The dewpoint temperature is a valuable tool to a forecaster. It is used to determine the amount of moisture in the air. It’s also used as a tool to forecast overnight low temperatures.
The dewpoint temperature simply put is the temperature at which dew forms. We see this happen through the night as the air temperature cools to the dewpoint temperature and dew forms into the early morning on the grass. The technical definition according to the American Meteorological Society is the temperature to which a given air parcel must be cooled at constant pressure and constant water vapor content in order for saturation to occur.
There is a relationship between dewpoint temperature and air temperature that determines how your body feels every day. The closer the air temperature is to the dewpoint temperature, the higher the relative humidity is. The larger the distance between the dewpoint temperature and the air temperature, the lower the relative humidity. You can use this as a forecasting tool to show if it is going to be hot and sticky or dry and pleasant.
If your high temperature is 90°F and the dewpoint is 45°F, it would not feel very humid. The air would be able to evaporate the sweat from your body to cool your system. If your high temperature for the day was 90°F and the dewpoint was 75°F, it would feel very humid. The air would have a hard time evaporating the sweat from your body and your system would have a harder time cooling off. The reaction from your sweat evaporating causes a cooling near the surface of your skin, allowing your body to cool. If the humid air makes evaporation hard, the body can’t cool as well.
You can also use the dewpoint temperature to get a rough idea of the overnight low temperature. If there is not a big change in the airmass from a frontal system, you can use this method. Take your dewpoint temperature from late afternoon and use it as your low temperature for the night. For example, if your 3 p.m. dewpoint temperature is 55°F, your low temperature for the night should be around 55°F. This is not always the perfect way to find the low temperature for the night ahead, but it is a good proxy. Clear skies can cause the temperature to fall lower. Wind can keep the temperature higher. Clouds can also keep the temperature higher.