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WIND
Wind is the horizontal movement of air in response to differences in pressure.
Winds are the means by which the atmosphere attempts to balance the uneven distribution of pressure over Earth’s surface. The movements of the wind also play a major role in correcting the imbalances in radiational heating and cooling that occur over Earth’s surface.
On average, locations below 38° latitude receive more radiant energy than they lose, whereas locations poleward of 38° lose more than they gain.
Our global wind system transports energy poleward to help maintain an energy balance. The global wind system also gives rise to ocean currents, which are another significant factor in equalizing the energy imbalance.
Thus, without winds and their associated ocean currents, the equatorial regions would get hotter and the polar regions colder over time. Besides serving a vital function in the evectional (horizontal) transport of heat energy, winds also transport water vapor from the air above bodies of water, where it has evaporated, to land surfaces, where it condenses and precipitates.
This allows greater precipitation over land surfaces than could otherwise occur. In addition, winds exert an influence on the rate of evaporation itself. Furthermore, as we become more aware of and concerned about the effect that the burning of fossil fuels has on our atmosphere, we look for alternate energy sources. Natural sources such as water, solar energy, and wind become increasingly attractive alternatives to fossil fuels. They are clean, abundant, and renewable.
Coriolis Effect
The Coriolis Effect and Wind Two factors, both related to our Earth’s rotation, greatly influence wind direction. First, our fixed-grid system of latitude and longitude is constantly rotating. Thus, our frame of reference for tracking the path of any free-moving object—whether it is an aircraft, a missile, or the wind—is constantly changing its position. Second, the speed of rotation of Earth’s surface increases as we move equatorward and decreases as we move toward the poles.
Because of these Earth rotation factors, anything moving horizontally appears to be deflected to the right of the direction in which it is traveling in the Northern Hemisphere and to the left in the Southern Hemisphere. This apparent deflection is termed the Coriolis Effect.
The degree of deflection, or curvature, is a function of the speed of the object in motion and the latitudinal location of the object. The higher the latitude, the greater will be the Coriolis Effect
Global Pressure Belts
These seasonal variations tend to be small at low latitudes, where there is little temperature variation, and large at high latitudes, where there is an increasing contrast in length of daylight and angle of the sun’s rays.
Furthermore, landmasses tend to alter the general pattern of seasonal variation.
This is an especially important factor in the Northern Hemisphere, where land accounts for 40% of the total Earth surface, as opposed to less than 20% in the Southern Hemisphere.
Because continents cool more quickly than the oceans, their temperatures will be lower in winter than those of the surrounding seas.
In the middle latitudes of the Northern Hemisphere this variation leads to the development of cells of high pressure over the land areas. In contrast, the subpolar lows develop over the oceans because they are comparatively warmer. Over eastern Asia, there is a strongly developed anticyclone during the winter months that is known as the Siberian High.
Trade Winds
A good place to begin our examination of winds and associated weather patterns as they actually occur is in the vicinity of the subtropical highs.
On Earth’s surface, the trade winds, which blow out of the subtropical highs toward the equatorial trough in both the Northern and Southern Hemispheres, can be identified between latitudes 5° and 25°. Because of the Coriolis Effect, the northern trades move away from the subtropical high in a clockwise direction out of the northeast.
In the Southern Hemisphere, the trades diverge out of the subtropical high toward the equatorial trough from the southeast, as their movement is counterclockwise. Because the trades tend to blow out of the east, they are also known as the tropical easterlies.
The trade winds tend to be constant, steady winds, consistent in their direction. This is most true when they cross the eastern sides of the oceans (near the eastern portion of the subtropical high).
The area of the trades varies somewhat during the solar year, moving north and south a few degrees of latitude with the sun. Near their source in the subtropical highs, the weather of the trades is clear and dry, but after crossing large expanses of ocean, the trades have a high potential for stormy weather. Early Spanish sea captains depended on the northeast trade winds to drive their galleons to destinations
The trade winds are one of the reasons that the Hawaiian Islands are so popular with tourists; the steady winds help keep temperatures pleasant, even though Hawaii is located south of the Tropic of Cancer.
Doldrums
The trade winds converge in the equatorial trough (or tropical low) lies a zone of calm and weak winds of no prevailing direction.
The air, which is very moist and heated by the sun, tends to expand and rise, maintaining the low pressure of the area.
These winds, which are roughly between 5°N and 5°S, are generally known as the doldrums.
This area is called the intertropical convergence zone (ITCZ), or the “equatorial belt of variable winds and calms.” Because of the converging moist air and a high potential for rainfall in the doldrums, this region coincides with the world’s latitudinal belt of heaviest precipitation and most persistent cloud cover.
Subtropical Highs
The areas of subtropical high pressure, generally located between latitudes 25° and 35°N and S, and from which winds blow poleward to become the westerlies and equatorward as the trade winds, are often called the subtropical belts of variable winds, or the “horse latitudes.”
The subtropical highs are areas, like the doldrums, in which there are no strong prevailing winds. However, unlike the doldrums, which are characterized by convergence, rising air, and heavy rainfall, the subtropical highs are areas of sinking and settling air from higher altitudes, which tend to build up the atmospheric pressure.
Weather conditions are typically clear, sunny, and rainless, especially over the eastern portions of the oceans where the high-pressure cells are strongest.
Westerlies
The winds that flow poleward out of the subtropical high-pressure cells in the Northern Hemisphere are deflected to the right and thus blow from the southwest.
Those in the Southern Hemisphere are deflected to the left and blow out of the northwest. Thus, these winds have been correctly labeled the westerlies.
They tend to be less consistent in direction than the trades, but they are usually stronger winds and may be associated with stormy weather.
The westerlies occur between about 35° and 65°N and S latitudes.
In the Southern Hemisphere, where there is less land than in the Northern Hemisphere to affect the development of winds, the westerlies attain their greatest consistency and strength.
Polar Winds
Accurate observations of pressure and wind are sparse in the two polar regions; therefore, we must rely on remotely sensed information (mainly by weather satellite imagery).
Pressures are consistently high throughout the year at the poles and that prevailing easterly winds blow from the Polar Regions to the subpolar low-pressure systems.
Polar Front Despite our limited knowledge of the wind systems of the Polar Regions,
The winds can be highly variable, blowing at times with great speed and intensity.
When the cold air flowing out of the Polar Regions and the warmer air moving in the path of the westerlies meet, they do so like two warring armies:
The denser, heavier cold air pushes the warm air upward, forcing it to rise rapidly.
The line along which these two great wind systems battle is appropriately known as the polar front. The weather that results from the meeting of the cold polar air and the warmer air from the subtropics can be very stormy. In fact, most of the storms that move slowly through the middle latitudes in the path of the prevailing westerlies are born at the polar front.
The Effects of Seasonal Migration
Insolation, temperature, and pressure systems migrate north and south as Earth revolves around the sun, Earth’s wind systems also migrate with the seasons.
In the Northern Hemisphere, maximum insolation is received north of the equator.
This condition causes the pressure belts to move north as well, and the wind belts of both hemispheres shift accordingly.
The various wind systems have migrated south in response to the migration of the pressure systems. Thus, seasonal variation in wind and pressure conditions is one important way in which actual atmospheric circulation differs from our idealized model.
The seasonal migration will most affect those regions near the boundary zone between two wind or pressure systems.
Two such zones in each hemisphere have a major effect on climate.
The first lies between latitudes 5° and 15°, where the wet equatorial low of the high-sun season (summer) alternates with the dry subtropical high and trade winds of the low-sun season (winter).
The second occurs between 30° and 40°, where the subtropical high dominates in summer but is replaced by the wetter westerlies in winter.
Longitudinal Differences in Winds
There are sizable latitudinal differences in pressure and winds.
There are significant longitudinal variations, especially in the zone of the subtropical highs.
The subtropical high-pressure cells, which are generally centered over the oceans, are much stronger on their eastern sides than on their western sides.
In the eastern portions of the oceans (west coasts of the continents) in the subtropics, subsidence, and divergence are especially noticeable.
The above-surface temperature inversions so typical of anticyclonic circulation are close to the surface, and the air is calm and clear. The air moving equatorward from this portion of the high produces the classic picture of the steady trade winds with clear, dry weather.
Ocean Currents
The planetary wind system, surface-ocean currents play a significant role in helping equalize the energy imbalance between the tropical and Polar Regions.
Surface-ocean currents greatly influence the climate of coastal locations.
Earth’s surface-wind system is the primary control of the major surface currents and drifts. Other controls are the Coriolis Effect and the size, shape, and depth of the sea or ocean basin.
Other currents may be caused by differences in density due to variations in temperature and salinity, tides, and wave action.
The major surface currents move in broad circulatory patterns, called gyres, around the subtropical highs. Because of the Coriolis Effect, the gyres flow clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere
The surface currents do not cross the equator.
Waters near the equator in both hemispheres are driven west by the tropical easterlies or the trade winds. The current thus produced is called the Equatorial Current.
At the western margin of the ocean, its warm tropical waters are deflected poleward along the coastline. As these warm waters move into higher latitudes.
Cyclones
Cyclones can be the most intense storms on Earth. A cyclone is a system of winds rotating counterclockwise in the Northern Hemisphere around a low-pressure center. The swirling air rises and cools, creating clouds and precipitation.
There are two types of cyclones:
- Middle latitude (mid-latitude) cyclones
- Tropical cyclones.
Mid-latitude cyclones are the main cause of winter storms in the middle latitudes.
Tropical cyclones are also known as hurricanes.
An anticyclone is the opposite of a cyclone. An anticyclone’s winds rotate clockwise in the Northern Hemisphere around a center of high pressure. Air comes in from above and sinks to the ground. High-pressure centers generally have fair weather.
Mid-Latitude Cyclones
Mid-latitude cyclones, sometimes called extratropical cyclones, form at the polar front when the temperature difference between two air masses is large.
These air masses blow past each other in opposite directions. Coriolis Effect deflects winds to the right in the Northern Hemisphere, causing the winds to strike the polar front at an angle. Warm and cold fronts form next to each other.
Most winter storms in the middle latitudes, including most of the United States and Europe, are caused by mid-latitude cyclones.
The warm air at the cold front rises and creates a low-pressure cell. Winds rush into the low pressure and create a rising column of air. The air twists, rotating counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Since the rising air is moist, rain or snowfalls.
Mid-latitude cyclones form in winter in the mid-latitudes and move eastward with the westerly winds. These two- to five-day storms can reach 1,000 to 2,500 km (625 to 1,600 miles) in diameter and produce winds up to 125 km (75 miles) per hour. Like tropical cyclones, they can cause extensive beach erosion and flooding. Mid-latitude cyclones are especially fierce in the mid-Atlantic and New England states where they are called nor’easters because they come from the northeast. About 30 nor’easters strike the region each year.
Hurricanes
Tropical cyclones have many names. They are called hurricanes in the North Atlantic and eastern Pacific oceans, typhoons in the western Pacific Ocean, tropical cyclones in the Indian Ocean, and will Willis in the waters near Australia.
They are the most damaging storms on Earth.
Hurricanes arise in the tropical latitudes (between 10 degrees and 25 degrees N) in summer and autumn when sea surface temperatures are 28 degrees C (82 degrees F) or higher.
The warm seas create a large humid air mass. The warm air rises and forms a low-pressure cell, known as a tropical depression. Thunderstorms materialize around the tropical depression.
If the temperature reaches or exceeds 28 degrees C (82 degrees F) the air begins to rotate around the low pressure (counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere).
