In Phoenix, we don't have many foggy days, but we do have a few. On some of those days, the fog becomes serious enough to be a true hazard when driving. During Phoenix summers, dewpoint (or dew point) is the indicator used by scientists to predict when our summer thunderstorm season (monsoon) has arrived.
Fog in Phoenix
When the temperature in Phoenix is cool enough to be at the dewpoint temperature, that moisture in the air condenses until it is thick enough to become fog.
But where did that moisture in the Phoenix air come from? After all, we don't have a large body of water nearby. Our fog events occur most often during the winter months after it has rained. That rain provides the moisture that creates the next morning's fog. As an aside, we almost never see fog in the summer in the Phoenix area because the air never cools down to the dewpoint long enough for fog to form.
If you are driving and hit a patch of dense fog that makes visibility difficult, slow your speed and proceed by using the white lines as a guide. If visibility is so poor that you don't want to drive in the fog, you might try to carefully make your way off the highway to park in a safe area until the fog lifts. If you pull over in your car to the side of the road, do not leave your lights on. Drivers with little or no visibility behind you may think you are still on the road and follow you.
Special thanks to the National Weather Service in Phoenix for providing the details!