## NFL Home Field Advantage by Climate

In this post I'll begin an analysis of home field advantage in the NFL and its relationship to climate. Others have examined the connection between weather and HFA previously, but here I'll attempt to present the data with a clear and novel approach. This post represents the 'clear' part. In following posts, the novel part will use logistic regression to account for team strength and determine the significance of each particular type of climate match-up.

I began by dividing each home city into four categories: dome, cold, moderate, and warm based on a combination of each city's average December high temperature and wind speed. The dome cities include STL, NO, MIN, DET, IND, and ATL. The cold cities are GB, BUF, CLE, CHI, KC, DEN, NYG, NYJ, NE, PHI, PIT, and CIN. Moderate cities include BAL, WAS, CAR, SEA, OAK, SF, TEN, and DAL. The warm cities are MIA, TB, JAX, HOU, SD, and ARI.

Based on all NFL regular season games from the 2002 through 2006 seasons, the winning percentage of the home team was calculated for each type of climate match-up. I divided the season into "early" and "late." The early season is defined as weeks 1 through 12 and the late season is defined as weeks 13 through 17. For example, the winning percentage of the home team in match-ups of cold teams at moderate cities in the early season is 57%, with n=77 examples of such cases.

The home winning percentage of all types of weather match-ups are presented below in a series of pairs of tables. Each table is presented the same way, with the visiting team climate on the left and the home climate on the top. The first table in each pair is for the early season (pre-December), and the second table is for the late season (December games).

Sample size is usually an issue when populations are divided up among several classes. For that reason, the first pair of tables lists the number of cases of each type. For example, the top right cell of the first table lists the number of games featuring dome teams playing at cold cities in the early season. The same cell in the second table lists the same type of match-up in the late season.

 Wk 1-12 Dome Warm Mod Cold Dome 31 29 44 62 Warm 34 20 50 62 Mod 40 49 51 78 Cold 59 66 77 128

 Wk 13-17 Dome Warm Mod Cold Dome 15 17 21 21 Warm 16 11 17 30 Mod 25 20 17 40 Cold 20 28 43 59

The second pair of tables simply lists the straight-up winning percentage of the home team in each type of match-up. For example, the bottom left cell of the first table lists the home team winning percentage when cold teams play at domes in the early season. The same cell in the second table lists home winning percentage of cold teams at domes in the late season.

 Wk 1-12 Dome Warm Mod Cold Dome 61% 45 61 51 Warm 68 50 66 66 Mod 60 59 59 76 Cold 53 52 57 50

 Wk 13-17 Dome Warm Mod Cold Dome 61% 65 52 86 Warm 63 64 59 63 Mod 52 35 53 63 Cold 45 54 42 58

What immediately stands out is the very high winning percentage of cold teams hosting dome teams late in the season. The most remarkable result, however, may be the 35% home winning percentage of warm teams hosting moderate teams. Note that there are only about 20 cases of each type of match-up in the past 5 years, so these results could be due to luck or due to general team strengths of the according type of teams. Perhaps a couple moderate teams have been relatively dominant over warm weather division rivals between '02 and '06.

The final table lists the difference in home winning percentage between late season and early season match-ups. Simply put, it is late season winning percentage minus early season winning percentage. A high positive number indicates cold weather may give an advantage to the home team. A negative number or near-zero number suggests otherwise. We'd expect to see a zero for dome teams at dome teams, because outdoor weather is obviously not a factor. This method begins to account for relative team strengths over the period studied.

 Difference Dome Warm Mod Cold Dome 0% 20 -9 35 Warm -5 14 -7 -3 Mod -8 -24 -6 -13 Cold -8 2 -15 8

Take dome teams for example. By reading across, we see that dome teams seem to have no greater HFA in late season than the early season against other dome teams--as we'd expect. We also see that they are at a 20% disadvantage playing at warm cities but, for some reason, have a 9% better advantage playing at moderate cities late rather than early. Lastly, we see that dome teams appear to be at a severe disadvantage playing in cold cities late in the season, apparently giving up 35% advantage to the cold.

As mentioned above, some of the observed differences in HFA due to weather may be due to luck and relative team strengths among the weather-classes. The final table is a simple way of accounting for team strength, but it does not address the possibility that the differences are primarily due to luck. The final part of this article will use logistic regression to more powerfully account for team strength and test for statistical significance.

### 6 Responses to “NFL Home Field Advantage by Climate”

1. Anonymous says:

Brian, I had written something last year looking at distance, climate, and home field advantage in divisional games.

http://www.pro-football-reference.com/blog/wordpress/?p=157

Now, I didn't look at when the games were played (assuming they were randomly distributed), and just reported the home team winning percentages in series, without separating it out to whether the dome team was at home or the cold team was at home.

One thing about your groupings is that there is more diversity within the group than between the teams on the edge of the group. Seattle is just on the edge of the cold teams, and a lot closer to most of those teams than to Dallas or Carolina. In fact, if I recall, Seattle is actually the coolest (or about even with Green Bay) during the first two months of the season. So, I am not sure they really belong in the same grouping.

2. Brian Burke says:

JKL-Thanks. Great stuff as always. I'll add your article to the ones referenced above.

Good point on Seattle. But based on the Late Season Bias paper I linked to, it's a good bet that weather doesn't really factor into things until it gets really cold. Although it's relatively cool year-round, Seattle rarely gets bitter cold. My theory is that it's the near-freezing temps that really affect the dome teams, not a 50 degree day in Sept or Oct.

My real problem is that my database goes back only 5 years. Using 4 climate categories and 2 "weather seasons" divides up the data pretty finely, even with 1280 games to consider. I'd add more years from PFR, but it would be quite a chore because I'm going to use team stats to account for relative team strengths in the regressions.

3. Anonymous says:

FYI - You transposed "moderate" and "warm" in the sentence, "The most remarkable result, however, may be the 35% home winning percentage of moderate teams hosting warm teams." And then based the following comment on the incorrect wording noted above: "Perhaps a couple warm teams have been relatively dominant over moderate weather division rivals between '02 and '06."

4. Brian Burke says:

Corrected. I must have misread my own table.

5. 81Trucolors says:

Interesting table. One thing that comes to mind is that home field advantage is an advantage but in many cases not enough to get a victory. It would be a lot of work but it might be fun to look at margin of victory/loss and apply it to climate and home field advantage. I love this site by the way.

6. Anonymous says:

This is excellent analysis that should be extended to future seasons to examine trends. Please give it a shot.

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