Weather, Climate and Climatic Elements
Weather is the instantaneous state of the atmosphere, or the sequence of the states of the atmosphere as time passes. The difference between climate and weather is usefully summarized by the popular phrase “Climate is what you expect, weather is what you get.” Weather and climate are meteorological terms that are related but not interchangeable.
Climate is usually defined as the average weather plus the extremes for a given time period and a given location. More technically, for a given location and time period, the climate is the probability distribution of each variable that characterizes the local weather. Climate changes from one location to another, and in a given location, climate can change from one time period to another. Weather is the sum total of the atmospheric variables at a given place for a brief period of time; it is an everyday experience. Thus, we speak of today’s weather, or of last week’s. Climate, on the other hand, refers to a more enduring regime of the atmosphere.
According to The Meteorological Glossary, published by the UKMO, ‘climate’ is defined as ‘the synthesis of the day-to-day values of the meteorological elements that affect its locality.’ Synthesis here means more than simple averages, as the climate also involves extreme values, and the ranges within which phenomena occur, and the frequencies of weather types with associated values of elements.
Climatic data are usually expressed in terms of an individual calendar month or season and are determined over a period of about 30 years. This is long enough to ensure that representative values for the month or season are obtained, and freaks or abnormal values do not exert too strong an influence.
Elements of Climate and Weather
Although weather and climate are not identical, both are described by combinations of the same atmospheric variables, called the elements of weather and climate. Primarily, these elements are Pressure, Temperature, Precipitation, Humidity, Wind, and Cloudiness. The atmospheric pressure is of particular importance in determining the characteristics of the other variables.
It is atmospheric pressure that, to an important degree, determines the direction and speed of the wind, and it is the wind that in turn moves air masses of contrasting temperature and moisture from one locality to another. While air movement is predominantly in a horizontal direction, there is also some slight upward or downward movement. Where the motion is upward, cloud and precipitation are likely, while downward air movement, or subsidence, favors fair skies.