TV weather reports may reinforce this way of thinking by telling us how many degrees today's temperatures deviated from normal. But the fact is that the idea of normal or average weather is purely a mathematical construction. Tomorrow's weather is never likely to be the average of what it's been in history. In fact, what weather records really show us is that variation is normal in weather, not just from day to day, week to week or one year to the next. Weather patterns can last for years.
Some decades are hot and dry, and others are cool and wet.
Yet most of us plan as though weather were more likely than not to be normal. Granted, there are a few places, such as Phoenix, Arizona, where we can anticipate average conditions for most of the year. But there are many other regions, such as the huge expanse of the North American Great Plains, where it's a safe bet that tomorrow's weather won't match a historic norm. The misconception that weather is usually normal becomes a serious problem when you consider that 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.