That’s the official definition; if the visibility is 5/8 sm to 6 sm, then the conditions are described as mist. In meteorological terminal air reports (METARs) and other weather reports and forecasts, fog is abbreviated as FG, and mist as BR.
With visibilities that low, pilots restricted to flight in visual meteorological conditions can’t safely—or legally—take off or land. Pilots who are instrument-rated and current can legally fly in these conditions, but they will have to follow strictly defined procedures to take off and land. Even then, if visibilities are lower than the limits published on instrument approach charts, you may not see the runway at all—and have to execute a missed approach, climb away, and seek out a destination with better weather. For these reasons and more it’s important to know some fog basics.
One main point to remember is that fog is associated with cooling temperatures, but cooling alone won’t do. The moisture content of the air mass is also important. Here we get into the relationship between an air mass’s temperature and its dew point—with dew point being the temperature to which an air mass must be cooled in order to create coalescence of its water vapor into water droplets. As a rule of thumb, fog forms when temperatures drop to within 5 degrees Fahrenheit (2 degrees Celsius) of the air mass’s dew point. By the way, dew points correspond to humidity. The higher the dew point, the more water vapor it can hold.
The more the temperature-dew point “spread” narrows, and especially when the two values are the same or “on top of each other” as the saying goes, fog is virtually guaranteed. Another means conducive to fog formation is the addition of water vapor to an air mass.
So how and where is fog most likely to form? Nighttime cooling is one common way. As the sun goes down, any daytime heat radiates away from the surface, which is why this type of fog is called radiation fog. Temperatures drop to the dew point, and fog forms at the surface during the coldest hours of the night. Clear nights and calm winds help assure its formation. For this reason, this type of fog is called radiation fog. With sunrise and the heat of the day, the temperature-dew point spread widens, and the fog evaporates.
Advection fog happens when moist air moves over a colder surface. (Advection means horizontal movement.) Here, the cold surface lowers the moist air to its dew point. This is common along the West Coast with its adjoining cold waters, and the southeastern and central states, where flows from the Atlantic Ocean move over the vast, rising Piedmont areas of Georgia and the Carolinas, cooling as they rise in elevation.
Upslope fog happens when moist air flows up the windward sides of mountains. As the air rises it cools to the dew point and creates fog that can obscure wide areas of high terrain.
Valley fog is sort of the reverse of upslope fog. Cold air is dense, and often sinks down mountainsides and pools above the rivers and streams that divide mountains and other high terrain. Moisture above the streams is cooled, the temperature and dew point converge, and the result is a dense fog that follows the rivers’ tracks, and the terrain along its shores. Many airports in mountainous terrain are situated along these riverbanks, which many times are the only areas flat enough to be suitable.
Frontal and precipitation fog occurs when warmer air advances over colder surfaces, as is the case with warm fronts. Any accompanying precipitation can add enough moisture to create large bands of dense fog. After a rainy or snowy frontal passage, another form of radiation fog can blanket the area until the cold, saturated earth wicks it away as clearing skies and rising temperatures drive the temperature and dew point values apart.