What is Weather?

The amount of water vapor in the air. It is often confused with relative humidity or dew point. Relative humidity is a ratio of the amount of atmospheric moisture present relative to the amount that would be in the air if it were saturated. Dewpoint is the temperature to which air must be cooled to in order to reach saturation. Humidity is measured by a hygrometer.

Air that flows in relation to the earth's surface, generally horizontally. There are four areas of wind that are measured: direction, speed, character (gusts and squalls), and shifts. Surface winds are measured by wind vanes and anemometers, while upper level winds are detected through pilot balloons, rawin, or aircraft reports.

Any and all forms of water, liquid or solid, that falls from clouds and reaches the ground. This includes drizzle, freezing drizzle, freezing rain, hail, ice crystals, ice pellets, rain, snow, snow pellets, and snow grains. The amount of fall is usually expressed in inches of liquid water depth of the substance that has fallen at a given point over a specified time period.

The measure of molecular motion or the degree of heat of a substance. It is measured on an arbitrary scale from absolute zero, where the molecules theoretically stop moving. It is also the degree of hotness or coldness. In surface observations, it refers primarily to the free air or ambient temperature close to the surface of the earth.

Atmospheric pressure is defined as the force per unit area exerted against a surface by the weight of the air above that surface. Changes in pressure lead to the development of winds, which in turn influence our daily weather. A barometer is used to measure the atmospheric pressure.