The Value of Homefield Advantage Part II - Weather

Editor's Note: 2005 Saints games and the ARI/SF Mexico City game are thrown out because of neutral site issues. The Saints played "home" games in a warm weather environment and a dome. They also had a faux home game in cold, grim New Jersey. The numbers have been subsequently corrected. 7/11/07

During the NFC Championship this year, they threw out some stat about dome teams being winless in road NFC championship games or something along those lines. And the Saints followed that trend by falling to the Bears 39-14. In the summer, players from cold-weather cities aren't used to the intense heat and humidity of places like Miami. In the winter, players from warm-weather cities aren't used to the icy winds, sleet, and snow of places like Green Bay and Cincinnati. Dome teams always seem to be at a disadvantage on the road when they don't have the luxury of A/C during games. Well, teams like the 1999 Rams were also built for the speed they could achieve on Astroturf. Anyway, the numbers bear out weather playing a factor in home field advantage. See how the spread accounts for different weather situations.

Average Result/Average Spread
	Cold @	Warm @	Dome @
@ Cold	2.2983/-2.4398	3.9104/-2.7412	3.4324/-2.4611
@ Warm	2.0283/-1.9379	2.4495/-2.4979	4.25/-2.7147
@ Dome	0.3688/-2.7079	2.2527/-3.0157	2.7407/-2.4198
% of games won by home team/% of games home team was favorite
	Cold @	Warm @	Dome @
@ Cold	56.358%/66.302%	64.792%/71.637%	60.81%/66.32%
@ Warm	59.109%/64.689%	55.37%/65.25%	61.92%/68.36%
@ Dome	52.13%/67.42%	56.41%/69.11%	58.64%/58.49%

Average Result/Average Spread, Weeks 1-8
	Cold @	Warm @	Dome @
@ Cold	2.1404/-2.4024	2.7937/-2.8548	2.6522/-1.9890
@ Warm	2.7115/-2.2007	3.2378/-2.2297	4.6964/-3.1594
@ Dome	-0.3846/-2.6625	2.5469/-2.8111	0.4091/-1.9222
% of games won by home team/% of games home team was favorite, Weeks 1-8
	Cold @	Warm @	Dome @
@ Cold	57.192%/67.619%	63.677%/72.903%	56.52%/65.93%
@ Warm	62.019%/65.789%	60.84%/64.86%	58.04%/72.46%
@ Dome	50.77%/63.75%	58.59%/71.11%	51.52%/55.56%

Average Result/Average Spread, Weeks 9-17
	Cold @	Warm @	Dome @
@ Cold	2.4319/-2.4717	4.8794/-2.6471	4.1139/-2.8824
@ Warm	1.5315/-1.7401	1.7622/-2.736	3.9122/-2.4306
@ Dome	1.0132/-2.7449	1.9931/-3.198	4.3438/-2.7896
% of games won by home team/% of games home team was favorite, Weeks 9-17
	Cold @	Warm @	Dome @
@ Cold	55.652%/65.182%	65.759%/70.588%	64.56%/66.67%
@ Warm	56.993%/63.861%	50.61%/65.60%	64.86%/65.74%
@ Dome	53.29%/70.41%	54.48%/67.33%	63.54%/60.66%

Note: The Houston Texans normally play with the roof closed, so they are considered a dome team. Idea from Football Outsiders. Arizona is still a warm weather team, though.

Weeks 1-8 cover roughly September and October, while Weeks 9-17 cover November-January. Warm-weather teams lose some home field advantage in the winter, and cold-weather teams gain some. Dome teams just get screwed against cold weather teams. Home field advantage is surprisingly worth next to nothing. The spread is mostly insensitive to home field climate matchups, leaving it particularly inefficient in dealing with dome teams. But can this be corrected for? Let's assume that the same inefficiencies show up in my model.

Adding binary variables for the home field climate matchups to my model slightly increases its accuracy on 2001-6, from 61.213% to 61.916%. The correlation coefficients of these variables to the final score margin are all very weak, ranging from -0.02 to 0.055. All 3 of the dome team home variables have negative correlations. Perhaps dome teams are just less talented on average because of the Lions and the Texans.

I also took the average results for each climate matchup and created a schedule difficulty score based on that. The score had 0.09735 correlation with win totals, suggesting that weather has a small overall effect on games.

Perhaps the climate factor is categorized too broadly. Gametime temperature and weather (wind, rain, etc.) could be compared with monthly average temperatures for both team's home cities. NFL gamebooks dating back to 2002 are available on NFL.com that include that sort of data. One could even calculate wind chill and heat index to account for wind speed and humidity. It's a project for the future.