The angle the sunlight strikes the earth's surface varies, depending on:
Location: sunlight strikes at a more direct angle in areas closer to the equator as compared with polar regions.
Season: sunlight strikes a area of the earth's surface more directly during the summer.
Time of Day: sunlight will be most direct when the sun is directly overhead.
The more directly the sunlight strikes the ground, the more it heats the ground. As a result, areas near the equator are tropical, while polar regions are cold. It is hotter in the summertime than in the wintertime. And, it generally is coldest in the early morning and hottest in the afternoon. The sun is heating every portion of the earth unevenly, depending on these factors.
Air is heated at the surface where it contacts the earth. Since the surface is heated unevenly, the air is also heated unevenly. Add to this the fact that hot air rises, while cold air sinks, and circulation of the atmosphere results.
Because of uneven heating, air would tend to naturally rise above the equator, travel at altitude to and sink at the north and south poles, then travel along the surface back to the equator. This flow is interrupted, however, by coriolis force.
Coriolis Force
Fluids are not fixed in place to the earth's surface. As a result, all moving fluids are deflected to the right in the northern hemisphere as the earth rotates underneath. This deflection is called coriolis force. For example, if air moves south, the earth slides underneath as it rotates, which results in the air moving south westerly.
Coriolis force is the greatest at the poles and least near the equator. It is also associated with the speed of the moving fluid. The faster the fluid flows, the more coriolis force acts to turn it.
Friction between the wind and the ground often slows the wind near the surface. Because of this lower wind speed closer to the ground, coriolis force is reduced. This often produces a difference in wind direction near the surface versus air aloft.
Cells of the General Circulation
As a result of coriolis force, earth's general circulation is broken up into thirds in each hemisphere, three cells in the northern hemisphere and three cells in the south. In the northern hemisphere, the cell closest to the equator, has an easterly flow. This means the winds blow from the east to the west. These easterly winds are known as the northeasterly trade winds. They exist from the equator to 30 degrees north latitude. The United States resides under the center cell, which covers the 30 to 60 degrees north latitude range. In this cell, the prevailing westerly winds act to push weather systems from the west to the east.
Smaller Scale Circulation
Water is difficult to heat or cool. As a result, something containing a lot of moisture will heat and cool more slowly when compared to something with less moisture. For example, small updrafts of air are common over plowed farm fields during the daytime. This is because the plowed fields contain dry dirt and heat more easily in the sunlight. The vegetation in areas adjacent to these fields hold moisture, causing them to heat more slowly. As a result, the plowed fields heat the air above them. That air has a lowered density and rises. The same things happen over paved areas, such as parking lots.
Another example is an area of land adjacent to a body of water, Florida for example. During the daytime, the land mass of Florida heats more easily, becoming warmer than the surrounding waters. This sets up a circulation pattern in which air rises over the land and moves out to sea aloft. The cooler air over the water is sucked inland, resulting in a sea breeze.
At night, the land mass of Florida cools quickly, while the water remains relatively warm. The opposite circulation pattern is setup, resulting in a land breeze. The warmer air over the ocean rises and moves inland aloft, while the cooler air over the land moves out to sea at low altitude and along the surface.
Closer isobars on a chart indicate stronger winds. Isobars are lines of equal pressure.
Wind does not parallel the isobars exactly up to about 2,000 feet AGL due to surface friction.
Coriolis force is at a right angle to wind direction and directly proportional to wind speed.
The major source of all weather is the sun. Weather patters are caused by unequal heating of the earth's surface.
Winds flow clockwise around high pressure systems and counterclockwise around low pressure systems in the northern hemisphere.