For some, fog is an atmospheric condition that almost never manifests itself. For others, it is an omnipresent nuisance that makes traveling anywhere slightly riskier than usual, if not downright hazardous. As annoying as you consider fog, though, how much do you actually know about the stuff?
In reality fog is little more than an earthbound cloud, one that formed near the surface of the planet rather than in the sky. And, unlike normal clouds, fog is usually caused via a water source near the foggy area.
The creation of fog begins in the same manner as clouds. The heat from the sun causes water in a particular area to evaporate into the air, or wind fronts carry water vapor into the area, and that vapor begins to rise. Because the air is already nearly saturated with water vapor, however - which explains why fog often occurs after rainfalls - the vapor doesn't have the chance to rise into the sky to become a cloud. Instead it undergoes condensation near the ground, the result of which is the fog cloud.
Fog doesn't happen everywhere, however, nor at all times of the year. As with rain fog requires a fairly warm air front to come through and mix with a cold one; consequently you won't see fog during the winter. Fog is also much more likely to form in areas that are naturally humid (swamps, for example) or serve as intersection points where currents often converge.
It's also worth noting that fog is functionally identical to a similar atmospheric phenomenon, mist. Both are formed through the same process; the only difference comes from the amount of water vapor in the air. Fog is very deep and typically restricts visibility to a few hundred meters at most. Mist, on the other hand, is wispier and allows a great deal more visibility. Mist is more likely to appear in areas with less natural humidity, whereas fog shows up often in the same areas where the conditions for its creation are just right.
Regardless of the cause, it's important to know that fog is dangerous. Yes, it doesn't create any physical irregularities, but its restriction of visibility can create unsafe transportation conditions. Avoid driving in fog whenever possible, especially on roads that aren't crowded with cars, as you may not see one up ahead until it's too late too stop. Given that fog typically appears after rain this also means that the roads will probably still be wet, upping the ante even more.