Dense fog blanketed the DC Metro area last night and all through the morning hours. In fact, its longevity seemed to even catch the National Weather Service by surprise, as the original Dense Fog Advisory was extended 5 hours past its original expiration time of 7am. Areas of reduced visibility could still be found throughout the Metro area at noon today.
Fog forms whenever the temperature cools down to its dewpoint. That means that the air is too cold (doesn't have enough energy) and can't hold all the moisture in the air, so the moisture starts condensing and forming tiny water droplets. In order to really understand fog, you need to picture the air at a microscopic level. You see, when air is hot, the molecules are moving around really fast, which allows them to bounce around a larger number of water molecules without dropping them. Individual water molecules, moving around within an airmass... you could consider that a definition of water vapor (water in gas form). As an airmass cools down, the air molecules slow down, and they're not as adept at keeping the individual water molecules apart from each other. As the molecules "find each other", they form loose bonds as liquid water droplets. This process is more commonly known as condensation! Fog is really just a cloud that forms at ground level. The condensation process that causes fog to form is the same process that forms clouds up in the sky.
It doesn't matter what time of year it is; fog can form in the winter, summer, or anytime in between. However, it's less common in cold weather because there is less moisture in the air overall, so it's less likely that the moisture will condense. Also, it's really tough to get fog to form when there's a breeze, which is why foggy days are usually really still and calm.