Fog

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Fog

Introduction

In meteorology, fog is defined as a mass of liquid water vapor or solid ice crystals that has condensed and suspended itself in the lower layers of the atmosphere just above the surface of Earth. Fog is different from a stratus cloud only in that the cloud's base has dropped down so that it is in contact or in close proximity with the ground, whether it is on level ground, on the top of a hill, or within a valley.

Because of fog's location near the ground, it often causes reduced visibility of a normally clear sky. Specifically, meteorologists define fog as a cloud that reduces visibility on the surface of Earth to less than 0.6 mi (1 km). They have found that fog forms when the difference between the dew point and the outside temperature is 5°F (3°C) or less. When the relative humidity is 100%, fog is much more likely to form.

Historical Background and Scientific Foundations

Fog is generally classified as four types, depending on how it is formed: advection, precipitation, radiation, and upslope. Advection fog is formed whenever wind or

current (formally called advection) of warm, moist air travels over a relatively cooler body of land or water. Precipitation fog, also called frontal fog, often develops when snowflakes or raindrops descend through an atmospheric layer that is cooler and drier. It also occurs frequently during the passage of warm and cold fronts, when the lower air near the surface is much different in temperature from the upper air.

Radiation fog, which forms only over land after sunset, is caused by the cooling of Earth—that is, land cools more rapidly than water; as a consequence, fog is formed over land. It often occurs in fall and early winter months. Radiation fog usually develops up to 4 feet (just over 1 meter) in depth, but winds and other atmospheric events can cause it to be larger in depth. Upslope fog is formed when air is uniformly cooled by rising and enlarging wind currents, such as when the wind blows up a mountain slope (what is called orographic lift), causing the moisture within it to condense.

Impacts and Issues

Changes in local and global climate make forecasting of fog difficult at best. Further meteorological studies are needed to decide if currently used forecasting tools provide accurate guidance to meteorologists and other professionals in various pursuits throughout the world. For instance, because aviation pilots and air traffic controllers need to be able to forecast the occurrence of fog when dealing with airplane flights, the Federal Aviation Administration (FAA) authorizes studies to find better methods and devices to assist these professionals.

According to the University Corporation for Atmospheric Research, the FAA provides research into soil and air temperature, cloud ceiling, visibility, rainfall, moisture levels, and other critical variables to better understand fog. Some of the research involves weather balloons and automated sensing systems placed in key locations in order to determine the behavior of fog.

The warming and cooling of Earth, locally and globally, is controlled to a large part by fog and clouds. They block the heat brought by the sun in the form of sunlight, reflecting it back into space. Such action helps to cool Earth. However, heat already beneath fog and clouds can become trapped by its water particles, which can quickly heat up the planet. Whether Earth becomes warmer or cooler depends in part by the frequency and amount of fog and clouds. A long-term change in their patterns could have a drastic impact on Earth's weather, either making it much colder or warmer.

Atmospheric physicist Anthony D. Del Genio, who is associated with the U.S. National Aeronautics and Space Administration's Goddard Institute for Space Studies, states that even a small change in cloud cover can dramatically change the temperature of Earth's atmosphere. Del Genio studies the relationship between water vapor in the atmosphere and meteorological phenomena. He researches whether a warmer climate will result in cloudier or clearer skies; more humid or drier atmosphere; and more or less frequent and/or violent storms. As the world adds larger concentrations of greenhouse gases to the atmosphere, Del Genio and many other scientists around the world are working on mathematical computer models based on satellite and ground-based observations to determine Earth's changing climate.

WORDS TO KNOW

BLACK CARBON: A type of aerosol (small, airborne particle) consisting mostly of carbon: includes soot, charcoal, and some other dark organic particles.

DEW POINT: The temperature to which air must be cooled for saturation to occur, exclusive of air pressure or moisture content change. At that temperature, dew begins to form, and water vapor condenses into liquid.

FOSSIL FUELS: Fuels formed by biological processes and transformed into solid or fluid minerals over geological time. Fossil fuels include coal, petroleum, and natural gas. Fossil fuels are non-renewable on the timescale of human civilization, because their natural replenishment would take many millions of years.

GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.

METEOROLOGY: The science that deals with Earth's atmosphere and its phenomena and with weather and weather forecasting.

OROGRAPHY: Branch of geology that deals with the arrangement and character of land altitude variations (hills and mountains); also, the average land altitude over a given region. In computer models of climate, orography in the second sense defines the lower bound of the atmosphere over land.

STRATUS CLOUD: Extensive, layer-like cloud form (“stratum” means layer in Latin). Stratus clouds occur worldwide but play a particularly important role in Arctic climate, where they are prevalent and affect vertical exchanges of heat, moisture, and momentum; this, in turn, affects global climate.

WATER VAPOR: The most abundant greenhouse gas, it is the water present in the atmosphere in gaseous form. Water vapor is an important part of the natural greenhouse effect. Although humans are not significantly increasing its concentration, it contributes to the enhanced greenhouse effect because the warming influence of greenhouse gases leads to apositive water vapor feed back. In addition to its role as a natural greenhouse gas, water vapor plays an important role in regulating the temperature of the planet because clouds form when excess water vapor in the atmosphere condenses to form ice and water droplets and precipitation.

Other scientists are working at research to determine how artificially made substances affect fog and clouds. For instance, black carbon is a primary ingredient in soot left over when fossil fuels are not completely combusted (burned). When black carbon gets into the atmosphere, it becomes easily trapped in water vapor. When this action happens, scientists contend that fog and clouds absorb more heat from Earth's surface than normal, causing warmer temperatures. Such research to learn more about how human-made substances affect fog and clouds is important to Earth's evolving climate.

BIBLIOGRAPHY

Books

Allaby, Michael. Fog, Smog, and Poisoned Rain. New York: Facts on File, 2003.

Entertainment Services and Technology Association (ESTA). Introduction to Modern Atmospheric Effects. New York: ESTA, 2000.

Lutgens, Frederick K. The Atmosphere: An Introduction to Meteorology. Upper Saddle River, NJ: Prentice Hall, 2004.

Pretor-Pinney, Gavin. The Cloudspotter's Guide: The Science, History, and Culture of Clouds. New York: Berkley Publishing Group, 2006.

Periodicals

McConnell, J.C., et al. “20th-Century Industrial Black Carbon Emissions Altered Arctic Climate Forcing.” Science 317 (September 7, 2007): 1381-1384.

Web Sites

“Clouds, Clouds, Clouds.” University Corporation for Atmospheric Research. < http://www.ucar.edu/news/features/clouds> (accessed November 4, 2007).

Jacobson, Mark Z. “Attribution of Regional and Global Climate Change: Relative Effects of Fossil-Fuel Soot, Methane, Other Greenhouse Gases and Particles, and Urbanization.” The Smithsonian/ NASA Astrophysics Data System. < http://adsabs.harvard.edu/abs/2006AGUFM.A43A0110J> (accessed November 4, 2007).

Climate Change: In Context