I Melt With You - Do the Broncos Really Play Poorly in Humidity?

Last week, as I was discussing the Broncos upcoming schedule, I made a comment that you hear from time to time:

Denver, humidity, and early games never mix.

After making this sort of a statement, I realized I was simply making like a member of the Black Hole--offering up an opinion without any data to back it up.  I had heard this mantra before (cold-weather teams don't play well in humidity), so I was simply reproducing something that seemed, at first, to be common sports-knowledge.  Also, I remembered well enough the first game of the 2005 season, in which Jake Plummer (who should just now be retiring a Bronco) led an excellent Broncos team into Miami, only to wilt under the oppressive heat and swamp-like conditions of south Florida.  The outcome was a 34-10 butt whoopin', the likes of which only Raiders teams dare to tread.  

So both my tendency to repeat what others have said, along with a selective memory, endeared me to the ever-ready generalization.  Luckily I decided to check and see if the generalization held swamp water.

I'm not recommending this to anyone, but I spent several hours gathering weather data from every Broncos regular-season game from 2000-2009.    Along with wins, losses, and scores, I recorded the temperature, the relative humidity level, and the wind speeds at kick-off.   If there was a correlation between what the Broncos were doing on the field and the weather, I would be sure to find it with 10 seasons of data.

So you'd like to know, wouldn't you?  Do the Denver Broncos wither under the soggy heat like a Sebastian Janikowski field-goal attempt in December?

Survive the jump, and I'll give you the results of what happens when you stop the world, lock yourself in a room with NFL game-books, and face Mother Nature mano a mano.

Moving Forward Using All My Breath

First, a little background about humidity.  Relative humidity, which is what you hear on the news each night, and what the NFL gives in their game-books, isn't necessarily a reflection of how muggy or oppressively "sticky" the weather is when playing.  So, in order to tell how muggy a day is, one needs a little more. 

The good news is that using this complex formula, one can estimate how "muggy"  a game is with temperature and relative humidity data, which the NFL game-books do provide.  So I was indeed able to calculate how sticky, hot, and oppressive each game was for the Broncos over the last decade.  This piece of data is actually called the Dew Point Temperature, but it really is a measurement that tells you how muggy it is.   And our friends over at the National Oceanic and Atmospheric Association (NOAA) have provided us this handy scale for determining "mugginess:"

Muggy Weather Scale - Dew Point Temperature in Degrees Fahrenheit

• Less than 50....Not Muggy
• 50 - 59...Slightly muggy
• 60 - 69...Moderately muggy
• 70 - 79...Very muggy
• Greater than 79...Unbearable!

The column on the far right is the Index, but there are a few other things one notices immediately from this table.  First, the Broncos have never played in Denver in the last decade in which the weather has ever been considered Moderately Muggy.  In fact, when looking at the data, I can only find one one home game (against Saint Louis in 2002) in the last 10 years in which the the game was even Slightly Muggy.   Second, the "muggiest" games were all in Florida, Cincinnati, New Orleans, or Buffalo.  If you've ever been to any of these cities, you can verify that it can get downright sweltering in these areas.    Third, it appeared as if San Diego, Kansas City, and Oakland, were close with respect to the intensity of muggy weather--at least during football season.  This fact surprised me, given Kansas City's reputation.  

On only 4 occasions in the past 10 years have the Broncos played in conditions the NOAA would consider Very Muggy.  The Broncos have gone 2-2 in those games.  Overall, in the 16 games over the last decade in which the Broncos have played in at least Moderately Muggy conditions, they are 10-6.  

It appears as if the Broncos don't melt after all. 

To further test this theory, I ran some good ol' fashioned correlation coefficients comparing how many points the Broncos scored in each game with how muggy (based on the Muggy Weather Scale) the game was.    The correlation coefficient over this 10-year-data set was -.060.  In other words, there is virtually no correlation whatsoever to how many points the Broncos scored to how muggy the weather was.  The same was true when I looked at points differential.  There just wasn't a significant correlation.

Just in case you were wondering about temperature and wind speeds, I ran correlations against these two variables and points scored as well.  The correlation coefficient for temperature and points scored was .0173.  And the correlation for wind speed and points scored was .085.  In other words, both variables had about as much correlation to the Broncos point totals as Jay Cutler does to team chemistry--none.  

In all three regression models, p-values were greater than .05, and thus, most stats guys (outside of Oakland) would reject the data as insignificant.  For a good and non-statistical explanation of this, click here.  Also, if you'd like my full data set, email me and I will send you the spreadsheet.  The data set includes from 2000-2009: 

• Date
• Score
• Opponent
• Points Differential (Broncos - Opponent)
• Away/Home Game
• Early/Late/Night Game
• Weather (Sunny/Clear/Showers, etc.)
• Relative Humidity @ Kickoff
• Temperature - Celsius @ Kickoff
• Temperature - Fahrenheit@ Kickoff
• Wind Speed @ Kickoff
• Muggy Index - Celsius @ Kickoff (Dew Point Temp)
• Muggy Index - Fahrenheit @ Kickoff (Dew Point Temp)
• Variables and Formulas used to calculate Dew Point Temp using Magnus-Tetens approximation (1974).

The Humidity Myth?

After looking at this data, I'm having a difficult time buying into the cold weather vs. hot weather team, or at the very least, that Denver is a team that will always be doomed when playing in hot and humid weather.  Yes, I've heard the rational before.  Teams like the Miami Dolphins have much better home records in September than they do in December, although after looking at the numbers, Miami's home record in December since 2000 is only 10-9.  So there goes that theory.  And NFL rosters are made up of players from a variety of climates and backgrounds, so I find it hard to believe that a guy like Tim Tebow is going to suddenly need an acclimation period if he returns to play in Florida.  

Brian Burke, over at Advanced NFL Stats, essentially came to a somewhat similar conclusion using a regression model that looked at team records by month and by category (warm teams, cold teams, and dome teams), although he did note an advantage for cold teams hosting dome teams late in the season.

Perhaps there is a slight hope, then, if the Broncos can ever host the Colts in the AFC Championship Game in January with freezing temperatures.  Until then, little Broncomaniacs every where can take their vitamins and go to sleep at night knowing that the Broncos won't melt under the heat because of the heat itself. 

It's only their execution that would make it so.