Weather is the state of the atmosphere at any given time and place, determined by such factors as temperature, precipitation, cloud cover, humidity, air pressure, and wind. The study of weather is known as meteorology. No exact date can be given for the beginnings of this science since humans have studied weather conditions for thousands of years. Weather conditions can be regarded as a result of the interaction of four basic physical elements: the Sun, Earth's atmosphere, Earth itself, and natural land-forms on Earth.
The driving force behind all meteorological changes taking place on Earth is solar energy. Only about 25 percent of the energy emitted from the Sun reaches Earth's surface directly. Another 25 percent reaches the surface only after being scattered by gases in the atmosphere. The remaining solar energy is either absorbed or reflected back into space by atmospheric gases and clouds.
Solar energy at Earth's surface is then reradiated to the atmosphere. This reradiated energy is likely to be absorbed by other gases in the atmosphere such as carbon dioxide and nitrous oxide. This absorption process—the greenhouse effect—is responsible for maintaining the planet's annual average temperature.
Humidity, clouds, and precipitation. The absorption of solar energy by Earth's surface and atmosphere is directly responsible for most of the major factors making up weather patterns. When water on the surface (in oceans, lakes, rivers, streams, and other bodies of water) is warmed, it tends to evaporate and move upward into the atmosphere. The amount of moisture found in the air at any one time and place is called the humidity.
When this moisture reaches cold levels of the atmosphere, it condenses into tiny water droplets or tiny ice crystals, which group together to form clouds. Since clouds tend to reflect sunlight back into space, an accumulation of cloud cover may cause heat to be lost from the atmosphere.
Humidity: The amount of water vapor contained in the air.
Meteorology: The study of Earth's atmosphere and the changes that take place within it.
Solar energy: Any form of electromagnetic radiation that is emitted by the Sun.
Topography: The detailed surface features of an area.
Clouds also are the breeding grounds for various types of precipitation. Water droplets or ice crystals in clouds combine with each other, eventually becoming large enough to overcome upward drafts in the air and falling to Earth as precipitation. The form of precipitation (rain, snow, sleet, hail, etc.) depends on the atmospheric conditions (temperature, winds) through which the water or ice falls.
Atmospheric pressure and winds. Solar energy also is directly responsible for the development of wind. When sunlight strikes Earth's surface, it heats varying locations (equatorial and polar regions) and varying topography (land and water) differently. Thus, some locations are heated more strongly than others. Warm places tend to heat the air above them, causing that air to rise upward into the upper atmosphere. The air above cooler regions tends to move downward from the upper atmosphere.
In regions where warm air moves upward, the atmospheric pressure tends to be low. Downward air movements bring about higher atmospheric pressures. Areas with different atmospheric pressures account for the movement of air or wind. Wind is simply the movement of air from a region of high pressure to one of lower pressure.
Earth's surface ranges from oceans to deserts to mountains to prairies to urbanized areas. The way solar energy is absorbed and reflected from each of these regions is different, accounting for variations in local weather patterns.
However, the tilt of Earth on its axis and it's varying distance from the Sun account for more significant weather variations. The fact that Earth's axis is tilted at an angle of 23.5 degrees to the plane of its orbit means that the planet is heated unevenly by the Sun. During the summer, sunlight strikes the Northern Hemisphere more directly than it does the Southern Hemisphere. In the winter, the situation is reversed.
At certain times of the year, Earth is closer to the Sun than at others. This variation means that the amount of solar energy reaching the outer atmosphere will vary from month to month depending on Earth's location in its path around the Sun.
Even Earth's rotation on its own axis influences weather patterns. If Earth did not rotate, air movements on the planet would probably be relatively simple. Air would move in a single overall equator-to-poles cycle. Earth's rotation, however, causes the deflection of these simple air movements, creating smaller regions of air movement that exist at different latitudes.
The terms weather and climate often are used in place of each other, but they refer to quite different phenomena. Weather refers to the day-today changes in atmospheric conditions. Climate refers to the average weather pattern for a region (or for the whole planet) over a much longer period of time (at least three decades according to some authorities).
[ See also Air masses and fronts ; Atmosphere, composition and structure ; Atmospheric circulation ; Atmospheric pressure ; Clouds ; Cyclone and anticyclone ; Drought ; El Niño ; Global climate ; Monsoon ; Thunderstorm ; Weather forecasting ; Wind ]