“Climate is what you expect. Weather is what you get.”

Weather is the condition of the atmosphere over a brief period of time. For example, we speak of today’s weather or the weather this week. Climate represents the composite of day-to-day weather over a longer period of time.

A climatologist attempts to discover and explain the impacts of climate so that society can plan its activities, design its buildings and infrastructure, and anticipate the effects of adverse conditions. Although climate is not weather, it is defined by the same terms, such as temperature, precipitation, wind, and solar radiation.

People in Minneapolis–St. Paul expect a white Christmas, and people in New Orleans expect very warm, humid summers. And a traveler going to Orlando, Florida, in March will not pack the same kind of clothing as a traveler going to Vail, Colorado, in March. These examples show how climate influences our daily lives. Additionally:

• Our houses are designed based on the climate where we live.
• Farmers make plans based on the length of the growing season from the last killing freeze in the spring to the first freeze in the fall.
• Utility companies base power supplies on what they expect to be the maximum need for heating in the winter and the maximum need for cooling in the summer.

Climate is usually defined by what is expected or “normal”, which climatologists traditionally interpret as the 30-year average for weather in an area.

By itself, “normal” can be misleading unless we also understand the concept of variability. For example, many people consider sunny, idyllic days “normal” in southern California. History and climatology tell us that this is not the full story. Although sunny weather is frequently associated with southern California, severe floods have had a significant impact there, including major floods in 1862 and 1868, shortly after California became a state. If you also consider severe droughts, most recently those of 1987–94, a more correct statement about sunny SoCal would be that precipitation in southern California is highly variable, and that rain is most likely between October and April.

Weather, in one form or another, is the source of water for irrigation, drinking, power supply, industry, wildlife habitat, and other uses. To ensure that our water supply, livelihoods, and lives are secure, it is essential that planners anticipate variation in weather, and that they recognize that drought and flood are both inevitable parts of the normal range of weather.