National Weather Service United States Department of Commerce


Systematic view of the components of the earth’s global weather and climate system which involves mutual interactions between components of the atmosphere, hydrosphere, lithosphere and biosphere. Figure from IPCC WGI AR4 Chapter 1.


Climate is what you expect, weather is what you get.

                                                                                                          ....Mark Twain


Weather is defined as the state of the atmosphere at a given time and place, with respect to variables such as temperature, moisture, wind speed and direction, and barometric pressure.

Climate is defined as the expected frequency of specific states of the atmosphere, ocean, and land including variables such as temperature (land, ocean, and atmosphere), salinity (oceans), soil moisture (land), wind speed and direction (atmosphere), current strength and direction (oceans). Climate encompasses the weather over different periods of time and also relates to mutual interactions between the components of the earth system (e.g., atmospheric composition, volcanic eruptions, changes in the earth’s orbit around the sun, changes in the energy from the sun itself).

To illustrate the difference between weather and climate, consider forecasts: Weather forecasts are event, location, and day or even minute specific. Climate forecasts take advantage of the slow evolution of the coupled atmosphere/ocean/land/cryosphere (ice) system, and are usually expressed in probabilistic terms (e.g. probability of warmer or wetter than average conditions) for periods such as weeks, months, or seasons. Climate forecasts never predict specific weather events. Or consider Normals: Climate Normals provide estimates of the maximum and minimum temperatures typical of a given location primarily based on analysis of historical data while the actual temperatures observed on that day in any particular year are considered weather.

While the climate definition emphasizes the mutual interactions between components of the earth system, it is important to note that these interactions also occur in weather.