Make a Sling Psychrometer - The Old Farmer's Almanac

MAKE A SLING PSYCHROMETER

YOU'LL NEED TWO thermometers and a wooden board that is wide enough to hold them side by side, plus a foot of string, a small wet cloth, and a rubber band.

Drill a hole into the center of the end of the board that will be the top. Tie the string through the hole. Tape the thermometers side by side onto the board, parallel to the length of the board, with the bulbs of the thermometers extending beyond the bottom edge. Wrap the wet cloth around one thermometer bulb, and secure it with the rubber band; leave the other bulb bare.

Holding onto the string, swing (or "sling") the device in front of you for a few minutes. Then stop swinging and read the temperature on both of the thermometers. The one wrapped in cloth will probably show a lower temperature than the dry-bulb thermometer because water in the cloth evaporated, cooling the cloth and the thermometer. The drier the air, the more water will evaporate from the wet cloth, and the greater the temperature difference will be between the two thermometers. For the most accurate reading, swing your psychrometer for several minutes, until the thermometer wrapped in cloth stays at a constant temperature. (That's when you'll know it's as cool as it's going to get.) Make a comparison of the two temperatures (see our chart) to determine the relative humidity.