Weather vs. Climate

Excerpt from the introduction to The West without Water: What Past Floods, Droughts, and other Climatic Clues Tell Us About Tomorrow by Frances Malamud-Roam

The terms weather and climate are often used interchangeably. But there are important distinctions. Climate refers to the statistical description of weather over a given period of time and for a given region, including the weather extremes. Weather has been monitored extensively for more than a century in the West, so we have a good idea of the region’s climate for this period. For example, we know that Phoenix, Arizona, is arid, with hot summers and cool winters; San Francisco, California, has cool, foggy summers and wet, mild winters. Average high temperatures in Phoenix are much different in summer (104 F) than in winter (about  65 F), but its ten inches of rain annually are more or less evenly distributed between winter and summer storms. In contrast, San Francisco has average high temperatures that vary little throughout the year, staying mostly within the narrow range of 60-70 F. The twenty inches of rain received annually falls almost entirely between November and March, and the spring and summer months in San Francisco are often foggy.