October 2005Departments

The Weather Never Sleeps

Fog formation

This weathermaker gets little respect

Fog is the Rodney Dangerfield of weather hazards: It gets no respect.

When you tune in The Weather Channel's "Storm Stories" or look for movies about weather disasters you'll find tornadoes, hurricanes, storms at sea, floods, blizzards, and even global warming--but not fog.

The deadly weather you see on television kills hardly any pilots. Even so, weather-related aviation accidents kill around 440 people a year in the United States, more deaths than are caused by the kinds of weather that make the 6 o'clock news, according to the Cooperative Program for Operational Meteorology, Education, and Training (COMET) in Boulder, Colorado, which offers online education and training courses for meteorologists.

Low ceilings and visibilities are the leading cause of aviation accidents, COMET says on its Web site. "Consider this sobering statistic: Of those general aviation pilots involved in low ceiling or low visibility accidents due to fog, more than half were fatal. Clearly, this is a user community that needs the best forecasts possible." It's also a community that needs to give fog some respect.

Fog forms when the air near the ground cools to its dew point, or when humidity is added to the air, increasing the dew point to the air's temperature.

The term dew point seems to have the power to fog the minds of those who haven't studied meteorology, but the concept isn't very complicated.

Humidity refers to water vapor--the invisible gas made of water molecules--that's mixed with the nitrogen, oxygen, and other gases that make up air. The cooler the air, the less water vapor it can have mixed in before some of the water vapor begins to condense into liquid water. When water vapor begins to condense, meteorologists say the air is saturated.

When condensation begins right at the ground, tiny drops of water called dew form on grass or other surfaces such as an airplane's wings. This is why the temperature at which condensation begins is called the dew point, whether the air is near the ground or high in the sky.

While the dew point is given as a temperature, it's really a measure of how much water vapor is actually in the air at a particular time and place. The more water vapor in the air, the higher the dew point. For instance, if an airport weather observation says the temperature is 85 degrees and the dew point is 50 degrees, the air would have to cool to 50 degrees before its humidity begins condensing. On a sticky, summer day, the temperature could also be 85, but the dew point could be 70 degrees. Condensation would begin when the temperature cools to 70 degrees, as it might do that night.

When condensation begins away from things such as grass or airplane wings, the tiny water drops form on minuscule particles of dust, smoke, or other materials floating in the air to create the tiny water drops that make up clouds. When the bottom of the cloud is touching the ground or is just a little above the ground, it's called mist or fog. The difference, as defined by the National Weather Service, is:

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 that are based at the Earth's surface, which reduces horizontal visibility to less than five-eighths of a statute mile.

More often than not, fog forms when air cools to the dew point. This happens when heat radiates away from the ground, humid air moves over cold water or ground, or humid air flows uphill into lower air pressure, which causes the air to expand and cool.

The ground cools the most at night when the sky is clear because on cloudy nights, the clouds absorb some of the heat leaving the earth and then radiate some of that heat back to the ground. Light winds also aid overnight cooling because strong winds create turbulence that mixes cold air from near the ground with warmer air higher up.

Fog that forms under these conditions is called radiation fog. It may also be called ground fog because it is usually less than about 300 feet thick. Radiation fog can begin forming early in the evening as the air begins to cool. 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. In general, if the temperature and dew point are within seven or so degrees of each other and the sky is mostly clear, you should be prepared for radiation fog.

If you don't know what to expect, a thin layer of radiation fog can give you a nasty surprise as you approach to land. Imagine flying over an airport that's covered by a 300-foot-deep fog layer. After looking down to clearly see the lights outlining the runway and taxiway, you enter the pattern and things still look good.

But, as you descend on final approach, the lights grow dimmer until you see nothing but white fog in the glare of your landing light.

What happened?

When you looked down from high above the airport, you were looking through only about 300 feet of fog. But, on final as you reached the top of the fog--assuming a normal 3-degree glideslope on your approach path--around 5,700 feet of fog was between you and the runway, with millions of tiny water drops blocking the view.

In hilly terrain, radiation fog tends to form in valleys because cool, dense air flows downhill. If there's a stream, pond, or lake in the low area, the fog could be thicker because water evaporates into the air, supplying more water to condense back into fog.

Meteorologists and pilots in the West talk about valley fog, especially in the winter. During the night, radiation fog forms in mountain valleys. During the day the low winter sun might not supply enough heat to evaporate the fog, which can hang around for days. Or, if enough solar warmth penetrates the fog to begin warming the ground, the fog can begin evaporating from the bottom, but low clouds remain over the airport all day until fog forms again at night.

Meteorologists use the word advection to describe the horizontal movement of air. Thus, when humid air is cooled to its dew point as it flows over cold ground or chilly water, the resulting fog is advection fog. Persistent, thick advection fog can form in the middle of the United States when the wind shifts to blow from the south, and air that's grown humid while sitting over the Gulf of Mexico flows in after a storm has covered the ground with snow.

Upslope fog can form on the Great Plains when southeast winds push humid air from Louisiana and Texas toward Colorado and Wyoming. As you travel from the Gulf Coast to the Plains just east of the Rocky Mountains, the elevation increases by about 5,000 feet. An elevation increase of this size would cool the air by more than 25 degrees, even if no other factors were cooling the air.

On a smaller scale, you sometimes see upslope fog forming on mountains. If you're looking up at such a mountain, you'd probably talk about the cloud that's covering the top. Hikers on the mountain would be talking about the fog that's hiding their view.

Fog can also form when water evaporates into the air, increasing its dew point--the more humidity in the air, the higher the dew point. This happens when rain or drizzle falls into air that's already close to the dew point and water evaporates from the falling precipitation. If the dew point is increased to the air's actual temperature, what's known as precipitation fog can form.

Talking about the different kinds of fog--radiation, advection, precipitation, and so forth--helps you to understand what's going on. But, fog is often much more complicated, with more than one of the processes at work to create dangerously low visibility. Also, fog can be very local, affecting airports relatively close to each other differently.

The COMET Web site notes that weather forecasting computer models have a hard time predicting fog and low clouds, making the job harder for the men and women who make aviation weather predictions. This, COMET says, "makes it all the more important that forecasters have a complete understanding of the processes involved in low ceiling and fog events and apply the latest techniques and approaches to the forecast problem."

It also means that pilots have to be especially alert when fog is forecast. Fortunately, student pilots and those with new pilot certificates usually do most of their flying during the day, while fog is most common at night or in the early morning.

While pilots who fly for recreation should find it easy to avoid potential fog, those who head for an aviation career by flying night cargo trips are sure to gain plenty of practical experience coping with poor visibility. Even as you gain experience, fog always has to be respected, as one of the most deadly crashes in history illustrates.

Fog covered the airport at Tenerife in Spain's Canary Islands on March 27, 1977, when a KLM Boeing 747 started its takeoff run without a clearance. The captain, who was one of the airline's most experienced pilots, thought he had been cleared to take off. The KLM 747 smashed into a Pan Am Boeing 747 that was taxiing on the runway--hidden by the fog. The crash killed 583 people on the two airplanes.

Even the most experienced pilots need to respect 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.