The root of all weather is the Sun, which heats the Earth. The heating is uneven, because of night and day, because different surfaces (such as rocks and trees) absorb and reflect sunlight in different amounts, and because sunlight hits the equator more directly than the poles. Uneven heat creates pressure differences, and wind flows between areas of high and low pressure.

Weather 101

  • High and Low Pressure
  • Weather Fronts
  • How Rain and Snow Form
  • The Jet Stream

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High and Low Pressure

Because the Earth is warmer at the equator than at the poles, major differences in pressure occur. Air moves north and south to try to equalize the pressure difference created by the temperature difference. The Earth rotates under this air, which deflects its direction (this is called the Coriolis Effect).

HIGH PRESSURE: When air cools, its molecules huddle closer together. The air becomes more dense (higher pressure) and it sinks. This is the nature of a high-pressure area.

The air pushes outward near the surface, seeking surrounding areas of lower air pressure. In the Northern Hemisphere, the Earth's rotation deflects these winds into a clockwise rotation.

In a high-pressure area, weather is generally fair and winds typically light.

LOW PRESSURE: When air warms, its molecules scatter. The air becomes lighter and rises.

The formation of low-pressure systems is more complicated, however, and involves a wavelike action that occurs between two areas of high pressure. The wave becomes stronger until it breaks and a low-pressure system is born, developing a rotation that is counterclockwise in the Northern Hemisphere.

In a low-pressure area, weather is generally cloudy and winds typically strong.


Weather Fronts

Fronts are the boundaries between areas of high atmospheric pressure and low atmospheric pressure that typically bring unsettled weather.

A COLD FRONT has cold air behind it that pushes abruptly underneath the warm air it encounters. As the warm air is forced upward, water vapor condenses into clouds and rain, which can sometimes be heavy along the front.

A cold front, noted on weather maps by this symbol, is often characterized by quick changes and steady precipitation, followed by showery precipitation.


A WARM FRONT has warm, moist air behind it that rises over the colder air it encounters. Rain may fall behind the front as the warm, rising air condenses.

A warm front, noted on weather maps by this symbol, generally has a long frontal boundary -- the area where changes occur.

STATIONARY FRONTS occur where warm and cold air meet but neither wins out. Unsettled weather can occur over a wide area near the frontal boundary.


How Rain and Snow Form

Not all raindrops are what you might think. Most of them, in fact, are never seen. Or, at least, they aren't seen until the very end of their life cycle.

{{adsense|premier|right}}Though a cloud might look like a giant cotton ball, it is actually made up of tiny ice crystals or water droplets that have condensed (turned from vapor to water) around even tinier bits of dust. Near the tops of clouds, even in summer, most of these little "raindrops" are ice rather than water, because it's so cold at the higher altitudes.

Clouds are often created when two different types of air masses run into each other -- a warm air mass and a cold air mass. Typically, the warm air gets pushed up over the cold air.

As warm air rises, condensation occurs; the air cools to a point where it will condense from the gas state into a water state. The rising air pulls the drop up, where it may freeze. All the while, more water is condensing onto it (or freezing onto it, a process called sublimation). So the drop gets bigger.

Finally, the updraft dies out and/or the drop is heavy enough to fall. When it falls, it may or may not turn from ice back into water. And it may get caught in another updraft and go through the whole cycle again. When this happens, the raindrop (or ice pellet) can get very large. This is how strong storms (where the air is going up and down rapidly and violently) create those huge raindrops or huge hailstones.

Eventually, the raindrop or hunk of ice is large enough that gravity overcomes whatever updrafts are in the system, and the raindrop, or whatever it has become, falls to Earth.

On the way down, it may melt or freeze, which determines what we finally call it when it hits the ground.


The Jet Stream

High-speed winds race around the globe between four and six miles above the earth, mostly from west to east. These rivers of air are often collectively referred to as the jet stream, and they form at the boundaries of warm and cold air.

Speeds average between 50 and 100 mph, but reach 250 mph. There are actually three major jet streams over North America in winter (and sometimes two), stretching from Canada to the subtropics. These separate bands of wind snake about, separating and combining at various times.

The course of the fast winds affect air masses, which in turn affect the course of the winds. Winter storms tend to track along the jet streams. A storm's energy, in the form of increased thunderstorm activity, alters the path of the polar jet stream, typically kicking it further north, where it can block Arctic air from moving into the East.