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Module #10 Air Currents Module Introduction: ► The unequal heating heating of Earth has wide-ranging effects on the climates of our planet not only because it determines regional temperatures, but also because it drives the circulation of air around our planet. ► These air currents bring warm and cold air to different regions. ► They also move moisture around the planet, which causes some regions to receive more precipitation than others. ► In this module, we will explore how air currents have a major impact on the climates of the world. ► We will begin by examining how the properties of air affect the amount of moisture it can carry. ► We then explore how the unequal heating of Earth causes air to circulate along the ground and up into the atmosphere. ► As we will see, the direction of this air movement is also affected by the rotation of Earth. ► Finally, we will discuss how air currents that travel over mountain ranges can cause different climates to exist on opposite sides of the mountains. Module #10: Air Currents Module #10 Review Questions: B, A, C, A, D, A, D Additional Resources to Review Review Essential Knowledge: 4.5, 4.8 Learning Objectives After this module you should be able to: ● Explain how the properties of air affect the way it moves in the atmosphere. ● Identify the factors that drive atmospheric convection currents. ● Describe how Earth’s rotation affects the movement of air currents. ● Explain how the movement of air currents over mountain ranges affects climates. 1. 2. Bozeman: The Atmosphere Nova: The Coriolis Effect Essential Knowledge 4.5 Global Wind Patterns (Module 10) ● Global wind patterns primarily result from the most intense solar radiation arriving at the equator, resulting in density differences and the Coriolis effect. 4.8 Earth's Geography and Climate (Modules 9, 10, 11) ● Weather and climate are affected not only by the sun’s energy but by geologic and geographic factors, such as mountains and ocean temperature. ● A rain shadow is a region of land that has become drier because a higher elevation area blocks precipitation from reaching the land. Properties of Air ► Air has several important properties that determine how it circulates in the atmosphere. ► Saturation point: The maximum amount of water vapor in the air at a given temperature. ► Adiabatic cooling: The cooling effect of reduced pressure on air as it rises higher in the atmosphere and expands. ► Adiabatic heating: The heating effect of increased pressure on air as it sinks toward the surface of Earth and decreases in volume. ► Latent heat release: The release of energy when water vapor in the atmosphere condenses into liquid water. ► Atmospheric convection current: Global patterns of air movement that are initiated by the unequal heating of Earth. ► Atmospheric convection currents move air and moisture around the globe. The Saturation Point of Air ► When air cools and its saturation point drops, water vapor condenses into liquid water that forms clouds. ► These clouds are ultimately the source of precipitation. Adiabatic Heating and Cooling ► As air rises, it experiences less air pressure and expands in volume. ► As the air expands it cools, thereby lowering the saturation point of the air mass, resulting in precipitation. This is known as adiabatic cooling. ► The same occurs as air descends, except in reverse: the air experiences greater air pressure causing it to compress and heat up. This is known as adiabatic heating. Air Pressure ► Warm air is less dense than cooler air. As air warms, it rises, generating low air pressure. ► Low air pressure is often associated with precipitation because as the air rises, it expands and cools lowering the saturation point. ► Cold air is more dense than warmer air. As air cools, it sinks, generating high pressure. ► High air pressure is associated with clear conditions. Atmospheric Currents ► Warming at Earth’s surface causes air to rise up into the atmosphere where it experiences lower pressures, adiabatic cooling, and latent heat release. ► The cool air near the top of the atmosphere is then displaced horizontally before it sinks back to Earth. ► As it sinks, the air experiences adiabatic heating and then moves horizontally along the surface of Earth to complete the cycle. Atmospheric Convection Currents ► Hadley cell: A convection current in the atmosphere that cycles between the equator and 30° N and 30° S. ► Intertropical convergence zone (ITCZ): The latitude that receives the most intense sunlight (highest angle of incidence), which causes the ascending branches of the two Hadley cells to converge. ► Polar cell: A convection current in the atmosphere, formed by air that rises at 60° N and 60° S and sinks at the poles, 90° N and 90° S. ► Ferrell cell: A convection current in the atmosphere that lies between Hadley cells and polar cells. The ITCZ is the latitude where the angle of incidence is closest to 90°. It moves between the Tropic of Cancer (23.5° N) and Tropic of Capricorn (23.5° S) over the course of the year. Hadley Cells ► Solar energy warms humid air in the tropics. ► The warm air rises and eventually cools below its saturation point. ► The water vapor it contains condenses into clouds and precipitation. ► The air, which now contains little moisture, sinks to Earth’s surface at approximately 30° N and 30° S. ► As the air descends, it is warmed by adiabatic heating. ► This descent of hot, dry air causes desert environments to develop at those latitudes. Hadley cells are atmospheric convection currents that operate between the equator and 30° N and 30° S. The Coriolis Effect ► Coriolis effect: The deflection of an object’s path due to the rotation of Earth. ► As Earth rotates (W → E), its surface moves much faster at the equator than in mid-latitude and polar regions. ► The faster rotation speeds found closer to the equator cause objects that are moving directly north or south to deflect. ► The prevailing winds of the world are produced by a combination of atmospheric convection currents and the Coriolis effect. ► National Geographic: Coriolis Effect (Video) Note how trade winds are deflected to the east and westerlies to the west by the Coriolis Effect. The Coriolis Effect ► ► (b) The different rotation speeds (a) A ball thrown from the North of Earth at different latitudes Pole toward the equator would be cause a deflection in the paths deflected to the west by the Coriolis of traveling objects. effect. Rain Shadow Effect ► Rain shadows cause mountains to be dry on one side. ► Rain shadow: A region with dry conditions found on the leeward side of a mountain range as a result of humid winds from the ocean causing precipitation on the windward side. ► Windward: The side of the mountain facing the wind, usually coming off of the ocean. ► Leeward: The side of the mountain opposite the windward side, in the rain shadow. Rain Shadows ► Air moving inland from the ocean contains a large amount of water vapor. When it meets the windward side of a mountain range (the side facing the wind), it rises and begins to experience adiabatic cooling as it expands under lower pressure. ► Because water vapor condenses as air cools, clouds form and precipitation falls. ► The presence of the mountain range causes large amounts of precipitation to fall on its windward side. ► The cold, dry air then travels to the other side of the mountain range (the leeward side), where it descends and experiences higher pressures, which cause adiabatic heating. ► This air is now warm and dry and produces arid conditions on the leeward side forming the region called a rain shadow. Rain Shadows ► Rain shadows occur where humid winds blowing inland from the ocean meet a mountain range. ► On the windward (wind-facing) side of the mountains, air rises and cools, and large amounts of water vapor condense to form clouds and precipitation. ► On the leeward side of the mountains, cold, dry air descends, warms via adiabatic heating, and causes much drier conditions. ► The rain shadow of the Rocky Mountains helped to form the Great Plains. Example Rainshadow ► Warm, wet air (blue arrows) blow in off the coast and contain water vapor before they hit the Cascade Mountain Range (black line). This causes the air to rise, expand, cool and generate precipitation. The resulting dry air masses (red arrows) provide little rainfall east of the Cascades. ► Note that the Colville, Yakama Indian and Warm Springs Reservations are situated in the rainshadow → environmental externalities and equity. Atacama Desert ► Deserts are described as places receiving less than an average of 250 mm of rain in a year. The Atacama, however, receives less than an average of 1mm a year. ► The Atacama desert is the driest hot desert on Earth, largely the result of being located in the rain shadow of the Andes Mountains. Module Review: ► In this module, we have seen that the properties of air affect how it moves in the atmosphere and how much water vapor it contains. ► These properties help us understand how the intense sunlight at latitudes near the equator drives the formation of Hadley cells, which are responsible for also producing the polar cells and Ferrell cells. ► These air convection currents, combined with the Coriolis effect, cause the predominant wind directions that exist around the planet. ► Air convection currents and the effects of rain shadows help to determine the distribution of heat and precipitation around the globe.