When reading a TAF, METAR or ARFOR weather report often you will see temperature given as two numbers separated by a /, eg: 21/20.
The first of the two numbers is temperature and the the second is the dew point. The dew point is the temperature at which the air can hold no more water vapour.
The significance of this is that whenever temperature and dew point are within 3°C there is a significant risk of fog, mist or low level cloud.
Be very careful flying too early on navigational flights due to both low freezing levels and higher chances of fog and low level cloud. Flying over fog puts you into a dangerous situation if you needed to perform a forced landing.