Meteorology: Understanding the Atmosphere            Ackerman and Knox

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US Snow Cover during El Nino

Changes in precipitation and temperature patterns caused by El Niņo affect snowfall in the United States (See Figure 8.15 in your text book). In the Southwest, there is a slight tendency toward cooler winters and a strong tendency toward wet winters, which makes higher-elevation snowpack deeper. In the Pacific Northwest, El Niņo winters are warmer and drier than usual, so that at a given elevation there is less precipitation and the freezing level is at a higher altitude. The type of precipitation is more likely to be rain, and the accumulation season is shorter. These factors produce a smaller snowpack accumulation by the end of winter in the Pacific Northwest. A significant reduction in total winter snowfall also occurs in the Midwest and New England regions during strong El Niņo events.

The applet below graphically demonstrates these impacts in two maps: The total snowfall in inches El Niņo winter minus all other winters. Negative values represent less snowfall in El Niņo winters. The ratio of snowfall during El Niņo winters to all other winters. Values less than 100% indicate regions where snowfall during El Niņo winters are less than normal.

Explain why, or when, it is useful to represent changes in snowfall by using both these maps. (Use the fader scrollbar to switch between maps.)