Fog Happens
by Dr. Sarah D. Oktay
Managing Director UMass Boston Nantucket Field Station
Well, the summer is slowly starting to wind down. Fortunately we have had several spectacular days of sun and heat and very little rain overall on Nantucket, although as I write this we are experiencing a thunderstorm. On Nantucket, fog is a normal part of life and trying to predict when to expect it and how long it may last is an essential part of travel to and from the island. In fact, Nantucket acquired one of its most often used nicknames, the “Grey Lady” due to its frequent blanket of fog.
Fog is essentially a cloud on land and occurs when water droplets condense out of moist air when the air temperature is equal to or close to the dew point. According to the American Meteorological Society (AMS), fog forms when the difference between temperature and dew point is generally less than 2.5 °C or 4 F. The dewpoint is the temperature at which the water vapor is at its saturation point within a parcel of air and condensation begins. Like the last person squeezed into a phone booth; the water droplets reach their max capacity and boom, you have fog (or people falling out of a phone booth, here fog is way more common).
Fog is distinguished from mist only by its density, as expressed in the resulting decrease in visibility: Fog reduces visibility to less than 1 km (5/8 statute mile), whereas mist reduces visibility to no less than 1 km (5/8 statute mile). Patchy fog may also occur, particularly where air parcels with different temperature and moisture contents begin to interact, which sometimes make these definitions difficult to apply in practice and sometimes can make their prediction difficult. Fogs of all types originate when the temperature and dewpoint of the air become identical (or nearly so). This may occur through cooling of the air to a little beyond its dewpoint (producing advection fog, radiation fog or upslope fog), or by adding moisture and thereby elevating the dewpoint (producing steam fog or frontal fog). When flying into an out of an airport; pilots are concerned with two things; the ceiling (height of the fog) and the visibility in miles.
Fog seldom forms when the dewpoint spread is greater than 4°F. According to U.S. weather observing practice, fog that hides less than 0.6 of the sky is called ground fog. If fog is so shallow that it is not an obstruction to vision at a height of 6 ft above the surface, it is called “shallow fog.” In aviation weather observations fog is encoded F, and ground fog GF. Fog is easily distinguished from haze by its higher relative humidity (near 100%, having physiologically appreciable dampness) and gray color. Haze does not contain activated droplets larger than the critical size according to Köhler theory. If you happen to be taking a cloud physics test this week, Köhler theory describes the process in which water vapor condenses and forms liquid cloud drops, and is based on equilibrium thermodynamics. Mist may be considered an intermediate between fog and haze; its particles are smaller (a few μm maximum) in size, it has lower relative humidity than fog, and does not obstruct visibility to the same extent as mentioned above. Near industrial areas, fog is often mixed with smoke, and this combination has been known as smog. However, fog droplets are usually absent in photochemical smog, which only contains unactivated (not forming actual water droplets) haze droplets.
Now that you know more than you probably want to about fog basics, let’s discuss the different types of fog, one of which at almost any given time might be affecting Nantucket.
The following information is adapted from the National Weather Service web site (http://www.crh.noaa.gov/jkl/?n=fog_types). Radiation Fog is a type of fog that forms at night under clear skies with calm winds when heat absorbed by the earth’s surface during the day is radiated into space. As the earth’s surface continues to cool, provided a deep enough layer of moist air is present near the ground, the humidity will reach 100% and fog will form. Radiation fog varies in depth from 3 feet to about 1,000 feet and is always found at ground level and usually remains stationary. This type of fog can reduce visibility to near zero at times and make driving very hazardous. Fog in cold temperatures can lead to hoar frost as the particles sublimate or precipitate out onto objects as the earth cools overnight.
Valley fog is a type of radiation fog that is very common in the mountains of eastern Kentucky. When air along ridgetops and the upper slopes of mountains begins to cool after sunset, the air becomes dense and heavy and begins to drain down into the valley floors below. As the air in the valley floor continues to cool due to radiational cooling, the air becomes saturated and fog forms. Valley fog can be very dense at times and make driving very hazardous due to reduced visibility. This type of fog tends to dissipate very quickly once the sun comes up and starts to evaporate the fog layer.
The type of fog we normally deal with here on island is advective fog which occurs when warm moist air moves (or advects) over a cold patch. Sea fogs (sometimes known as haar or fret) are always advection fogs, because the oceans don’t radiate heat in the same way as land and so never cool sufficiently to produce radiation fog. Sea fog is the fog that forms at sea (shockingly obvious) when warm air associated with warm current drifts over a cold current and condensation takes place. Sometimes such fogs are drawn inland by low pressure, as often occurs on the Pacific coast of North America. Advection fog may also form when moist maritime, or ocean, air drifts over a cold inland area. The presence of salt particles in the air near coastlines where waves break can provide condensation particles (nuclei) for water vapor to condense around. Condensation on salt particles has been observed to occur at humidities as low as 70%, thus fog can occur even in relatively dry air in suitable locations such as the California coast. Typically, such lower humidity fog is preceded by a transparent mistiness along the coastline as condensation competes with evaporation, a phenomenon that is typically noticeable by beachgoers in the afternoon. Another recently-discovered source of condensation nuclei for coastal fog is kelp. Researchers have found that under stress (intense sunlight, strong evaporation, etc.), kelp release particles of iodine which in turn become nuclei for condensation of water vapor.
Upslope fog forms when light winds push moist air up a hillside or mountainside to a level where the air becomes saturated and condensation occurs. This type of fog usually forms a good distance from the peak of the hill or mountain and covers a large area. Upslope fog occurs in all mountain ranges in North America. This usually occurs during the winter months, when cold air behind a cold front drifts westward and encounters the eastward facing slopes of the Rocky Mountains. As the cold, moist air rises up the slopes of the mountains, condensation occurs and extensive areas of fog form on the lower slopes of the mountains.
Ice fog forms when the air temperature is well below freezing and is composed entirely of tiny ice crystals that are suspended in the air. Ice fog will only be witnessed in cold Arctic/Polar air. Generally the temperature will be 14 F or colder in order for ice fog to occur. Freezing fog occurs when the water droplets that the fog is composed of are "supercooled." Supercooled water droplets remain in the liquid state until they come into contact with a surface upon which they can freeze. As a result, any object the freezing fog comes into contact with will become coated with ice. The same thing happens with freezing rain or drizzle.
Nantucket sometimes experience evaporation or mixing fog in the fall. This type of fog forms when sufficient water vapor is added to the air by evaporation and the moist air mixes with cooler, relatively drier air. The two common types are steam fog and frontal fog. Sometimes you will see steam fog which forms when cold air moves over warm water and mixes with the warm moist air then cools until its humidity reaches 100% and fog forms. This type of fog takes on the appearance of wisps of smoke rising off the surface of the water that you can often see over ponds in the fall or spring. Sea smoke is a type of evaporation fog I have witnessed on island that can be a freezing fog. The other type of evaporation fog is known as frontal fog or precipitation fog. This type of fog forms when warm raindrops evaporate into a cooler drier layer of air near the ground. Once enough rain has evaporated into the layer of cool surface, the humidity of this air may eventually reach 100% and, you got it, fog forms. So there are a lot of different ways that fog can form and really all you need is a lot of moist air and two different temperatures to create an air current.
Wikipedia (http://en.wikipedia.org/wiki/Fog) tells us that the foggiest parts of the world are very near by on the Grand Banks off the island of Newfoundland, which is the meeting place of the cold Labrador Current from the north and the much warmer Gulf Stream from the south. Some of the foggiest land areas in the world include the little town of Argentia, Newfoundland and Point Reyes, California, each with over 200 foggy days per year. Even in generally warmer southern Europe, thick fog and localized fog is often found in lowlands and valleys, such as the lower part of the Po Valley and the Arno and Tiber valleys in Italy or Ebro Valley in northeastern Iberia, as well as on the Swiss plateau, especially in the Seeland area, in late autumn and winter. Other notably foggy areas include coastal Chile (in the south), coastal Namibia, and the Severnaya Zemlya islands. According to the World Meteorological Association, the 10 foggiest cities in Europe in order (London does not make the list) are: Milan, Rome, Zurich, Switzerland, Amsterdam, Copenhagen, Denmark, Prague, Czech Republic, Frankfurt, Germany, Helsinki, Finland, Lisbon, Portugal, and Munich, Germany.
You might recall a July 29th 2010 article in Yesterday’s island (www.yesterdaysisland.com/2010/science/13-hot.php) when we discussed frost pockets in a desperate attempt to cool off by thinking cool thoughts. Frost pockets lead to “foggy bottoms” which exist around Nantucket and are simply cooler depressions in the ground where fog can collect as the cold moist air is denser than the surrounding warmer air.
It’s hard to believe there are foggier places in America, but the geography map from City University in NY shows that we are in the 2nd foggiest zone: www.geography.hunter.cuny.edu/~tbw/wc.notes/5.cond.precip/clouds/fog_map.htm
Fun fog facts from the San Francisco Chronicle’s online site (http://articles.sfgate.com/2009-07-08/entertainment/17216111_1_california-central-valley-s-highway-czech-republic-foggiest): “On occasion at Candlestick Park, the San Francisco Giants or their opponents had to be pulled from the field because they couldn't spot the white baseball in the fog. In 2008, the California Highway Patrol spent $12 million on the first Fog Pilot detection system that alerts drivers along a 12-mile stretch of the California Central Valley's Highway 99 if conditions are dangerous because of fog. In 1579, British explorer Sir Francis Drake might have discovered San Francisco Bay if not for the fog that obscured his view.”
From Nantucket.net facts page (www.nantucket.net/links/facts.php) about Nantucket we learn about Sheep Storms: “Sheep storms were what the islanders called those periods of intense fogginess which often preside over the moors in late June and early July. In the 1800s, the islanders knew these days were coming, and they also knew that after several days the fog would drench the thick coats of the many sheep grazing on the moors (in pasturing sections called the Sheep Commons). They also knew that after the fog inevitably came hot, sunny days during which the coats would dry. That was the time for shearing, which meant the famous Sheep-Shearing festivals, with fun and games and food for one and all.“
Fog has caused many a tragedy at sea nearby. From the Town of Nantucket web site we can find a snippet from a long and enduring story of the loss of the Andréa Doria; “On the night of July 25, 1956, in heavy fog roughly 50 miles south of Nantucket, the Italian ocean liner SS Andrea Doria, inbound to New York, was rammed by the SS Stockholm bound for Europe. Survivors including the Gifford family of Nantucket were put on rescue ships and some were flown to Nantucket before moving on to hospitals in Boston. The Andrea Doria sank the following morning and now rests 250 feet below the surface.”
I loved this simple page of crew quotes from the Lightship Nantucket LV-112 which many of you might have seen in the harbor this past week. (http://www.nantucketlightshiplv-112.org/crew_quotes.htm). I found this quote by Bernard Webber, former LV-112 crew member (1958-1960) especially vivid; “The only terror I felt was when on Nantucket Station in rough foggy weather a Radar Target would be observed headed directly towards the Lightship as it got close you could hear its engines and soon out of the fog so close you could spit on it would come one of the great liners sailing the seas at the time like the S.S. United States or S.S. France etc."
Switching from safety at sea to safety on land, some folks might not remember that you should always use your low beams (and not your “brights” or high beams) when driving in fog. Fog droplets form multiple reflective surfaces that disperse high beams and reduce visibility. Last but not least, for an excellent description of our island companion, advective fog, check out Blair Perkins’ blog at: http://nantucket.net/blog_waves/?p=49
