What is a dew point?
The dew point is a measure of the amount of moisture in the air and an indicator of whether clouds or fog are likely to form. Let's examine each in turn.
The dew point is expressed as a temperature. As dew points increase, the moisture content of the atmosphere also increases, and as dew points decrease, the moisture content of the atmosphere decreases. For example, if the atmosphere has a dew point of 60, it will hold roughly twice as much moisture or water vapor as when the dew point is just 40.
Next, when the temperature cools to the dew point, the atmosphere becomes saturated, and clouds or fog often form. That's why meteorologists keep a close eye on whether the temperature is likely to cool to the dew point. If that's likely to happen at the surface, fog is likely (though not guaranteed) to form...if it's likely to happen higher in the atmosphere, clouds are likely to form.