This is how the jet stream forms. This current of air snakes around the planet at speeds greater than kilometers miles per hour. The more rapidly it moves, the more the northern jet stream curves east.
But as the air moves poleward, it never actually gets to the poles. As a result, the jet stream meanders as it circles the Earth in each hemisphere. In the North, it moves air west to east in a circle around the mid-latitudes and the opposite in the Southern Hemisphere , changing its path from season to season. Poleward of the jet stream, the atmosphere is turbulent.
Along this temperature boundary, a fierce atmospheric battleground develops. Colliding air masses of different temperatures spin up cyclones and other severe weather. The position of the jet stream influences the type of weather a region encounters. Consider the Northern Hemisphere, for instance. This allows an extensive dome of super-cold air to bank up nearby. Atmospheric scientists refer to this flowing pool of cold air and low pressure as the polar vortex. It swells in size during winter.
And when this flow of cold air surges southward, it pushes the jet stream into southern Canada and the northern United States. That can bring seemingly endless snowstorms to the upper Midwest and Northeast during the dead of winter. In summer, the poles warm. This weakens the temperature gradient between these zones and the equator.
The jet stream responds by retreating some 1, kilometers a thousand miles northward. Now, the weather in the lower 48 U. Sure, scattered thunderstorms erupt from time to time. But there are no huge storm systems spanning 1, kilometers or more to influence day-to-day events. Ordinarily, air would flow from high pressure to low pressure. It would move across a pressure gradient. So the driving force would be known as the pressure gradient force.
Because warm air rises, it leaves behind an area of low pressure behind it. Gases move from high-pressure areas to low-pressure areas. And the bigger the difference between the pressures, the faster the air will move from the high to the low pressure. That rush of air is the wind we experience.
You might be wondering why the air would move from high pressure to low pressure in the first place. This is something that happens in nature all the time: things always try to even out.
Even people do it! The tip of South America and Australia, as well as the islands of New Zealand, are the only large landmasses to penetrate the Roaring Forties. The westerlies of the Roaring Forties were very important to sailors during the Age of Exploration , when explorers and traders from Europe and western Asia used the strong winds to reach the spice markets of Southeast Asia and Australia.
Westerlies have an enormous impact on ocean current s, especially in the Southern Hemisphere. Driven by westerlies, the powerful Antarctic Circumpolar Current ACC rushes around the continent from west to east at about 4 kilometers per hour 2. The ACC is the largest ocean current in the world, and is responsible for transporting enormous volumes of cold, nutrient-rich water to the ocean, creating healthy marine ecosystems and food webs.
Horse Latitudes The horse latitude s are a narrow zone of warm, dry climate s between westerlies and the trade winds. Horse latitudes are about 30 and 35 degrees north and south. Many deserts, from the rainless Atacama of South America to the arid Kalahari of Africa, are part of the horse latitudes. The prevailing winds at the horse latitudes vary, but are usually light. Even strong winds are often short in duration. Trade Winds Trade wind s are the powerful prevailing winds that blow from the east across the tropics.
Trade winds are generally very predictable. They have been instrumental in the history of exploration, communication, and trade. Ships relied on trade winds to establish quick, reliable route s across the vast Atlantic and, later, Pacific Oceans. Even today, shipping depends on trade winds and the ocean currents they drive.
In , Norwegian explorer Thor Hyerdahl and a small crew used trade winds to travel from the coast of Peru to the coral reefs of French Polynesia, more than 6, kilometers 4, miles , in a sail-powered raft. The expedition, named after the raft Kon-Tiki aimed to prove that ancient mariners could have used predictable trade winds to explore wide stretches of the Pacific. Trade winds that form over land called continental trade winds are warmer and drier than those that form over the ocean maritime trade winds.
The relationship between continental and maritime trade winds can be violent. Most tropical storm s, including hurricanes, cyclones, and typhoons, develop as trade winds. Differences in air pressure over the ocean cause these storms to develop. As the dense, moist winds of the storm encounter the drier winds of the coast, the storm can increase in intensity.
Strong trade winds are associated with a lack of precipitation , while weak trade winds carry rainfall far inland. The most famous rain pattern in the world, the Southeast Asian monsoon, is a seasonal, moisture-laden trade wind. Besides ships and rainfall, trade winds can also carry particles of dust and sand for thousands of kilometers. Particles from Saharan sand and dust storms can blow across islands in the Caribbean Sea and the U.
Dust storms in the tropics can be devastating for the local community. Valuable topsoil is blown away and visibility can drop to almost zero. Across the ocean, dust makes the sky hazy.
These dust storms are often associated with dry, low-pressure areas and a lack of tropical storms. Doldrums The place where trade winds of the two hemispheres meet is called the intertropical convergence zone ITCZ. The area around the ITCZ is called the doldrums. Prevailing winds in the doldrums are very weak, and the weather is unusually calm. In fact, the low-pressure doldrums are created as the sun heats the equatorial region and causes air masses to rise and travel north and south.
This warm, low-pressure equatorial wind descends again around the horse latitudes. Some equatorial air masses return to the doldrums as trade winds, while others circulate in the other direction as westerlies. Although monsoons impact tropical as well as equatorial regions, the wind itself is created as the ITCZ moves slightly away from the Equator each season. This change in the doldrums disturbs the usual air pressure, creating the moisture-laden Southeast Asian monsoon.
Results of Wind Wind traveling at different speeds, different altitudes, and over water or land can cause different types of patterns and storms. Jet Streams Jet stream s are geostrophic winds that form near the boundaries of air masses with different temperatures and humidity. The rotation of the Earth and its uneven heating by the sun also contribute to the formation of high-altitude jet streams. These strong, fast winds in the upper atmosphere can blow kph mph. There is little turbulence in the stratosphere, which is why commercial airline pilots like to fly in this layer.
Riding with jet streams saves time and fuel. Have you ever heard someone talk about a headwind or tailwind when they are talking about airplanes? These are jet streams. If they are behind the plane, pushing it forward, they are called tailwind s. They can help you get to your destination more quickly.
If the winds are in front of the plane, pushing it back, they are called headwind s. Strong headwinds can cause flight delays. Hurricane A hurricane is a giant, spiraling tropical storm that can pack wind speeds of over kph mph and unleash more than 9 trillion liters 2.
These same tropical storms are known as hurricanes in the Atlantic Ocean, cyclone s in the northern Indian Ocean, and typhoon s in the western Pacific Ocean. These tropical storms have a spiral shape. The spiral swirling counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere develops as a high-pressure area twists around a low-pressure area.
Wind conditions that can lead to hurricanes are called tropical disturbances. They begin in warm ocean waters when the surface temperatures are at least If the disturbance lasts for more than 24 hours and gets to speeds of 61 kph 38 mph , it becomes known as a tropical depression. When a tropical depression speeds up to kph mph , it is known as a tropical storm, and is given a name.
Meteorologist s name the storms in alphabetical order, and alternate with female and male names. When a storm reaches kph 74 mph , it becomes a hurricane and is rated from 1 to 5 in severity on the Saffir Simpson scale. A Category 5 hurricane is the strongest storm possible on the Saffir-Simpson scale. Winds of a Category 5 blow at kph mph. Hurricane Ethel, the strongest hurricane in recorded history, roared across the Gulf of Mexico in September Winds were sustained at kph mph.
However, Hurricane Ethel quickly dissipate d. Although its winds ultimately blew as far north as the U. Hurricanes bring destruction to coastal ecosystems and communities. When a hurricane reaches land, it often produces waves that can reach 6 meters 20 feet high and be pushed by high winds kilometers miles inland.
These storm surge s are extremely dangerous and cause 90 percent of all hurricane deaths. The deadliest hurricane on record is the Great Hurricane of Although sophisticated meteorological equipment was not available at that time, winds may have reached kph mph as the hurricane hit Barbados and other islands in the Caribbean Sea.
This may have been enough to strip the bark from trees. More than 20, people died as a result of the hurricane as it made its way across Barbados, St. Although it decreased in intensity, the hurricane was tracked through the U. Hurricanes can be destructive in other ways. High winds can create tornadoes.
Heavy rains contribute to floods and landslides, which may occur many kilometers inland. Damage to homes, businesses, schools, hospitals, roads, and transportation systems can devastate communities and entire regions. Hurricane Katrina, which blew through the Gulf of Mexico and into the southern U. New Orleans, Louisiana, was almost completely devastated by Hurricane Katrina.
New Orleans, as well as Mobile, Alabama, and Gulfport, Mississippi, took years to recover from the damage done to their structures and infrastructure. The best defense against a hurricane is an accurate forecast that gives people time to get out of its way. The National Hurricane Center issues hurricane watches for storms that may endanger communities, and hurricane warnings for storms that will reach land within 24 hours. Cyclones Cyclones blow through the Indian Ocean in the same way hurricanes blow across the Atlantic.
Cyclones blow in with air masses from the east, often the South China Sea, or the south. The most powerful and devastating cyclone in recorded history was the Bhola Cyclone. Its winds were about kph mph as it made landfall along the coast of the Bay of Bengal, in what is today Bangladesh. More than , people died, and more than a million were made homeless. Cyclone winds devastated fishing villages, and storm surges drowned crops.
Typhoon Typhoons are tropical storms that develop over the northwest Pacific Ocean. The doldrums is an area of calm weather. The trade winds coming from the south and the north meet near the equator. These converging trade winds produce general upward winds as they are heated, so there are no steady surface winds. What are the prevailing westerlies? Between thirty and sixty degrees latitude, the winds that move toward the poles appear to curve to the east.
Because winds are named from the direction in which they originate, these winds are called prevailing westerlies. Prevailing westerlies in the Northern Hemisphere are responsible for many of the weather movements across the United States and Canada. What are the polar easterlies? At about sixty degrees latitude in both hemispheres, the prevailing westerlies join with the polar easterlies to reduce upward motion.
The polar easterlies form when the atmosphere over the poles cools. This cool air then sinks and spreads over the surface. As the air flows away from the poles, it is turned to the west by the Coriolis effect. Again, because these winds begin in the east, they are called easterlies. What is a sea breeze? On a warm summer day along the coast, this differential heating of land and sea leads to the development of local winds called sea breezes.
As air above the land surface is heated by radiation from the Sun, it expands and begins to rise, being lighter than the surrounding air. To replace the rising air, cooler air is drawn in from above the surface of the sea. This is the sea breeze, and can offer a pleasant cooling influence on hot summer afternoons.
What is a land breeze? A land breeze occurs at night when the land cools faster than the sea. In this case, it is air above the warmer surface water that is heated and rises, pulling in air from the cooler land surface. How is wind helpful to Earth? Wind is the fastest growing source of electricity in the world. It's often one of the least expensive forms of renewable power available.
Some experts say it can sometimes be the cheapest form of any kind of power. Generating power from the wind leaves no dangerous waste products behind. Best of all, its supply is unlimited. How do windmills work?
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