To attempt to come up with an approximate answer to this question, I used the home team wins and losses in all games since 1986, sorted by climate and distance. I eliminated games involving dome teams (for now) because I wanted to isolate the effect without the noise coming from the advantage of outdoor teams vs. dome teams. I also (for now) eliminated games involving Denver.
Denver clearly has a strong home field advantage. Denver plays all of its games against opponents from different time zones, and some of the advantage is likely due to this. But Denver also plays opponents from different elevations and weather patterns as well. So, for now, we will look at how time zone changes affect other teams, to get a sense of how much of Denver's advantage is due to this effect.
Here is the data for games between outdoor teams from the same time zone versus one time zone difference, versus two or more time zone differences.
- Same Time Zone -- 759-603-2 (.557)
- 1 Time Zone Diff -- 329-219-3 (.600)
- 2+ Time Zone Diff-- 451-317-1 (.587)
The home field advantage is weaker when two outdoor teams are from the same time zone. However, while the advantage increases with a change in time zones, it does not appear to continue to increase with additional time zone changes. When we weight the time zone changes by cross-referencing against climate changes, the effect of playing an opponent from a different time zone versus one from the same time zone is roughly an increase in expected winning percentage of +0.035, but the effect of additional changes in time zone is negligible.
In games involving Denver and another outdoor team, the home team is 167-87 (.657) since 1986. In all other outdoor vs outdoor games since 1986, the home team is 1539-1139-6 (.575). Thus, Denver has had an advantage of +.082 above a normal outdoor vs. outdoor situation. Roughly +0.047 of this, then, is due to either random chance due to the smaller sample size, or to legitimate factors other than time zone changes, such as elevation changes and environmental changes between Denver and the opponent.
This research also supports that the effect of cross-country travel is overstated. Changing from the Eastern to the Pacific Time Zone has been no less a disadvantage than changing from the Eastern to Central Time Zones.
Conversely, the effect that coming from a similar environment and the same time zone has on reducing home field advantage is understated. In a future post, I will look at this further by looking at "against the spread" data.