WEATHER, WEATHER ALL AROUND Part 1

BIG IDEA 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.

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 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 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.

MEASURING PRESSURE Barometric pressure changes as low and high pressure systems move across the Earth's surface. It is this fact that makes barometers one of the most important instruments used to predict future weather conditions. Today, surface pressure measurements are used along with temperature, humidity, and wind observations to locate and forecast pressure systems. With a vast network of pressure stations, meteorologists are able to develop a complete picture of the location and movement of weather systems. Severe thunderstorms can even be identified and tracked, helping to assess the risk of tornado-producing thunderstorms.

Humidity Humidity is the amount of water vapor in the air and can be described in different ways. The term that you'll hear most often to describe the humidity is relative humidity. Relative humidity is the amount of water in the air compared to the amount that the air can hold at that temperature. Relative humidity is generally expressed as a percentage.

Another common measurement of humidity is the dew point, which is the temperature that the air would have to be cooled to in order for the water vapor in the air to condense back into a liquid. The higher the dew point, the more uncomfortable and humid the air feels to humans.

Unless you are a weather forecaster, or a scientist or engineer involved in work involving the amount of water vapor in the air, you can easily go through life without understanding relative humidity. On the other hand, it is nice to know what a TV meteorologist means by dew point.