While some emerging technologies are beginning to show promise of one day enabling pilots to "see" down a foggy runway, most pilots, especially general aviation pilots, must find ways to avoid fog, just as those with whom Rogers flew did some 70 years ago.
The basic meteorology of fog is simple; understanding what causes fog to form and dissipate is much easier than grasping what goes on inside thunderstorms. While the basic meteorology is relatively simple, forecasting when fog is likely to slow air traffic is infinitely complex. For instance, during the summer of 2001 forecasters and researchers began testing a system that includes a network of special sensors around San Francisco Bay with the aim of better forecasting when fog that slows arrivals at San Francisco International Airport will begin and end. From May through September, fog and low clouds frequently move in, cutting arrivals at San Francisco from 55 an hour to 30 an hour. Arrivals are slowed because pilots must maintain visual separation for simultaneous landings on the closely spaced parallel runways. Better forecasts would enable controllers to better meter the flow of traffic to San Francisco.
Learning how fog forms and dissipates doesn't enable pilots to forecast when fog is likely to form and lift. It does, however, help the pilot to recognize when to be on the watch for fog to avoid being caught in the air as fuel runs low and fog hides nearby airports.
Fog forms in the same way that clouds form, because fog is nothing but a cloud on the ground. Imagine being in the air and seeing clouds cover the top of a distant mountain; you might point out the cloud cover to a passenger. Hikers on the mountain would be talking about the thick fog.
Clouds and fog form when the air becomes saturated; that is, there is so much water vapor in the air in relation to the air's temperature that some of the vapor begins condensing into tiny water droplets. The temperature at which air will become saturated is called its dew point. The dew point temperature depends on the amount of vapor in the air.
Air becomes saturated through one of two processes: The air is cooled to its dew point, or water vapor is added to the air, which increases the dew point. Cooling the air leads to formation of the most common kinds of fogs.
Let's begin with radiation fog, which forms overnight when the sky is clear and the wind is light. Heat radiates from the ground into space. This process cools the ground and the air next to the ground. If the air cools to its dew point, patches of fog begin to form. Such fog is usually less than about 300 feet thick, but it also can be a thin layer at the ground no more than five or six feet deep. Radiation fog is often called ground fog, but this term, strictly speaking, refers to fog that is only about 20 feet thick.
While radiation fog is usually thickest during the coolest part of the day, normally around dawn, it can begin forming early in the evening as the air begins to cool off. If the temperature and dew point are only a few degrees apart as you're getting ready to take off for a night flight, you should be prepared for fog to form. For instance, if the temperature is 60 degrees and the dew point is 56 degrees, fog will start forming anywhere that the temperature cools by only four degrees to 56 degrees. That's not much of a margin on a clear night. A good general rule is that radiation fog is likely to form if the temperature and dew point are fewer than seven degrees apart, the sky is clear, and the winds are close to being calm.
Valley fog is a kind of radiation fog. As air cools it becomes denser � heavier - and flows downhill if there is a hill to flow down. This is one of the reasons you are more likely to see fog in low places than on the tops of hills. During the winter when the sun is low in the sky and not up as long as in the summer, valley fog can sometimes linger for days, because the air doesn't get a chance to warm above the dew point.
In addition to being cooled by heat radiating away from the ground, air can be chilled as it moves over cold ground. Meteorologists use the word advection to describe the horizontal movement of air. Thus, when air is cooled to its dew point by moving over cold ground, the fog that forms is called advection fog. While radiation fog normally tends to "burn off" or evaporate as the sun rises higher in the sky, advection fog can hang around for days. Some of the thickest and most persistent advection fog forms in winter when warm, rather humid air from the south flows northward over snow-covered ground.
In addition to radiation and advection, air can be cooled when it is forced to move to a higher altitude. As air rises, it expands so that its pressure matches the pressure of the air around it. As the air expands it cools. This means that air blowing up a hill, even a gentle slope, will cool. If it cools to its dew point, the resulting fog is called upslope fog.
To begin making sense of the weather, you have to put different things into different boxes. That's why fog is classified by the way it is formed - radiation, advection, or upslope. In the real world, however, fogs are often the result of two or even all three processes cooling the air. For instance, advection fog might thicken around sunrise as radiation cools the air even more.
While most fog forms when the air is cooled, fog created when moisture is added to the air can also sometimes trouble pilots. As rain falls, some of the water in the raindrops evaporates into the surrounding air. If the air's temperature is already close to the dew point, the moisture added by such evaporation can increase the dew point to match the temperature. When this happens, some of the moisture in the air begins condensing into tiny drops - much smaller than the falling raindrops - to form patchy clouds above the ground and fog on the ground.
Such precipitation fog is sometimes called frontal fog because it is most common ahead of an advancing warm front, when the rain is falling from warm air aloft into cooler air below. The main danger to pilots from precipitation fog is that it can reduce visibility even more in areas where the weather is already marginal.
Most of us are familiar with the other kind of fog formed when evaporation increases the amount of water vapor in the air. On cool fall days, you may see wisps of fog rising from ponds, lakes, and rivers. Because of its appearance - like steam rising from hot water - such fog is called steam fog. As cool air flows over warmer water some of the water evaporates into the air, increasing its dew point to match the air temperature, and fog begins forming.
Any kind of fog can be freezing fog when the air temperature is below 32 degrees Fahrenheit or 0 degrees Celsius. As the temperature drops below freezing, water droplets do not begin turning to ice. Instead, they become supercooled - that is, they remain liquid even though they are below freezing. When supercooled water drops hit anything, such as an airplane's wings, the water can instantly turn into ice. The layer of ice left on an airplane by freezing fog might be very thin, but it can be enough to reduce the lift of the wings, making it impossible for the airplane to take off or to gain altitude if it does manage to struggle into the air.
While fog is still a problem for pilots, emerging technology could help to make Will Rogers' dream of "licking" fog come true. Enhanced vision systems, some of which use infrared sensors, are being developed as ways to see through obstructions such as fog, smoke, or dust. The aim is to use the infrared energy that objects emit to create video-like images of those objects, including runways hidden by fog.
Within a few years such devices are expected to begin showing up in business jets, military aircraft, and airliners. As the technology develops and the cost and size of the systems decrease, general aviation pilots can look forward to having a head-up display that projects an image of a fog-obscured runway onto the windshield of their airplanes.
Until then, pilots will have to continue avoiding fog by using their knowledge of how it forms to make sense of weather reports and forecasts.
National Weather Service Definitions Of Obstructions To Visibility
Mist: A visible aggregate of minute water particles suspended in the atmosphere that reduces visibility to less than seven statute miles but greater than or equal to five-eighths of a statute mile.
Fog: A visible aggregate of minute water particles (droplets) which are based at the Earth's surface and reduce horizontal visibility to less than five-eighths of a statute mile. Unlike drizzle, it does not fall to the ground.
Smoke: A suspension in the air of small particles produced by combustion. A transition to haze may occur when smoke particles have traveled great distances (25 to 100 miles or more) and when the larger particles have settled out and the remaining particles have become widely scattered through the atmosphere.
Haze: A suspension in the air of extremely small, dry particles invisible to the naked eye and sufficiently numerous to give the air an opalescent appearance.
National Weather Service Definitions Of The Nature Of Fog
Shallow: The descriptor shallow shall only be used to further describe fog that has little vertical extent (fewer than six feet).
Partial and Patches: The descriptors partial and patches shall only be used to further describe fog that has little vertical extent (normally greater than or equal to six feet but under 20 feet) and reduces horizontal visibility and to a lesser extent vertical visibility. The stars may often be seen by night and the sun by day.
Freezing: When fog is occurring and the temperature is below 0 degrees C, freezing is used to further describe the phenomena.
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