Historical Weather Data
In a 2010 blog post, we outlined how we populated historical weather data:
The weather info right now is daily, and not specific to the hour. It is based on the weather data from the airport for the city in question, and not every stadium is right by the airport. The reported temperature is the daily high if it was an afternoon game, and adjusted for night games using the daily average and low temperatures. The wind conditions reported are based on daily averages and maximum sustained winds on the day in question. I would view it as very good approximate weather info for the game in question, and it goes back to every game (except for 3 random holdouts) since 1960.
Since then, we have acquired a number of gamebooks which list the official weather conditions at the start of each game. That means that since 1999, the weather conditions listed in the boxscores are the official NFL game weather conditions. Games before that date are still the general area weather conditions on that day aside from a few special cases such as the 1967 Ice Bowl game where we made sure to fill in the gamebook weather for that game.