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How Global Air Movements Affect Weather and Climate
Summary: The atmosphere near the equator receives more direct sunlight than the atmosphere at higher latitudes. Therefore, near the equator tends to be warm. The warm air (low pressure) rises and moves away from the equator toward higher latitudes. This low pressure belt along the equator is called The Doldrums. At the same time, cooler air sinks (high pressure) at the higher latitudes and then moves toward the equator. The high pressure belts at 30º north and south latitudes are called The Horse Latitudes. This movement of air forms convection cells, or huge regions of circulating air. Similar convection cells occur at higher latitudes as well.
Prevailing Winds and Convection Cells Produce Global Climate Bands:
Global convection cells produce global belts of high and low pressure.
- The Doldrums: The Doldrums is a belt low pressure (rising air) that runs along the equator. As the air rises along the equator, it cools. This causes water vapor in the air to condense, forming clouds and increased precipitation. As a result, most areas near the equator are warm and wet. Most tropical rain forests are near the equator.
- The Horse Latitudes: The horse latitudes are belts of high pressure (sinking air) located at 30˚ north and south latitudes. Along these latitudes, warm, dry air sinks toward the ground. This sinking air produces belts of high pressure. Remember, that high-pressure systems are associated with dry weather. Most of the world’s deserts are located along these latitudes.
Prevailing winds are large-scale winds that blow in the same direction most of the time. Prevailing winds can significantly affect an area’s climate. There are three main groups of prevailing winds:
- Trade Winds: The Trade winds are prevailing winds that blow westward (from east to west) carrying warm, dryair from the equator toward 30º north and south latitude.
- Prevailing Westerlies: The Prevailing Westerlies blow eastward (from west to east) between 30˚ to 60˚. These winds carry warm air to the middle latitudes creating temperate climates. In the Northern Hemisphere, most of the United States lies within these latitudes.
- Polar Easterlies: The Polar Easterlies are prevailing winds that blow westward (from east to west) from the poles to 60˚ latitude. At 60˚ latitude, the Westerlies collide with the Polar Easterlies. The warm air is forced up over the cold air. Water vapor in the warm air condenses, producing precipitation. The poles are extremely dry and cold areas because the air sinks there.
Effect of Bodies of Water on Weather and Climate
Summary: Large bodies of water, such as oceans, can store a great deal of heat. This heat moves from the water to the cooler air above it. As a result, warm water to bring warm air to cold areas. In a similar way, cold water brings cooler air to warm areas.
- Land areas next to large bodies of water: Because water warms and cools so slowly relative to land, areas near large bodies of water tend to have more stable temperatures with cooler summers and warmer winters. In contrast, places located in the middle of continents are far away from the moderating effects of the sea. As a result, these places tend to have colder winters and hotter summers.
- Warm and cold ocean currents and their effect on climate: Currents of warm and cold water flow along the surface of the ocean. These currents affect the climates of areas near them. Example: The Gulf Stream is a warm ocean current that runs northward along the east coast of North America, across the Atlantic, and past the British Isles. The air above the Gulf Stream absorbs heat from the warm water. This makes the air warmer. Without the Gulf Stream to moderate land temperatures, Western Europe would be a much colder place. Much of the farming carried out in this part of the world would be far more difficult or impossible without the Gulf Stream’s presence. Another example is Iceland’s climate. Parts of Iceland are above the Arctic Circle, but because these areas are near the Gulf Stream, the climate there is warmer than other regions at the same latitude.
- Land and Sea Breezes:
Large bodies of water can affect the weather of an area as well as the area’s climate. During the day, bodies of water are cooler than nearby land because water heats up more slowly. The air above the water is therefore cooler than the air above the land.
Cool air is denser than warm air. Therefore, the air pressure over the water is higher. Air always moves from areas of high-pressure to areas of low-pressure. During the day, wind will blow from the water onto the land. This is called a sea breeze, ocean breeze, or a lake breeze. During the night, the air over the ocean is warmer than the air over the land. So at night, land breezes blow from the land toward the water.
4. Lake-Effect Snow: Lake-effect snow forms when cold, arctic air blows over the Great Lakes. The cold air closest to the lake surface becomes warm and moist. As the air rises, the water vapor in it condenses to form clouds. As the clouds move over the opposite shore, they release large amounts of snow. Lake-effect snow is a familiar phenomenon in Erie, Pennsylvania.