The Weather Never Sleeps

Forecasting fog triggers task saturation

Some kinds of weather-tornadoes, for instance-are obviously sexier than others. Think of how many times you've seen video of tornadoes on television.

Tornadoes are thrilling and obviously dangerous, but they don't kill as many people each year as heat waves or floods, and pilots do a good job of avoiding them. (Only two aircraft accidents have been blamed on tornadoes. See "The Weather Never Sleeps: Gone with the wind," April 2002 AOPA Flight Training.)

Fog, on the other hand, is not at all sexy. Yet, the worst accident in aviation history happened in fog. In March 1977, a Boeing 747 started its takeoff on a fog-shrouded runway in the Canary Islands as another 747 was taxiing across the runway. The crash killed 570 people.

When the U.S. Navy studied 235 Navy and Marine Corps aviation accidents occurring in 1990 in which crew error was a factor, it found that 19 percent were weather-related, and more than half of these involved poor visibility caused by low clouds or fog. The Navy concluded that nearly two-thirds of all weather-related accidents are "preventable with a perfect weather forecast believed by the aircrew."

Since weather forecasts aren't perfect, especially for fog, pilots should stay alert and skeptical, always looking for signs that the weather isn't acting as advertised. Doing this, of course, requires having at least a general idea of what causes fog. Otherwise, you don't know what to be looking for. Let's start with the very basics.

Fog, which is really a cloud that's sitting on the ground-or very near to the ground-is made of tiny water droplets floating in the air. These droplets begin forming where the air becomes saturated and the water vapor in it begins condensing into liquid water droplets.

Let's take a brief look at saturation. The maximum amount of water vapor in the air depends on the air's temperature; the warmer the air, the more water vapor it can have in it. If you cool the air or add water vapor to it, you can reach a temperature at which the water vapor will begin condensing-the air has become saturated. In ordinary weather observations, the point at which the air at a particular time and place would become saturated is given with a dew point temperature.

All this means is that if the air is cooled to its dew point temperature, condensation will begin. If the air is at the ground when this happens, you get fog. If it's aloft, you get clouds.

This is the reason for the general rule pilots often hear: "If the temperature and dew point are within 5 degrees Fahrenheit of each other, and it's getting colder, such as it usually does overnight into the early morning, watch out for fog." For instance, you're flying at night to land around dawn. The temperature at your arrival airport is now 59 degrees F and the dew point there is 55. You should be ready to find fog, possibly reducing the visibility to less than a quarter-mile when you arrive.

You've been told that if the sky is clear and the winds are nearly calm, fog is more likely. A clear sky allows the Earth's heat to better radiate away into space, while clouds absorb some heat and radiate it back toward the ground, slowing cooling. Thus, the kind of fog that forms on clear nights is called radiation fog.

Wind can mix the cold air near the ground with warmer air a little bit higher, also reducing the amount of cooling. Anything that cools the air to its dew point can cause fog to form. For instance, relatively warm, humid air flowing over cold ground, especially snow-covered ground, can be cooled to the dew point, and advection fog forms.

Rising air cools, which means air flowing uphill with the wind can cool to the dew point, forming upslope fog.

In addition to cooling the air to the dew point, the dew point temperature can be increased by evaporating water into the air. This happens, for instance, when rain or drizzle falls into air that's already close to the dew point. Some of the falling rain evaporates, adding more water vapor to the air and increasing the dew point. If the dew point is increased to the ambient temperature, what is known as precipitation fog can form.

Water vapor can also be added to the air by evaporation of water on the ground from rain that fell earlier in the day, or from a river or lake. This is why airports in river valleys or near lakes are usually the first to turn foggy.

The use of words such as "radiation," "advection," or "precipitation" fog helps you to understand what's going on. But the real world is often more complicated. Much of the time more than one of the fog-forming processes is involved. In fact, it gets even more complicated than this, which is why fog and other causes of poor visibility are notoriously hard to forecast.

One of the reasons is that fog can occur on a small scale. A patch of fog maybe two miles long, 2,000 feet wide, and maybe 200 feet deep in a valley could close an airport in the valley. An airport two miles away could be clear.

Forecasting when and where fog will form and how long it will last requires detailed information about the atmosphere, especially how much water vapor is where. Such information is hard to come by.

The meteorologists who work for United Parcel Service's airline have put extra efforts into improving fog forecasts because hauling overnight packages around the country means the company's airplanes make many dawn arrivals, when fog is most likely. The UPS meteorologists have shared their techniques with the National Weather Service, the military, and other airlines. At an American Meteorological Society aviation weather meeting in 2002 they discussed how complicated fog forecasting can be. "Standard forecasting practice calls for clear skies as a requirement for the development of radiation fog," they said. But, at times a layer of stratus clouds can "build down"-thicken and lower to the ground to create fog. In other words, you can't always expect to descend through what had been reported as a layer of clouds to find clear visibility below them.

The UPS meteorologists also noted that a meteorologist will normally forecast fog if the surface temperature is expected to cool to or below the dew point and the winds are expected to be light. But fog doesn't always form under these conditions. It turns out that the amount of humidity in the air not only right at the ground, but also at higher levels-along with other information that weather observers don't usually collect-is needed to more accurately forecast fog.

The message for pilots is that fog doesn't follow simple rules. You should be ready for fog when the temperature and dew point are close together, the air is cooling off-or having more water vapor added such as from evaporating drizzle-and the winds are calm or nearly calm. If the expected fog doesn't form, take it as another example of how tricky forecasting can be.

But the next time the conditions seem to be the same, you should be ready for fog, maybe with a plan to divert to an airport the fog is leaving alone. Just when you think you have fog figured out, it could leave you in the murk.

Jack Williams is weather editor of Usatoday.com. An instrument-rated private pilot, he is the author of The USA Today Weather Book and The Complete Idiot's Guide to the Arctic and Antarctic, and co-author with Bob Sheets of Hurricane Watch: Forecasting the Deadliest Storms on Earth.