I like to shoot in morning fog or fog in general. Is there a way to predict when it'll occur?
Awesome question, I am studying for my Private Pilot license and (as a photographer) found the chapter on Weather Theory facinating. Among other things, it gives a very reasonable description on the predictors for fog (and other meteorological events). It describes 4 kinds of fog and when each may/will occur:
Radiation Fog: "On clear nights, with relatively little to no wind present, radiation fog may develop. Usually, it forms in low-lying areas like mountain valleys. This type of fog occurs when the ground cools rapidly due to terrestrial radiation, and the surrounding air temperature reaches its dew point. As the sun rises and the temperature increases, radiation fog lifts and eventually burns off"
Advection Fog: "When a layer of warm, moist air moves over a cold surface, advection fog is likely to occur. Unlike radiation fog, wind is required to form advection fog. Winds of up to 15 knots allow the fog to form and intensify. Advection fog is common in coastal areas where sea breezes can blow the air over cooler landmasses."
Upslope Fog: "...occurs when moist, stable air is forced up sloping land features like a mountain range. This type of fog also requires wind for formation and continued existence. Upslope and advection fog, unlike radiation fog, may not burn off with the morning sun, but instead can persist for days. They can also extend to greater heights than radiation fog."
Steam Fog (my favorite): "(AKA Sea Smoke) forms when cold, dry air moves over warm water. As the water evaporates, it rises and resembles smoke. This type of fog is common over bodies of water during the coldest times of the year. Low-level turbulence and icing are commonly associated with steam fog"
I really liked how they've given you the tools to predict when fog will occur, it all makes a lot of sense. It goes on to discuss how temperature, atmospheric pressure and moisture are related to weather patterns and how to predict what different land (or water) masses will impact winds and weather in the area.
You're looking for very humid conditions followed by a (somewhat) rapid cooling, relative to temperature. In practice, Spring and Autumn tend to feature the sorts of temperature swings needed, but fog can obviously occur any time of the year.
The humidity needed to form fog will most likely come from recent rainfall, but again, this can vary. A fast snow melt can release the sort of humidity you're looking for, and standing bodies of water can form localized mist or fog, which is another sort of look, as well, as you'll see the fog hanging over the water. Another factor, of course, is wind conditions, as any sort of air movement at all will tend to disperse fog before or shortly after it forms.
Perhaps the easiest way to predict fog is to cheat: sign up for weather alerts in whatever place you're interested -- you'll see any dense fog advisories that are issued for that area. Beyond that, just watch weather conditions, including wind conditions. A weather map will show you wind conditions in specific places, and over larger areas, watch for widely-spaced isobar lines which will normally accompany light winds.
I check the weather forecast. Of course many people are interested in this information so weather centers predict it. It is especially critical for pilots.
Try http://www.intellicast.com/National/Humidity/FOGcast.aspx or similar.
They also define it on that very website as "Fog forms when the difference between the air temperature and dewpoint is 5°F (3°C), or less."