If you've wondered about fog, and how and why it occurs, perhaps you'd be interested in a science fair project that explores when and how fog develops.
Basically, fog is just a cloud that hangs very low to the ground. When the temperature drops below the dew point-that's the temperature at which air becomes saturated with water vapor, and can't hold any more-the vapor condenses out of the moist air near the ground and forms fog. This usually occurs when warm air moves into a region over ground that is much colder. There are various types of fog that form for various reasons.
A suggested science fair project dealing with fog would be to keep a weather journal over the course of two months. It's best to do this in a season when the temperature is changeable, such as in the spring and fall.
Record the temperature at the same time each night and each morning, note the presence or absence of fog, whether skies are clear or cloudy, the speed and direction of the wind, and other meteorological factors.
After you've summarized and charted your observations, you should see some clear patterns of when fog occurred. Additional research will be necessary to help you figure out why fog occurs under certain conditions.