Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloudy. Most weather phenomena occur in the lowest level of the atmosphere, the troposphere, just below the stratosphere. Weather refers to day -to- day temperature and precipitation activity, whereas climate is the term for the averaging of atmospheric conditions over longer periods of time. When used without qualification, "weather" is generally understood to mean the weather of Earth.
Weather is driven by air pressure, temperature and moisture differences betweenone place and another. These differences can occur due to the sun 's angle at any particular spot, which varies with latitude. The strong temperature contrastbetween polar and tropical air gives rise to the largest scale atmospheric circulations: the Hadley Cell, the Ferrel Cell, the Polar Cell, and the jet stream. Weather systems in the mid-latitudes, such as extratropical cyclones, are caused by instabilities of the jet stream flow. Because the Earth 's axis is tilted relative to its orbital plane, sunlight is incident at different angles at different times of the year. On Earth 's surface, temperatures usually range ±40 °C (−40 °F to 100 °F) annually.
