We have seen that heat is transferred vertically from the earth to the air by convection. But wind is defined as the horizontal movement of air relative to the earth's surface. What causes this movement? Differences in temperature. Air temperature varies because the earth's surface heats up at different rates. Latitude and season, as noted before, cause temperature variations. Large bodies of water warm and cool at a slower rate than large bodies of land, creating a disparity in the atmosphere above. Because heat decreases with altitude, mountain peaks are cooler than cities at sea level. These are a few of the causes of differing temperatures.
Differences of temperature cause differences in pressure. A difference in pressure across distances is called a pressure gradient, and this drives the wind. Here are two examples of air flow caused by pressure gradient. When a can of coffee is "vacuum packed," air is removed from the can. When the can is opened you can hear air rush in from the outside higher pressure. Similarly, when you blow up a balloon you create a high pressure area because you compress the air and increase its density within the balloon. When the balloon is punctured the air rushes outward to the lower pressure. In both examples air moves from higher to lower pressure, and the greater the difference the faster the air travels.
Once the air has begun to move (surplus heat to the poles and surplus cold to the equator) another force comes into play. This is called the Coriolis force, and is caused by the rotation of the earth. Imagine yourself in a fixed position in space, looking down at the earth. You would observe that the wind moving from the equator to the north pole was traveling in a straight line, with the earth's rotating surface moving beneath it. Now place yourself at a location on the earth's surface and observe the wind again. The wind would appear to be curving to the right. Why?
The earth rotates on its axis at the rate of 1041 miles per hour at the equator. The speed decreases with increasing latitude until it is virtually zero at the poles. This is because the latitude circles grow smaller. Place an object on the equator and allow 24 hours to go by. When the object returns, it will have traveled more than 24,000 miles - in other words, to travel that distance in 24 hours its linear speed was 1041 mph. Now place the object at 60 degrees north and let it make its circle. In 24 hours it will travel about 12,500 miles at 521 mph. At the north pole the linear speed would be zero because there would be no distance traveled.
As an object (such as a piece of wind, or a rocket) starts to move in a straight path from the equator to the north pole, its eastward speed (the earth rotates from west to east) will be 1041 mph. As it travels northward, its eastward movement will be faster than the eastward movement of the surface of the earth at higher latitudes. It will run ahead of any object at higher latitudes, and appear to an earth based observer to be curving to the right. Similarly, if the object traveled from the north pole to the equator it would have no eastward movement, and would fall behind a lower latitude object whose eastward movement would be faster. To an earth based observer the curve would again appear to be to the right of the direction of motion.
Why is it, then, that in the southern hemisphere this apparent motion is reversed - that is, the Coriolis deflection is to the left? Imagine yourself once again in space. This time you are hovering just above the north pole. When you look down at the rotation of the earth you see it moving counterclockwise. Now relocate yourself to just above the south pole. When you glance down, the earth is rotating clockwise. This explains why the apparent curve is to the right in the northern hemisphere and to the left in the southern. In fact, as we continue to study wind motion, we'll see that each hemisphere is a mirror image of the other. Now one more imaginary placement of yourself. If you straddled the equator you would see neither clockwise or counterclockwise movement. Because of this, the Coriolis force is not in effect at the equator.
General Wind Patterns
As mentioned, local wind patterns are the result of pressure differences in the immediate area: land, sea, mountain, etc. But there are global patterns that we can observe as well. Let's start by following movement in the northern hemisphere. Hot air rises from the equator, creates a low pressure area, and flows towards the north pole. The upper wind flow is deflected to the right by the Coriolis effect, which causes it to pile up and move from west to east. The piled up air cools, creating a high pressure area, and sinks; and as it accumulates on the surface it flows towards both the equator and north pole. The air moving toward the equator is influenced by the Coriolis effect and moves from the northeast, and because of its direction is called the northeast trade winds. (Wind is classified according to the direction from which it is blowing.) The poleward moving air also moves to the right and is called the prevailing westerlies. The third wind belt develops as cold polar air sinks and moves south, is deflected to the right, and is therefore called the polar easterlies. The same air pattern occurs in the latitudes of the southern hemisphere, except that the deflection of the wind is to the left rather than right. (In the southern hemisphere the trades are called the southeast trade winds.)
Roughly speaking, trade winds occupy the area between 0 (the equator) and 30 degrees latitude; prevailing westerlies the area between 30 and 60 degrees; and polar easterlies the region between 60 and 90 degrees (the pole). The zones that separate these three major wind belts are also identified. Near the equator is a region called the doldrums, literally meaning "stagnation or listlessness". The area was given this name by 16th century English merchant sailors, who found themselves trapped by lack of a "trade" wind to carry them onward to the next port of call. A more explanatory, if less colorful, name is the intertropical convergence, for it is here that the trade winds of both hemispheres meet. It is known for its extremely low pressure, frequent thunderstorms, and very calm wind. At about 30 degrees is a high pressure area where the trades and westerlies diverge and go toward the equator and pole, respectively. Like the doldrums, it is an area with little wind; unlike the doldrums, there are no cloud formations, just blue skies and warm temperatures. The sailors gave this region a graphic name, the horse latitudes. The origin of the name varies: some say that horses being transported from the old to new world (from Europe to America) grew extremely restless under the dry, still skies, panicked, and had to be pushed overboard; others say it was the sailors who grew terrified of being stranded forever in still seas and jettisoned their cargo in hopes of movement with a lighter load. In any event, ships tried to avoid this zone and its lack of wind. The third zone lies at about 60 degrees latitude, and is called the polar front. Its location varies with the seasons, since the polar front moves south in winter and north in the summer. The cold polar easterlies meet the warm prevailing westerlies in this zone, and because of the extreme differences in pressure, dramatic weather conditions occur.
With the development of high flying aircraft, an additional wind pattern was discovered. The jet stream is a band of fast moving, high altitude air. Friction from the earth's surface slows down air movement, but at higher altitudes friction has no effect and air travels faster. Another factor contributing to jet streams is pressure gradient. Recall that pressure gradient is the difference between areas of pressure, and the greater the difference the faster the wind speed. Above the polar front, where cold polar easterlies meet warm prevailing westerlies, the polar jet stream exists. This band of turbulent air moves about erratically, changing in width from 25 to 100 miles. Wind speed, which is generally westerly, can exceed 300 mph. Pilots traveling west to east try to hitch a ride on the polar jet to reduce their flight time, but aircraft moving east to west would be delayed by encountering the jet stream. There are other jet streams, such as the one around 30 degrees, but because of the greater and constant pressure gradient the polar jet is the best known.
To sum up at this point: Convected energy from the earth's surface rises upward and becomes wind as it begins to move horizontally. This movement is caused primarily by pressure gradient force, as high pressure air moves toward low pressure areas. Curvature of the wind is caused by the rotation of the earth and is called the Coriolis effect. In the area from the equator to 30 degrees a large circular overturning of air occurs as warm low pressure air rises, moves poleward, cools, and returns to the surface. The cool surface air that moves back towards the equator and completes the circle is called the tradewinds (northeast and southeast, depending upon the hemisphere). The surface wind that flows toward the pole is called the prevailing westerlies. Movement from the high pressure areas around the pole is called the polar easterlies. While the air between the equator and 30 degrees moves in a circular motion, air between 30 and 60 degrees moves in gigantic waves, rising in one limb as warm tropical air moves poleward and sinking as cold polar air moves equatorward. The three wind belts are separated by zones near 0, 30 and 60 degrees: the doldrums, horse latitudes, and polar front. The polar jet is the high speed, erratic high altitude wind that is the most pronounced of the jet streams.
In the northern hemisphere, air flows around a low pressure area in a counterclockwise direction. At the earth's surface, the air spirals toward the center and converges. This convergence and rotation around a low pressure area is called a cyclone. The circulation forces the air upward, and these rising convection currents carry heat and moisture which results in cooling, clouds, and precipitation. In contrast, high pressure areas in the northern hemisphere circulate wind in an outward, clockwise direction. The air moving down to replace the diverging air results in drier air, fewer clouds, and clear crisp weather. Because it is the opposite of a cyclone, this movement of air around a high pressure area is called an anticyclone.
We mentioned earlier the weather disturbances that are created near the polar front. Let's continue that discussion. An air mass is a large volume of air that has the same general character, such as temperature and moisture. It usually forms over a large body of land or sea with uniform characteristics. A front is the area where two air masses collide. As cold air pushes down from the pole during the winter, it dominates the warm tropical air. The warm air retreats as the polar front moves southward. An anticyclone can never form around a front because its nature is to push air away; but a front provides an ideal place for a cyclone to arise and sweep over thousands of square miles while intensifying its low pressure and speed. Most cyclonic lows occur between 30 and 60 degrees, and are therefore called mid-latitude lows.
A mid-latitude cyclone and a tropical cyclone have only two things in common: They each have a low pressure area, and they each create bad weather. A mid- latitude cyclone can occur at any time of the year; a tropical cyclone occurs most often in the summer. A mid-latitude cyclone moves in a westerly direction and takes an elliptical shape; a tropical cyclone moves from east to west and is circular in shape. A mid-latitude cyclone is accompanied by an anticyclone and frontal boundaries; a tropical cyclone has neither.
A tropical cyclone originates off the coast of large continents and over very warm water. The rising and release of water vapor as latent heat provides the energy that intensifies the storm. Interestingly, tropical cyclones do not originate over the doldrums because there is no Coriolis force to help develop sufficient rotary motion. A distinguishing feature of a tropical cyclone is its eye, a calm in the center of the storm that is the result of subsiding warm air; this calmness is in sharp contrast to the eye wall, an upward whirl of thunderstorms that sometimes reach into the stratosphere. A tropical cyclone has different names in different parts of the world, such as typhoon and willy-willy. In the United States, when a tropical storm establishes a closed wind circulation that reaches 72 mph, it is called a hurricane. Hurricane Camille, which caused massive destruction in 1969, had wind speeds over 200 mph.
A tornado is the most devastating of cyclonic winds. Tornadoes usually form near fast-moving cold fronts, and/or from within severe thunderstorms. The clash of air masses with differing layers of temperature, moisture, wind flow and density produces unstable, turbulent air movement. From within a thundercloud a funnel suddenly appears and bobs up and down as it moves toward the earth's surface. Whatever it touches on the ground is instantly destroyed: The intensely spiraling air and extreme pressure gradient cause buildings to literally explode as the air within them seeks to rush outward. Most tornadoes occur in the midwest and southeast United States in the spring and early summer as cold dry air from the pole meets warm moist air from the tropics.
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