Does your head stay warmer when you have a hat on? Mothers everywhere have been telling us for years that 40% to 99% of body heat (the actual number varies depending on which mother is talking) is lost through the head when we don't wear our hats in cold weather. (Perhaps you weren't listening.) But it works just the same with cooking. Without a lid, more of the heat that you're applying at the bottom of the pot escapes at the top. With a lid, the heat collects more quickly, raising the temperature and getting to a faster boil.
Those brave individuals who grill outdoors through the cold winter months have learned this lesson. They are generally obliged to close the lids on their grills in order to collect enough heat to cook their food - even though the steaminess caused by the closed lid is a hindrance to great grilling. But do they wear their hats while grilling? We think you can guess the answer.