Time and temperature are important, but so is the moisture content of the air. The degree of humidity can have significant effects on the human body. If the air is dry, as in the desert, then the heat doesn’t feel as uncomfortable even at a higher temperature. That makes it easy to dehydrate. A thermometer and a clock can help you remember to refresh your body fluids at frequent intervals – preferably with something non-caffeinated and with minimal sugar. At the other end of the scale, humidity can make your lungs work harder and can make the day feel at least ten degrees hotter than it really is, especially if the air is still. On a still, muggy day perspiration seems to pour off your face and down your back but doesn’t do a lot to help you cool off. The hygrometer will let you know that the feeling that you are swimming through the air isn’t your imagination, it’s the humidity.