You can measure humidity in your back yard with a wet/dry bulb thermometer (it takes practice to use this, and some math skills). You have to wet the bulb of one thermometer and leave the other one dry. Then you have to fan the air over them both at the same time. A big flat piece of cardboard works well for this. After a few minutes, read the thermometers. You will need a relative humidity chart to determine the relative humidity in the air. The dry numbers appear vertically, and the wet numbers appear horizontally. Use the x/y intersection to find the relative humidity using a wet/dry bulb psychrometer.
A hair hygrometer is an early humidity measuring device and though primitive, is an accurate instrument to measure humidity. As the humidity in the air increases, hair will tend to stretch and get a bit longer. As the humidity in the air decreases (gets dryer) the hair will then tend to shorten or draw up.
Begin your experiment by making a hair hygrometer with a human hair (blond is best) some poster board and a penny. Use the penny as a weight and a pointer. Make a mark on your poster board right under the penny on a relatively dry day. Then wait until some clouds roll in...then make a second mark on your poster board on a cloudy day. Be sure to write...dry day...cloudy day...and then rainy day. Keep your poster board in a shed so that it won't get ruined by the rain. But keep measurements over several weeks so that you can actually see the changes in the height of the penny on various types of days. As the humidity increases, the pointer will drop.