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.