Yes, in hot weather you typically need to drink more because you sweat more, especially if you’re exercising. Humidity also increases your water needs. When it’s humid and warm—a double whammy—you may need as much as two times more water than when it’s drier. And although we tend to skimp on water when it’s chilly out—don’t. You also lose more water while breathing frigid, dry air. Fluid needs don’t change at altitude (11,500 to 17,500 feet): in fact, your body finds a new normal at 2 percent dehydrated and actually functions better.