US Climate Extremes Index & Drought

By Paul Homewood

One of the indicators we can use to measure drought in the US is the US Climate Extremes Index, which is NOAA’s official method of assessing climate extremes.

One of the indicators used in the Index is the “Days without precipitation”, defined as :

The sum of (a) percentage of the United States with a much greater than normal number of days with precipitation and (b) percentage of the United States with a much greater than normal number of days without precipitation.

(In each case, we define much above (below) normal or extreme conditions as those falling in the upper (lower) tenth percentile of the local, period of record)

The Index currently shows this:

http://www.ncdc.noaa.gov/extremes/cei/graph/5/01-12

As the index looks at above and below normal separately, it avoids the problem of ups and downs averaging themselves out.

It can be seen clearly that extremely dry periods were much more widespread up to about 1960, and that the situation has been reasonably stable since then.

The Index is also available by the nine Climate Regions listed below.

In none of the regions is there any evidence that dry days are becoming more common, and in most regions the trend is declining.