Weather

WEATHER IS THE physical condition or state of the atmosphere at any given time. It is what is happening in the atmosphere at any time or over any short period of time. If there were no atmosphere, there would be no weather. The principal elements of weather are temperature, pressure, winds, moisture, and precipitation. Thus, weather of any place is the sum total of its temperature, pressure, winds, moisture, and precipitation conditions for a short period of a day or a week. Temperature expresses intensity of heat. Unequal distribution of temperature over the Earths surface causes differences in atmospheric pressure, which causes winds. Moisture is present in the atmosphere as water vapor, often condensed into clouds. It maybe precipitated in the form of rain, hail, sleet, or snow. The capacity of air to gather and retain water vapor is largely dependent on its temperature. The higher the temperature, the greater the capacity of air to hold moisture. On cooling, the air is not able to retain all the moisture it gathers while warm. This leads to condensation and precipitation.

Weather is not synonymous with climate. Weather changes from day to day, whereas climate is something more stable, commonly defined as the average weather. Climate is the composite weather conditions over a considerable period of time. Weather conditions can change suddenly. Today may be warm and sunny, tomorrow may be cool and cloudy. Weather conditions include clouds, rain, snow, sleet, hail, fog, mist, sunshine, wind, temperature, and thunderstorms. Weather is driven by the heat stored in the Earth's atmosphere, which comes from solar energy. When heat is moved around the Earths surface and in the atmosphere because of differences in temperature between places, this makes winds. Winds form part of larger weather systems, the most powerful of which is the hurricane. Other weather features like the thunderstorm also develop because of the movement of heat in the atmosphere. Some thunderstorms result in tornadoes. The Earth's water cycle plays an important role in the development of many weather features such as dew, fog, clouds, and rain.

Weather can be described using terms such as wet or fine, warm or cold, windy or calm. The science of studying weather is called meteorology. Meteorologists measure temperature, rainfall, air pressure, humidity, sunshine, and cloudiness, and they make predictions and forecasts about what the weather will do in the future. This is important for giving people advance notice of severe weather such as floods and hurricanes. Temperature is measured with a mercury thermometer in degrees Celsius. Temperature is the hotness or coldness of an object. Rainfall [Page 1084]is usually measured by collecting what falls in rain gauges and is expressed as a depth of water that has fallen, in millimeters. Wind can be observed with a weather vane, but to measure its speed, more technical equipment is needed. Alternatively, the Beaufort scale can be used to make a judgment of the strength of the wind by observing how it affects objects outdoors, such as trees. The relative humidity and dew point temperature of the air can be determined by making measurements with a hygrometer and reading a table of numbers.

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