But the more you fly, the more you’ll realize the importance of this term. Think of the dew point as a kind of threshold for fog formation. When the air temperature drops to within 5 or so degrees Fahrenheit (or about 2 degrees Celsius) of the dew point, fog is likely. So, when you hear someone say that there’s a close temperature-dew point “spread,” you’ll know what they mean. And when you hear that the temperature and dew point are “on top of each other,” you’ll know that the temperature and dew point are the same. At times like that, it’s almost guaranteed that fog—or some form of precipitation—is present.
Fog is basically a cloud on the ground, and it’s one of the biggest adverse weather events you’ll face as your flying career evolves. It can make surface visibilities low enough to prevent safe takeoffs and landings, and it can close even the largest, most well-equipped airports if visibilities go to zero. The National Weather Service defines fog in terms of yielding surface visibilities at or below 5/8 statute mile. Under visual flight rules (VFR)—the rules you’ll fly under as a student pilot—you can’t legally take off if the visibility drops to three statute miles or less.
Cooling is the main driver in fog creation. That’s because cooling—whether caused by a frontal passage, rain, snow, or simply the presence of a cooler air mass—promotes condensation. Which is exactly what happens to water vapor when the temperature-dew point spread closes. And if water vapor condenses into tiny, suspended fog droplets when temperatures cool, they evaporate as temperatures climb and the temperature-dew point spread widens.
Nightfall is one way of cooling surface temperatures to the dew point. After a night’s worth of cooling, temperatures at dawn can be low enough for a layer of fog to form. Clear nights further aid cooling by allowing plenty of surface heat to dissipate into the atmosphere.
Cold air is denser than warm, so it can easily sink into valleys and riverbeds, other great spots where fog is prevalent. You’ll also want to watch out for fog when rain occurs. Rain cools the air through evaporation as it falls, creating what’s called precipitation fog. During the daylight hours, a rain-soaked surface can be cooler than it ordinarily would be, because the sun’s energy is spent more on evaporating rainwater than cooling the ground. At night, there’s cooling, both by evaporation and the usual nocturnal temperature drop. A layer of snow is also great for making fog, because it chills the air above it. Did it snow during the night before your flight? Then those cold nighttime temperatures add to the chill factor.
There are seasonal factors in the climatology of fog. In the autumn months, air temperatures trend cooler while bodies of water, such as lakes and river, retain the warmth accumulated during the preceding months. The warmer water evaporates into the cooler air above it, then condenses into a shallow layer of fog. In the winter and spring, moist flows from the Atlantic and Pacific oceans move inland over colder, and often rising, terrain to form advection fog. (Advection simply means the horizontal movement of air). Another type of fog is ice fog, sometimes caused in subzero regions when aircraft exhaust gases rapidly cool.
Fog in freezing temperatures? It can happen. When fog droplets remain in what’s called a supercooled state—still liquid, but below freezing—it’s called freezing fog. This means that dew point temperatures below freezing are possible—and, in fact, common. For example, snow, freezing rain, and frost can have temperatures and dew points below the freezing point.
For all these reasons, temperatures and dew points are posted in all hourly aviation weather observations at a large number of airports. Called METARs (for meteorological aerodrome reports), they show the observed weather using a standardized format.