September 2007Departments

The Weather Never Sleeps

Obscure details

Facts about fog

"Blind Plane Flies 15 Miles and Lands; Fog Peril Overcome," declared the headline in the September 25, 1929, edition of The New York Times. The story explained how Army Lt. James H. Doolittle had taken off, flown 15 miles, and landed "without seeing the ground or any part of his plane but the illuminated instrument board."

What did Doolittle really accomplish that day? With a canvas cover completely hiding his view from the rear cockpit of an open-cockpit Consolidated NY-2 Husky biplane, he demonstrated that the combination of a sensitive altimeter, gyroscopic instruments, and radio navigation make what we call instrument flying possible. (In 1929 they called it "blind" flying.) During the flight Doolittle's safety pilot, Lt. Ben Kelsey, kept both hands in view on the cowling of the front cockpit, assuring observers that Doolittle was flying the airplane even when, for a moment, the landing seemed to be going awry.

While those involved in the 1920s instrument-flying experiments talked about "fog" flying, the techniques they pioneered enable pilots with the proper training and equipment to safely fly through clouds, including clouds that are very close to the ground. Fog is a cloud whose bottom touches the ground.

Doolittle's success enabled the nascent airline industry to eventually become as reliable as the trains that carried almost all long-distance travelers of the time. But the flight had not really "conquered" fog--"man's greatest enemy in the air," as the New York Times story said. Even today, only pilots qualified to make Category III C instrument landings, flying an airplane meeting stringent equipment requirements, and using an approved airport can legally and safely mimic Doolittle's 1929 feat: landing with zero ceiling and zero visibility; that is, completely blind to anything outside the cockpit.

Despite all of the advances in weather and aviation technology since 1929, fog is still a peril that pilots must avoid. The worst accident in aviation history demonstrates this. On March 27, 1977, at Tenerife in Spain's Canary Islands, a KLM Boeing 747 started its takeoff run without a clearance. The captain--one of the airline's most experienced--thought he had been cleared to take off. The KLM 747 smashed into a Pan Am Boeing 747 that was taxiing across the runway--hidden by the fog--killing a total of 570 people on the two airplanes.

On October 8, 2001, an SAS MD-87 taking off from Linate Airport in Milan, Italy, hit a Cessna Citation that had turned onto the wrong taxiway in the fog and started across the runway. All 114 people aboard the two airplanes were killed, along with four others on the ground.

Taking off in poor visibility is legal and safe when all the pilots and air traffic controllers involved are qualified and properly doing their jobs. A pilot without an instrument rating and current experience shouldn't attempt a takeoff in poor visibility. This doesn't mean, however, that a new pilot doesn't have to worry about fog--the most likely scenario is to be caught by fog that formed after takeoff.

As with any weather hazard, a thorough preflight briefing is the best defense. Knowing how and when fog can form is equally important. While the basic meteorology of how fog forms is easy to understand, the details can become quite complex, which often makes forecasting exactly where and when fog will form difficult.

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 the air can become saturated is called the dew point. When the air touching anything, such as grass or an airplane, becomes saturated, dew begins to form. Thus, the temperature at which the water vapor in the air at a particular time and place will begin condensing is the air's dew point. If the air isn't touching something, tiny drops begin forming on very small particles in the air, such as dust, called condensation nuclei. In other words, the air's dew point is really its condensation point.

By the way, when the air is saturated we say its relative humidity is 100 percent. Very clean air with very few condensation nuclei can have a relative humidity a little above 100 percent, but this is uncommon.

The most common kind of fog forms overnight under a clear or mostly clear sky and relatively light winds. After the sun goes down, solar energy no longer replaces the infrared energy that's always radiating away from the ground. If the air grows cold enough, fog begins forming. Meteorologists call such fog radiation fog. It's also called ground fog.

As you can imagine, radiation fog is most common in the morning around sunrise when the air is usually the coolest. As the sun rises and begins warming the air the fog will begin evaporating.

If the temperature and dew point are already close to each other, however, ground fog can form at other times of the day, especially in the evening as temperatures begin to cool. This is why when you obtain your preflight weather briefing, you should ask for the current temperature and dew point. If they are seven degrees or less apart, you should be alert for the formation of fog or low clouds.

Valley fog refers to radiation fog that forms as cool air moves down into a valley. If you've ever driven on foggy nights you've probably noticed fog covering the low places in the road while the hilltops are clear. During the winter when the sun is both low in the sky and not up as long as in the summer, valley fog can linger for days because the air never warms above the dew point.

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. 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.

Along the west coast of North America, advection fog forms when warm, humid air flows over the chilly water offshore. (Even in the summer, Pacific Ocean water off Southern California warms only into the low and mid-60s.) When winds from the tropics flow across this cool water, fog and low clouds--called the marine layer--form and move inland. When the marine layer moves inland low clouds and fog can shut airports along the coast while the sky stays clear a few miles inland.

When air is moving uphill the fog that forms is called upslope fog. As air rises, it cools and if it rises far enough it will cool to the dew point--this is how clouds form. If the air is humid enough and is flowing up a hill, the cloud can form with its bottom touching the hill; it's fog. Upslope fog is commonly seen on hillsides, but it also forms on places that might look flat to you, such as the Great Plains between the Mississippi River and the Rocky Mountains--the terrain gradually rises 5,846 feet between Memphis, Tennessee, and Colorado Springs, Colorado.

Basic meteorology texts describe radiation fog and advection fog as separate phenomena, but in the real world they can combine. For instance, advection fog might thicken around sunrise as radiation cools the air.

National Weather Service definitions of fog and mist

Fog: A collection of small water droplets suspended in the air at the Earth's surface that reduces horizontal visibility to less than five-eighths of a statute mile. Unlike drizzle, it does not fall to the ground.

Mist: A collection of small water droplets suspended in the air at the Earth's surface that reduces horizontal visibility to less than seven statute miles but greater than or equal to five-eighths of a statute mile.

Shallow fog: Fog that is less than six feet deep. Partial fog, patches of fog: Fog that is between six and 20 feet deep and reduces horizontal visibility, but to a lesser extent vertically. The stars may often be seen by night and the sun by day.

Fog can also form when enough humidity is added to the air to increase the dew point until it reaches the air's actual temperature--for example, if rain falls in air that is already extremely humid. As rain falls it can cool the air to the dew point as it evaporates. Soon, the dew point matches the temperature, and fog begins forming. Such precipitation fog could endanger a pilot who doesn't have an instrument rating who is flying through rain that's light enough for visual flight rules (VFR) flying. Precipitation fog could lower the visibility below legal and safe VFR minimums.

Rivers, streams, ponds, and lakes--or puddles left by recent rain--also evaporate water vapor into the air. This is why a river valley will often have thicker fog than nearby but drier low areas. If an airport is located next to a river or lake you should be alert for fog to form before it forms in other nearby places.

While warm air flowing over cold water creates advection fog, cold air flowing over warm water creates what is called steam fog because it looks like steam rising from the water. Water vapor evaporates from the warm water into the cold air, increasing the air's dew point to match its temperature. This causes the water vapor to condense back into water--tiny fog drops. Steam fog is usually light and isn't much of a concern for pilots. But steam fog can drift from over a lake to reduce visibility at a nearby airport, maybe adding to radiation fog that's forming there.

NASA and several private companies are developing and deploying technologies including synthetic visibility, enhanced visibility, or combinations of the two. Synthetic visibility uses a terrain database and the aircraft's GPS navigation system to project a 3-D graphic on a pilot's head-up display showing the scene ahead even though clouds or fog hide it. Enhanced visibility systems use infrared sensors or radar to "see" what's actually on the ground, such as runway lights or an airplane taxing onto a runway.

These systems offer the promise of finally overcoming the fog's peril as promised in the 1929 headline. Still, even when such systems come into wide use, pilots will continue to have to make decisions based on their knowledge of weather elements, including fog.

Jack Williams is coordinator of public outreach for the American Meteorological Society. 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.