Ocean during El Niño events create strong Hadley cell circulation, resulting in a persistent subtropical jet stream that brings cooler, weather, often stormy weather to the southern United States and northern Mexico. The polar jet stream weakens, and warm, dry conditions settle in to the northern states and southern Canada and Alaska.
Global wind patterns interact with other features of the landscape to create precipitation. Mountains force air upward, causing it to cool and lose its moisture as precipitation on the windward side. As the dry air descends the leeward slope and travels across the lowlands beyond, it picks up moisture and creates arid environments called rain shadows.
Topography and geology can modify the environment on a local scale within regions of otherwise uniform climate. In hilly areas, the slope of the land and its exposure to the sun influence the temperature and moisture content of the soil. Soils on steep slopes may drain well; causing drought stress for plants on the hillside at the same time water saturates the soils of nearby lowlands. In arid regions, stream bottomlands and seasonally dry riverbeds may support well-developed riparian forests, which accentuate the contrasting bleakness of the surrounding desert. In the Northern Hemisphere, south-facing slopes receive more sunlight, and its warmth and drying power limit vegetation to shrubby, drought-resistant xeric forms.
The tilt of the earth’s axis causes seasonal variation in climate. Because of that tilt, the orientation of the earth’s axis relative to the sun, and thus the incident solar radiation at each latitude, changes as the earth orbits the sun. The position of the solar equator also changes with the seasons.
The earth’s climate tends to be cold and dry toward the poles and hot and wet toward the equator. On a global scale, this pattern originates in the greater intensity of sunlight at the equator than at higher latitudes.
Seasonal variation in temperature increases with distance from the equator, especially in the Northern Hemisphere, where there is less area of ocean to moderate temperature changes (Figure 4.3). At high latitudes in the Northern Hemisphere, mean monthly temperatures vary by an average of 30°C over the year, and extremes vary by more than 50°C annually.
The rising tropical air mass cools as it expands under the lower pressure of the upper atmosphere and radiates heat into space. By the time this air has extended to about 30° north and south of the equator, it has become dense enough to sink back to the earth’s surface and spread out to the north and south, thus completing a cycle within the atmosphere (Figure 4.5). This type of circulation pattern is called Hadley circulation, and the closed cycle of rising and falling air within the tropics is referred to as a Hadley cell.
One Hadley cell forms immediately to the north of the equator and another to the south, like a pair of giant waistbands girdling the earth. The sinking air of the tropical Hadley cells drives less distinct secondary cells, called Ferrel cells, in temperate regions, which circulate in the opposite direction
Differential warming of the earth’s surface creates Hadley circulation. Warm, moist air rises in the tropics, and cool, dry air moves toward the tropics from subtropical latitudes to replace it, forming hadley cells. this circulation pattern drives the secondary Ferrel cells and polar cells at higher latitudes.
The region where surface currents of air from the northern and southern subtropics meet near the equator and begin to rise under the warming influence of the sun is referred to as the intertropical convergence. As the moisture- laden tropical air rises and begins to cool, the moisture condenses to form clouds and precipitation. Thus, the tropics are humid not because there is more water at tropical latitudes than elsewhere, but because water cycles more rapidly through the tropical atmosphere.
Ocean Currents: In large ocean basins, cold surface water circulates toward the tropics along the western coasts of continents, and warm surface water circulates pole ward along the eastern coasts of continents (Figure 4.8). The direction of ocean circulation is another manifestation of the Coriolis effect: ocean currents tend to veer to the right (clockwise) in the Northern Hemisphere and to the left (counterclockwise) in the Southern Hemisphere.
Any upward movement of ocean water is referred to as upwelling. Upwelling occurs wherever surface cur- rents diverge, as in the western tropical Pacific Ocean. As surface currents move apart, they tend to draw water upward from deeper layers. Because deep water tends to be rich in nutrients, upwelling zones are often regions of high biological productivity.
Southern California, at the same latitude, lies to the west of the summer rainfall belt and has a winter- rainfall, summer-drought climate, often referred to as a Mediterranean climate. Named for the Mediterranean region of Europe, which has the same seasonal pat- tern of temperature and rainfall, Mediterranean climates are also found in western South Africa, Chile, and Western Australia— all regions lying along the western sides of continents at about the same latitude north or south of the equator.
The upper layer of warm water above the thermocline is called the epilimnion, and the deeper layer of cold water below it is called the hypolimnion.
An El Niño event appears to be triggered by a reversal of these pressure areas (the so-called Southern Oscillation) and of the winds that flow between them. As a result, the westward-flowing equatorial currents stop or even reverse, upwelling off the coast of South America weakens or ceases, and warm water— the El Niño current— piles up along the coast of South America.
The warm tropical waters that dominate the eastern Pacific Ocean during El Niño events create strong Hadley cell circulation, resulting in a persistent subtropical jet stream that brings cooler, weather, often stormy weather to the southern United States and northern Mexico. The polar jet stream weakens, and warm, dry conditions settle in to the northern states and southern Canada and Alaska.
Global wind patterns interact with other features of the landscape to create precipitation. Mountains force air upward, causing it to cool and lose its moisture as precipitation on the windward side. As the dry air descends the leeward slope and travels across the lowlands beyond, it picks up moisture and creates arid environments called rain shadows.
Topography and geology can modify the environment on a local scale within regions of otherwise uniform climate. In hilly areas, the slope of the land and its exposure to the sun influence the temperature and moisture content of the soil. Soils on steep slopes may drain well; causing drought stress for plants on the hillside at the same time water saturates the soils of nearby lowlands. In arid regions, stream bottomlands and seasonally dry riverbeds may support well-developed riparian forests, which accentuate the contrasting bleakness of the surrounding desert. In the Northern Hemisphere, south-facing slopes receive more sunlight, and its warmth and drying power limit vegetation to shrubby, drought-resistant xeric forms.