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.
The distinctions between weather and climate often catch people off-guard, occasionally with tragic results. Mark Twain once remarked that “climate is what we expect, weather is what we get,” and the Donner Party is a famous and tragic example of “getting” weather. This group of pioneers was surprised by spectacularly abundant snow that came unusually early one year. The Illinois pioneers were headed for San Francisco in the fall of 1846 but were stopped short in the Sierra Nevada near the Nevada-California border on October 20 by a mountain range blanketed by the first snowfall of the year. Statistically, the first heavy snow storms should not have arrived so early, yet the possibility is always there. The group managed to make it as far as Truckee Lake (now called Donner Lake) where another heavy snowstorm–the second of ten major snowstorms that winter–shrouded the area, rendering these unfortunate pioneers snowbound. Another attempt to cross the summit was blocked in early November when yet another major storm increased the snowpack to ten feet. Now considered one of the most spectacular tragedies of the western migration, only 48 of the original 87 members survived, and they did so by resorting to cannibalism.
In modern parlance, the doom of the Donner Party was a result of extreme weather. Of course, during the mid-nineteenth century, just how variable and extreme the weather in the West could be was virtually unknown to pioneers entering the region. We know much more today–not just about the recent historical past but also about the longer-term climate history of the West.