In this activity, you will build a sling psychrometer and determine the relative humidity of your local area. Weather observers use sling psychrometers to measure relative humidity. Relative humidity is the amount of water vapor in the air compared with maximum saturation. Estimated time: 2 hours You will need the following materials: plastic soda or water bottle scissors or knife string or yarn (1.5 feet) masking tape or duct tape 2 alcohol (not mercury) thermometers cotton medical gauze water rubber band safety goggles Follow these steps to build your sling psychrometer: Poke a hole near the neck of a plastic bottle. You can use scissors or a knife to do this. Slip the string through the hole. Tie the ends of the string securely around the neck, as shown in the image. Tape the string down for extra security. Wet the gauze with water. Wrap the wet gauze around the bulb of one thermometer. Secure the gauze in place with a rubber band. Leave the other thermometer as it is. Tape the thermometers to opposite sides of the bottle. Make sure they are taped securely, because you are going to swing the bottle.