What is atmospheric visibility?
Atmospheric visibility is basically a measure of how clear the air is. It is often defined in terms of a visual range (how far an observer can see) and is expressed in nautical miles, miles or kilometers. Precipitation, clouds, and air pollution can reduce atmospheric visibility by increasing the number of small particles in the air that absorb and scatter light.
Gulf of Maine atmospheric visibility
Fog (clouds at or near ground or sea level) is a common sight in the Gulf of Maine and its coastal regions. Fog forms under a variety of conditions; however, each results from same process: air cools and condenses water vapor. Advection fog forms when warm moist air hits cooler water or land. As the warmer air cools, water vapor condenses and fog is formed. Advection fog is particularly common in the gulf during summer. The satellite image at right shows foggy conditions over the Gulf of Maine and the Gulf Stream's deep blue waters. When warm southerly winds that drive the Gulf Stream meet cool Gulf of Maine waters, the air cools and water vapor in the air condenses to form fog.
"Sea smoke" is another type of fog that forms regularly in the Gulf of Maine. During fall, gulf surface waters are warm because of summer heating. Fall winds often blow cold dry air over these warm waters; this process adds moisture and heat to air just above the sea surface. When this warm, moist air mixes with cooler air above it, a fog can form that "hugs" the sea surface. "Sea smoke" conditions can be described as a photographer's "dream" and a mariner's "nightmare."
Gulf of Maine visibility can also be reduced by air pollution from a variety of natural and manmade sources. These can include smoke and soot from wildfires, windblown dust, and emissions from vehicles, utility and industrial fuel burning, and manufacturing operations. The satellite image at left shows the Gulf of Maine and North Atlantic under a haze produced by power plants and vehicle emissions.
Note: All satellite images on this page were taken by the SeaWiFS instrument and are available from NASA's "Earth Observatory" website.
Visibility sensors on GoMOOS moorings work by shining infrared (IR) light into the air above ocean's surface and then detecting the amount the IR light is scattered. At this wavelength (i.e., 880 nanometers), the amount of light scattered forward (i.e., the direction of the IR light's propagation) is proportional to the amount of water vapor in the air. From this estimate of the amount of water vapor in the air, atmospheric visibility is derived in miles or kilometers.
It is well established that Gulf of Maine visibility can significantly affect boating conditions. This is the reason why there are over 60 lighthouses on Maine's coast alone! In addition, ships utilize their own radars to detect other ships or landmasses in their paths.
The clarity of the atmosphere also affects Earth's heat budget. Under low visibility conditions (e.g., clouds, haze), the sun's heat may be reflected and absorbed before reaching ground or sea level; this serves to cool the lower atmosphere. At the same time, low clouds or haze help to trap heat that emanates from the Earth's surface; this serves to warm the lower atmosphere. Many things complicate this simple scenario: including cloud structure and height. Much research is being dedicated to better understand relationships between global temperature and atmospheric conditions.
Beyond the Gulf: How might fog form away from coasts?
Radiation fog is commonly seen on cool, clear nights when there is little wind. Why? Cloudless conditions allow a large amount of heat to be radiated from land. As the ground cools, so does air just above it. If the air has a lot of moisture, it will condense and form fog, especially in the absence of wind mixing. Nighttime rain may also promote the formation of fog. Rain increases the amount of moisture on the ground and in the air above it; cool temperatures can cause condensation and fog formation.
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- Environmental Protection Agency. (2002). What is Visibility Impairment? URL: http://www.epa.gov/air/visibility/what.html.
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