The amount of moisture the air around us can hold depends on the air's temperature. Because a parcel of air can hold more moisture as the temperature increases, a fixed amount of moisture becomes a smaller percentage of the total amount the air can hold as the air temperature goes up. As the air's temperature decreases, a fixed amount of moisture becomes a larger percentage of the total amont the air can hold as its temperature goes down. That amount of moisture is called relative humidity , because the percentage is relative to the air temperature.
Relative humidity is measured with a device called a sling psychrometer.
We'll do this in class and outside in today's lab. We'll measure the relative humidity in the classroom first. Moisten the cloth sleeve on the wet bulb thermometer. Swing the psychrometer until the temperatures become stable. Split into groups of three or four.