Humans are heat factories. If the average human being produces heat at a rate of 80ish watts, then 19 humans hunched together would radiate as much heat as my 1,500-watt space heater. Humans' surplus heat warms a housing project in Paris, a 13-story office building in Stockholm and the Mall of America in sub-freezing Minnesota.
A baseball stadium in the Midwest in October is, unlike a shopping mall, outdoors. But for a few hours it gathers an astounding quantity of hot mass -- roughly 7 million pounds of it, equivalent to 2,500 space heaters -- into a relatively dense seating area.