If you know what your dew point is, you will have a better grasp on the humidity you're experiencing in a particular temperature. It's the point at which water vapor in the air will start to condense into visible moisture, like fog or dew. In summer, high dew points combine with high air temperatures to make it feel sticky and humid despite the actual temperature. Dew points can also help you predict the possibility of precipitation, rain, thunderstorms and tornadoes.

The dew point is determined by the amount of water vapor in the air at a given temperature and how cool the surface can get before that moisture begins to form. A device called a hygrometer is used to measure the dew point over a wide range of temperatures. It works by letting the air pass over a polished metal mirror that is cooled to below the dew point. If the surface of the mirror is able to drop to the dew point, moisture forms on it. A hygrometer can be found in most home improvement stores and is often used to calibrate other types of humidity sensors, such as automatic ones used by weather stations or in industrial processes.

When the air is warm enough to hold all of its available water vapor and it drops below its dew point, that moisture becomes condensation that can be seen on grass and leaves, in fog or as rain. It's the main bit of fuel a storm needs to grow and develop, although other factors also contribute.