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Tuesday, September 21, 2004

The Skies Look Good For Kerry?

A few weeks ago, I discussed the research of Princeton political Scientists Achen and Bartels, who found that voters often punish incumbents in November for bad weather from May to October. According to their data, bad weather cost Gore half a percentage point in 2000, compared to average weather, or 2.5 percentage points compared to perfect weather. The reasons for this are pretty mysterious: perhaps rain or drought puts the voters in a bad mood. My earlier posts speculate about the possible reasons why the weather should matter [post one, post two, older version of Achens & Bartels paper]

So, how's the weather been this year? I've updated Achens & Bartels' "weather index" using data from May through August of this year. It turns out that the weather's been a little worse than average, especially in swing states. The weather's been especially bad in Colorado, Nevada, and Arizona (drought) and also Ohio, Pennsylvania, West Virginia, and Wisconsin (rain and flooding). Florida has had good weather, according to the Achen & Bartels measure, but presumably the hurricanes have changed that.

The table below lists the swing states and their "weather index." Higher numbers mean more drought or more rain/flooding. A weather index higher than 1.8 is worse than average. The next column multiplies the weather index by the factor estimated by Achen & Bartels for the 2000 election. The factor translates the weather into the number of percentage points the weather costs the incumbent.

You can also see the National Climatic Data Center's discussion of the weather here, and a cool map here. The map shows areas with a long term moisture deficit or surplus, which is the measure Achens & Bartels used. Basically, white areas count as good weather, and anything colored counts as worse than average. An awful lot of the map is colored in, so things look pretty good for Kerry.

I also plotted the Bush's gains this year against the weather, to see if Achens & Bartels' measure could "predict out of sample." The vertical axis shows Bush's standing in the polls as of September 13, less his percentage in the same state in 2000. For example, Bush is polling 3 points ahead of his 2000 showing in Arizona, and about the same as 2000 in Florida. The horizontal axis shows the predicted points lost due to the weather (again, compared to perfect weather).

If you fit a line through the scatterplot, it's upward sloping and statistically significant. But that's the wrong slope! So far, Bush has been doing better (exceeding his 2000 results by more) in states with worse weather! Of course, the race could change a lot in the next six weeks and probably will, but so far the weather doesn't seem to be having the impact predicted by the political scientists.

Percentage Points Lost
Weather by Bush (compared
Index* to perfect weather)
------- ----------------------
Missouri 1.2 1.6
Arkansas 1.2 1.6
Florida 1.2 1.7
Maine 1.4 1.9
North Carolina 1.5 2.0
New Hampshire 1.6 2.1
Minnesota 1.6 2.3
20th Century Average 1.8 2.5
Iowa 1.9 2.6
Oregon 1.9 2.7
Washington 2.0 2.8
New Mexico 2.1 2.9
Tennessee 2.3 3.2
Michigan 2.4 3.3
Virginia 2.6 3.6
Wisconsin 3.0 4.1
Colorado 3.1 4.2
Nevada 3.2 4.4
Ohio 3.3 4.5
Pennsylvania 3.5 4.8
Arizona 3.8 5.3
West Virginia 4.2 5.8

Note: Weather Index calculated based on PHDI for May-August,
2004 as in Achen & Bartels (2004).

Source: Authors calculations, data from NOAA.

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