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81-417: Diamond dust is similar to fog in that it is a cloud based at the surface; however, it differs from fog in two main ways. Generally fog refers to a cloud composed of liquid water (the term ice fog usually refers to a fog that formed as liquid water and then froze, and frequently seems to occur in valleys with airborne pollution such as Fairbanks , Alaska , while diamond dust forms directly as ice). Also, fog

162-620: A cloud seed . More and more water accumulates on the seed until a visible cloud is formed. In the case of ship tracks, the cloud seeds are stretched over a long narrow path where the wind has blown the ship's exhaust, so the resulting clouds resemble long strings over the ocean. The warming caused by human-produced greenhouse gases has been somewhat offset by the cooling effect of human-produced aerosols. In 2020, regulations on fuel significantly cut sulfur dioxide emissions from international shipping by approximately 80%, leading to an unexpected global geoengineering termination shock. Aerosols in

243-717: A cloud seed . More and more water accumulates on the seed until a visible cloud is formed. In the case of ship tracks, the cloud seeds are stretched over a long narrow path where the wind has blown the ship's exhaust, so the resulting clouds resemble long strings over the ocean. The warming caused by human-produced greenhouse gases has been somewhat offset by the cooling effect of human-produced aerosols. In 2020, regulations on fuel significantly cut sulfur dioxide emissions from international shipping by approximately 80%, leading to an unexpected global geoengineering termination shock. The liquid or solid particles in an aerosol have diameters typically less than 1 μm . Larger particles with

324-409: A histogram with the area of each bar representing the proportion of particles in that size bin, usually normalised by dividing the number of particles in a bin by the width of the interval so that the area of each bar is proportionate to the number of particles in the size range that it represents. If the width of the bins tends to zero , the frequency function is: where Therefore, the area under

405-452: A slope (called orographic lift ), adiabatically cooling it as it rises and causing the moisture in it to condense. This often causes freezing fog on mountaintops, where the cloud ceiling would not otherwise be low enough. Valley fog forms in mountain valleys , often during winter. It is essentially a radiation fog confined by local topography and can last for several days in calm conditions. In California's Central Valley , valley fog

486-466: A convective phenomenon, resulting in fog that can be very dense and deep and looks fluffy from above. Arctic sea smoke is similar to sea smoke but occurs when the air is very cold. Instead of condensing into water droplets, columns of freezing, rising, and condensing water vapor is formed. The water vapor produces the sea smoke fog and is usually misty and smoke-like. Garúa fog near the coast of Chile and Peru occurs when typical fog produced by

567-452: A principal source of water, particularly in otherwise desert climes, as along many African coastal areas. Some coastal communities use fog nets to extract moisture from the atmosphere where groundwater pumping and rainwater collection are insufficient. Fog can be of different type according to climatic conditions. Artificial fog is man-made fog that is usually created by vaporizing a water- and glycol - or glycerine -based fluid. The fluid

648-534: A short wavelength. To transmit a high frequency wave, air must move back and forth very quickly. Short-wavelength high-pitched sound waves are reflected and refracted by many separated water droplets, partially cancelling and dissipating their energy (a process called " damping "). In contrast, low pitched notes, with a low frequency and a long wavelength, move the air less rapidly and less often, and lose less energy to interactions with small water droplets. Low-pitched notes are less affected by fog and travel further, which

729-506: A significant effect on Earth's climate: volcanic, desert dust, sea-salt, that originating from biogenic sources and human-made. Volcanic aerosol forms in the stratosphere after an eruption as droplets of sulfuric acid that can prevail for up to two years, and reflect sunlight, lowering temperature. Desert dust, mineral particles blown to high altitudes, absorb heat and may be responsible for inhibiting storm cloud formation. Human-made sulfate aerosols , primarily from burning oil and coal, affect

810-506: A significant effect on Earth's climate: volcanic, desert dust, sea-salt, that originating from biogenic sources and human-made. Volcanic aerosol forms in the stratosphere after an eruption as droplets of sulfuric acid that can prevail for up to two years, and reflect sunlight, lowering temperature. Desert dust, mineral particles blown to high altitudes, absorb heat and may be responsible for inhibiting storm cloud formation. Human-made sulfate aerosols , primarily from burning oil and coal, affect

891-429: A significant settling speed make the mixture a suspension , but the distinction is not clear. In everyday language, aerosol often refers to a dispensing system that delivers a consumer product from a spray can . Diseases can spread by means of small droplets in the breath , sometimes called bioaerosols . Aerosol is defined as a suspension system of solid or liquid particles in a gas. An aerosol includes both

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972-466: A snowpack can continue to generate advection fog at elevated velocities up to 80 km/h (50 mph) or more – this fog will be in a turbulent, rapidly moving, and comparatively shallow layer, observed as a few centimetres/inches in depth over flat farm fields, flat urban terrain and the like, and/or form more complex forms where the terrain is different such as rotating areas in the lee of hills or large buildings and so on. Fog formed by advection along

1053-508: Is IC. Note that images are different from the naked eye in that they capture out-of-focus crystals which are shown as large, blurred objects. The Science Behind Diamond Dust: How It Reflects Solar Radiation Fog Fog is a visible aerosol consisting of tiny water droplets or ice crystals suspended in the air at or near the Earth 's surface. Fog can be considered a type of low-lying cloud usually resembling stratus and

1134-410: Is a dense-enough cloud to significantly reduce visibility, while diamond dust is usually very thin and may not have any effect on visibility (there are far fewer crystals in a volume of air than there are droplets in the same volume with fog). Because mist is often classified to be more transparent than fog, diamond dust has often been referred to as ice mist . However, diamond dust still can often reduce

1215-436: Is a stable cloud deck which tends to form when a cool, stable air mass is trapped underneath a warm air mass. Fog normally occurs at a relative humidity near 100%. This occurs from either added moisture in the air, or falling ambient air temperature. However, fog can form at lower humidities and can sometimes fail to form with relative humidity at 100%. At 100% relative humidity, the air cannot hold additional moisture, thus

1296-553: Is any kind of fog where the droplets have frozen into extremely tiny crystals of ice in midair. Generally, this requires temperatures at or below −35 °C (−31 °F), making it common only in and near the Arctic and Antarctic regions. It is most often seen in urban areas where it is created by the freezing of water vapor present in automobile exhaust and combustion products from heating and power generation. Urban ice fog can become extremely dense and will persist day and night until

1377-519: Is common as a warm front passes over an area with significant snow-pack. It is most common at sea when moist air encounters cooler waters, including areas of cold water upwelling , such as along the California coast . A strong enough temperature difference over water or bare ground can also cause advection fog. Although strong winds often mix the air and can disperse, fragment, or prevent many kinds of fog, markedly warmer and humid air blowing over

1458-422: Is created by cold air passing over warmer water or moist land. It may cause freezing fog or sometimes hoar frost . This situation can also lead to the formation of steam devils , which look like their dust counterparts . Lake-effect fog is of this type, sometimes in combination with other causes like radiation fog. It tends to differ from most advective fog formed over land in that it is (like lake-effect snow )

1539-409: Is dense enough to be illuminated by light that passes through gaps in a structure or tree, but thin enough to let a large quantity of that light pass through to illuminate points further on. As a result, object shadows appear as "beams" oriented in a direction parallel to the light source. These voluminous shadows are created the same way as crepuscular rays , which are the shadows of clouds. In fog, it

1620-586: Is derived from the Shoshone word paγi̵nappi̵h , which means "cloud". In The Old Farmer's Almanac , in the calendar for December, the phrase "Beware the Pogonip" regularly appears. In his anthology Smoke Bellew , Jack London describes a pogonip which surrounded the main characters, killing one of them. The phenomenon is common in the inland areas of the Pacific Northwest, with temperatures in

1701-406: Is fog that obscures less than 60% of the sky and does not extend to the base of any overhead clouds. However, the term is usually a synonym for shallow radiation fog; in some cases the depth of the fog is on the order of tens of centimetres over certain kinds of terrain with the absence of wind. Advection fog occurs when moist air passes over a cool surface by advection (wind) and is cooled. It

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1782-529: Is formed as water vapor condenses on bits of salt. Fog is similar to, but less transparent than, mist . The term fog is typically distinguished from the more generic term cloud in that fog is low-lying, and the moisture in the fog is often generated locally (such as from a nearby body of water, like a lake or ocean, or from nearby moist ground or marshes ). By definition, fog reduces visibility to less than 1 km (0.62 mi), whereas mist causes lesser impairment of visibility. For aviation purposes in

1863-401: Is heavily influenced by nearby bodies of water, topography, and wind conditions. In turn, fog affects many human activities, such as shipping, travel, and warfare. Fog appears when water vapor (water in its gaseous form) condenses. During condensation , molecules of water vapor combine to make tiny water droplets that hang in the air. Sea fog , which shows up near bodies of saline water ,

1944-415: Is injected into a heated metal block which evaporates quickly. The resulting pressure forces the vapor out of a vent. Upon coming into contact with cool outside air, the vapor condenses in microscopic droplets and appears as fog. Such fog machines are primarily used for entertainment applications . The presence of fog has often played a key role in historical events, such as strategic battles. One example

2025-447: Is not of the "frostless" or "frost-free" type. The term "freezing fog" may also refer to fog where water vapor is super-cooled , filling the air with small ice crystals similar to very light snow. It seems to make the fog "tangible", as if one could "grab a handful". In the western United States , freezing fog may be referred to as pogonip . It occurs commonly during cold winter spells, usually in deep mountain valleys. The word pogonip

2106-503: Is often associated with halos , such as sun dogs , light pillars , etc. Like the ice crystals in cirrus or cirrostratus clouds , diamond dust crystals form directly as simple hexagonal ice crystals — as opposed to freezing drops — and generally form slowly. This combination results in crystals with well defined shapes - usually either hexagonal plates or columns - which, like a prism, can reflect and/or refract light in specific directions. While diamond dust can be seen in any area of

2187-405: Is often referred to as tule fog . Sea fog (also known as haar or fret ) is heavily influenced by the presence of sea spray and microscopic airborne salt crystals. Clouds of all types require minute hygroscopic particles upon which water vapor can condense. Over the ocean surface, the most common particles are salt from salt spray produced by breaking waves. Except in areas of storminess,

2268-437: Is solid objects that cast shadows. Sound typically travels fastest and farthest through solids, then liquids, then gases such as the atmosphere. Sound is affected during fog conditions due to the small distances between water droplets, and air temperature differences. Though fog is essentially liquid water, the many droplets are separated by small air gaps. High-pitched sounds have a high frequency, which in turn means they have

2349-662: Is the 1776 Battle of Long Island when American General George Washington and his command were able to evade imminent capture by the British Army, using fog to conceal their escape. Another example is D-Day (6 June 1944) during World War II , when the Allies landed on the beaches of Normandy, France during fog conditions. Both positive and negative results were reported from both sides during that battle, due to impaired visibility. Under "[ ^ "Federal Meteorological Handbook Number 1: Chapter 8 – Present Weather" (PDF). Office of

2430-413: Is why foghorns use a low-pitched tone. A fog can be caused by a temperature inversion where cold air is pooled at the surface which helped to create the fog, while warmer air sits above it. The inverted boundary between cold air and warm air reflects sound waves back toward the ground, allowing sound that would normally radiate out escaping into the upper atmosphere to instead bounce back and travel near

2511-511: The mass concentration ( M ), defined as the mass of particulate matter per unit volume, in units such as μg/m . Also commonly used is the number concentration ( N ), the number of particles per unit volume, in units such as number per m or number per cm . Particle size has a major influence on particle properties, and the aerosol particle radius or diameter ( d p ) is a key property used to characterise aerosols. Aerosols vary in their dispersity . A monodisperse aerosol, producible in

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2592-559: The Seeland area, in late autumn and winter. Other notably foggy areas include coastal Chile (in the south); coastal Namibia ; Nord, Greenland ; and the Severnaya Zemlya islands. Redwood forests in California receive approximately 30–40% of their moisture from coastal fog by way of fog drip . Change in climate patterns could result in relative drought in these areas. Some animals, including insects, depend on wet fog as

2673-522: The power function distribution , occasionally applied to atmospheric aerosols; the exponential distribution , applied to powdered materials; and for cloud droplets, the Khrgian–Mazin distribution. For low values of the Reynolds number (<1), true for most aerosol motion, Stokes' law describes the force of resistance on a solid spherical particle in a fluid. However, Stokes' law is only valid when

2754-427: The skewness associated with a long tail of larger particles. Also for a quantity that varies over a large range, as many aerosol sizes do, the width of the distribution implies negative particles sizes, which is not physically realistic. However, the normal distribution can be suitable for some aerosols, such as test aerosols, certain pollen grains and spores . A more widely chosen log-normal distribution gives

2835-514: The terminal velocity of a particle undergoing gravitational settling in still air. Neglecting buoyancy effects, we find: where The terminal velocity can also be derived for other kinds of forces. If Stokes' law holds, then the resistance to motion is directly proportional to speed. The constant of proportionality is the mechanical mobility ( B ) of a particle: A particle traveling at any reasonable initial velocity approaches its terminal velocity exponentially with an e -folding time equal to

2916-542: The 10 to 30 °F (−12 to −1 °C) range. The Columbia Plateau experiences this phenomenon most years during temperature inversions , sometimes lasting for as long as three weeks. The fog typically begins forming around the area of the Columbia River and expands, sometimes covering the land to distances as far away as La Pine, Oregon , almost 150 miles (240 km) due south of the river and into south central Washington. Frozen fog (also known as ice fog )

2997-490: The 20 μm range show a particularly long persistence time in air conditioned rooms due to their "jet rider" behaviour (move with air jets, gravitationally fall out in slowly moving air); as this aerosol size is most effectively adsorbed in the human nose, the primordial infection site in COVID-19 , such aerosols may contribute to the pandemic. Aerosol particles with an effective diameter smaller than 10 μm can enter

3078-442: The California coastline is propelled onto land by one of several processes. A cold front can push the marine layer coast-ward, an occurrence most typical in the spring or late fall. During the summer months, a low-pressure trough produced by intense heating inland creates a strong pressure gradient, drawing in the dense marine layer. Also, during the summer, strong high pressure aloft over the desert southwest, usually in connection with

3159-455: The Earth's atmosphere can influence its climate, as well as human health. Volcanic eruptions release large amounts of sulphuric acid , hydrogen sulfide and hydrochloric acid into the atmosphere. These gases represent aerosols and eventually return to earth as acid rain , having a number of adverse effects on the environment and human life. When aerosols absorb pollutants, it facilitates

3240-527: The Federal Coordinator for Meteorology. 1 September 2005. pp. 8–1, 8–2. Retrieved 9 October 2010. ] " …. Actually use the following link- http://www.ofcm.gov/publications/fmh/FMH1/FMH1.pdf and proceed to Chapter 8, etc. Aerosol An aerosol is a suspension of fine solid particles or liquid droplets in air or another gas . Aerosols can be generated from natural or human causes . The term aerosol commonly refers to

3321-496: The United Kingdom, a visibility of less than 5 km (3.1 mi) but greater than 999 m (3,278 ft) is considered to be mist if the relative humidity is 95% or greater; below 95%, haze is reported. Fog forms when the difference between air temperature and dew point is less than 2.5  °C (4.5  °F ). Fog begins to form when water vapor condenses into tiny water droplets that are suspended in

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3402-442: The air will become supersaturated if additional moisture is added. Fog commonly produces precipitation in the form of drizzle or very light snow. Drizzle occurs when the humidity attains 100% and the minute cloud droplets begin to coalesce into larger droplets. This can occur when the fog layer is lifted and cooled sufficiently, or when it is forcibly compressed from above by descending air. Drizzle becomes freezing drizzle when

3483-518: The air. Some examples of ways that water vapor is condensed include wind convergence into areas of upward motion; precipitation or virga falling from above; daytime heating evaporating water from the surface of oceans, water bodies, or wet land; transpiration from plants; cool or dry air moving over warmer water; and lifting air over mountains. Water vapor normally begins to condense on condensation nuclei such as dust, ice, and salt in order to form clouds. Fog, like its elevated cousin stratus ,

3564-454: The appearance of haze to almost zero visibility. Many lives are lost each year worldwide from accidents involving fog conditions on the highways, including multiple-vehicle collisions . The aviation travel industry is affected by the severity of fog conditions. Even though modern auto-landing computers can put an aircraft down without the aid of a pilot, personnel manning an airport control tower must be able to see if aircraft are sitting on

3645-451: The behavior of clouds. Although all hydrometeors , solid and liquid, can be described as aerosols, a distinction is commonly made between such dispersions (i.e. clouds) containing activated drops and crystals, and aerosol particles. The atmosphere of Earth contains aerosols of various types and concentrations, including quantities of: Aerosols can be found in urban ecosystems in various forms, for example: The presence of aerosols in

3726-431: The behavior of clouds. When aerosols absorb pollutants, it facilitates the deposition of pollutants to the surface of the earth as well as to bodies of water. This has the potential to be damaging to both the environment and human health. Ship tracks are clouds that form around the exhaust released by ships into the still ocean air. Water molecules collect around the tiny particles ( aerosols ) from exhaust to form

3807-439: The bronchi, while the ones with an effective diameter smaller than 2.5 μm can enter as far as the gas exchange region in the lungs, which can be hazardous to human health. For a monodisperse aerosol, a single number—the particle diameter—suffices to describe the size of the particles. However, more complicated particle-size distributions describe the sizes of the particles in a polydisperse aerosol. This distribution defines

3888-514: 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 seaweed. Researchers have found that under stress (intense sunlight, strong evaporation, etc.), kelp releases particles of iodine which in turn become nuclei for condensation of water vapor, causing fog that diffuses direct sunlight. Sea smoke , also called steam fog or evaporation fog ,

3969-449: The deposition of pollutants to the surface of the earth as well as to bodies of water. This has the potential to be damaging to both the environment and human health. Aerosols interact with the Earth's energy budget in two ways, directly and indirectly. Ship tracks are clouds that form around the exhaust released by ships into the still ocean air. Water molecules collect around the tiny particles ( aerosols ) from exhaust to form

4050-424: The dew point, forming fog. In perfect calm, the fog layer can be less than a meter thick, but turbulence can promote a thicker layer. Radiation fog occurs at night and usually does not last long after sunrise, but it can persist all day in the winter months especially in areas bounded by high ground. Radiation fog is most common in autumn and early winter. Examples of this phenomenon include tule fog . Ground fog

4131-444: The diameter of the spherical particle with a density of 1000 kg/m and the same settling velocity as the irregular particle. Neglecting the slip correction, the particle settles at the terminal velocity proportional to the square of the aerodynamic diameter, d a : where This equation gives the aerodynamic diameter: One can apply the aerodynamic diameter to particulate pollutants or to inhaled drugs to predict where in

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4212-520: The droplets freeze, they can remain liquid to −39 °C (−38 °F), at which point even very tiny, pure water droplets will freeze. In the interior of Antarctica diamond dust is fairly common at temperatures below about −25 °C (−13 °F). Artificial diamond dust can form from snow machines which blow ice crystals into the air. These are found at ski resorts. Diamond dust may also be observed immediately downwind from manufacturing facilities or chilled water plants that produce steam. Diamond dust

4293-558: The frequency curve between two sizes a and b represents the total fraction of the particles in that size range: It can also be formulated in terms of the total number density N : Assuming spherical aerosol particles, the aerosol surface area per unit volume ( S ) is given by the second moment : And the third moment gives the total volume concentration ( V ) of the particles: The particle size distribution can be approximated. The normal distribution usually does not suitably describe particle size distributions in aerosols because of

4374-411: The front passes. Hail fog sometimes occurs in the vicinity of significant hail accumulations due to decreased temperature and increased moisture leading to saturation in a very shallow layer near the surface. It most often occurs when there is a warm, humid layer atop the hail and when wind is light. This ground fog tends to be localized but can be extremely dense and abrupt. It may form shortly after

4455-404: The hail falls; when the hail has had time to cool the air and as it absorbs heat when melting and evaporating. Freezing fog occurs when liquid fog droplets freeze to surfaces, forming white soft or hard rime ice . This is very common on mountain tops which are exposed to low clouds. It is equivalent to freezing rain and essentially the same as the ice that forms inside a freezer which

4536-403: The laboratory, contains particles of uniform size. Most aerosols, however, as polydisperse colloidal systems, exhibit a range of particle sizes. Liquid droplets are almost always nearly spherical, but scientists use an equivalent diameter to characterize the properties of various shapes of solid particles, some very irregular. The equivalent diameter is the diameter of a spherical particle with

4617-437: The mixture of particulates in air, and not to the particulate matter alone. Examples of natural aerosols are fog , mist or dust . Examples of human caused aerosols include particulate air pollutants , mist from the discharge at hydroelectric dams , irrigation mist, perfume from atomizers , smoke , dust , sprayed pesticides , and medical treatments for respiratory illnesses. Several types of atmospheric aerosol have

4698-402: The most common areas of breaking waves are located near coastlines, hence the greatest densities of airborne salt particles are there. 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

4779-542: The number frequency as: where: The log-normal distribution has no negative values, can cover a wide range of values, and fits many observed size distributions reasonably well. Other distributions sometimes used to characterise particle size include: the Rosin-Rammler distribution , applied to coarsely dispersed dusts and sprays; the Nukiyama–Tanasawa distribution, for sprays of extremely broad size ranges;

4860-519: The particles and the suspending gas, which is usually air. Meteorologists and climatologists often refer to them as particle matter, while the classification in sizes ranges like PM2.5 or PM10, is useful in the field of atmospheric pollution as these size range play a role in ascertain the harmful effects in human health. Frederick G. Donnan presumably first used the term aerosol during World War I to describe an aero- solution , clouds of microscopic particles in air. This term developed analogously to

4941-401: The phenomenon its name since it looks like many tiny diamonds are flashing in the air. These ice crystals usually form when a temperature inversion is present at the surface and the warmer air above the ground mixes with the colder air near the surface. Since warmer air frequently contains more water vapor than colder air, this mixing will usually also transport water vapor into the air near

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5022-433: The pressure is high and conversely may expand upwards when the pressure above it is lowering. Fog can form multiple ways, depending on how the cooling occurred that caused the condensation. Radiation fog is formed by the cooling of land after sunset by infrared thermal radiation in calm conditions with a clear sky. The cooling ground then cools adjacent air by conduction , causing the air temperature to fall and reach

5103-436: The relative amounts of particles, sorted according to size. One approach to defining the particle size distribution uses a list of the sizes of every particle in a sample. However, this approach proves tedious to ascertain in aerosols with millions of particles and awkward to use. Another approach splits the size range into intervals and finds the number (or proportion) of particles in each interval. These data can be presented in

5184-471: The relative humidity can cause either fog or diamond dust. This is because very small droplets of water can remain liquid well below the freezing point, a state known as supercooled water. In areas with a lot of small particles in the air, from human pollution or natural sources like dust, the water droplets are likely to be able to freeze at a temperature around −10 °C (14 °F), but in very clean areas, where there are no particles ( ice nuclei ) to help

5265-401: The relaxation time: where: To account for the effect of the shape of non-spherical particles, a correction factor known as the dynamic shape factor is applied to Stokes' law. It is defined as the ratio of the resistive force of the irregular particle to that of a spherical particle with the same volume and velocity: where: The aerodynamic diameter of an irregular particle is defined as

5346-461: The respiratory tract such particles deposit. Pharmaceutical companies typically use aerodynamic diameter, not geometric diameter, to characterize particles in inhalable drugs. The previous discussion focused on single aerosol particles. In contrast, aerosol dynamics explains the evolution of complete aerosol populations. The concentrations of particles will change over time as a result of many processes. External processes that move particles outside

5427-678: The runway awaiting takeoff. Safe operations are difficult in thick fog, and civilian airports may forbid takeoffs and landings until conditions improve. A solution for landing returning military aircraft developed in World War II was called Fog Investigation and Dispersal Operation (FIDO). It involved burning enormous amounts of fuel alongside runways to evaporate fog, allowing returning fighter and bomber pilots sufficient visual cues to safely land their aircraft. The high energy demands of this method discourage its use for routine operations. Shadows are cast through fog in three dimensions. The fog

5508-421: The same value of some physical property as the irregular particle. The equivalent volume diameter ( d e ) is defined as the diameter of a sphere of the same volume as that of the irregular particle. Also commonly used is the aerodynamic diameter ,  d a . People generate aerosols for various purposes, including: Some devices for generating aerosols are: Several types of atmospheric aerosol have

5589-415: The same way as stratus cloud near a front when raindrops, falling from relatively warm air above a frontal surface, evaporate into cooler air close to the Earth's surface and cause it to become saturated. The water vapor cools and at the dewpoint it condenses and fog forms. This type of fog can be the result of a very low frontal stratus cloud subsiding to surface level in the absence of any lifting agent after

5670-415: The sea travels inland but suddenly meets an area of hot air. This causes the water particles of fog to shrink by evaporation, producing a "transparent mist". Garua fog is nearly invisible, yet it still forces drivers to use windshield wipers because of condensation onto cooler hard surfaces. Camanchaca is a similar dense fog. Depending on the concentration of the droplets, visibility in fog can range from

5751-506: The summer monsoon , produces a south to southeasterly flow which can drive the offshore marine layer up the coastline; a phenomenon known as a "southerly surge", typically following a coastal heat spell. However, if the monsoonal flow is sufficiently turbulent, it might instead break up the marine layer and any fog it may contain. Moderate turbulence will typically transform a fog bank, lifting it and breaking it up into shallow convective clouds called stratocumulus . Frontal fog forms in much

5832-505: The surface, causing the relative humidity of the near-surface air to increase. If the relative humidity increase near the surface is large enough then ice crystals may form. To form diamond dust the temperature must be below the freezing point of water, 0 °C (32 °F), or the ice cannot form or would melt. However, diamond dust is not often observed at temperatures near 0 °C (32 °F). At temperatures between 0 °C (32 °F) and about −39 °C (−38 °F) increasing

5913-425: The surface. A temperature inversion increases the distance that lower frequency sounds can travel, by reflecting the sound between the ground and the inversion layer. Particularly foggy places include Hamilton, New Zealand and Grand Banks off the coast of Newfoundland (the meeting place of the cold Labrador Current from the north and the much warmer Gulf Stream from the south). Some very foggy land areas in

5994-506: The temperature at the surface drops below the freezing point. The thickness of a fog layer is largely determined by the altitude of the inversion boundary, which in coastal or oceanic locales is also the top of the marine layer , above which the air mass is warmer and drier. The inversion boundary varies its altitude primarily in response to the weight of the air above it, which is measured in terms of atmospheric pressure. The marine layer, and any fog-bank it may contain, will be "squashed" when

6075-425: The temperature rises. It can be associated with the diamond dust form of precipitation, in which very small crystals of ice form and slowly fall. This often occurs during blue sky conditions, which can cause many types of halos and other results of refraction of sunlight by the airborne crystals. Ice fog often leads to the visual phenomenon of light pillars . Up-slope fog or hill fog forms when winds blow air up

6156-625: The term hydrosol , a colloid system with water as the dispersed medium. Primary aerosols contain particles introduced directly into the gas; secondary aerosols form through gas-to-particle conversion. Key aerosol groups include sulfates, organic carbon, black carbon, nitrates, mineral dust, and sea salt, they usually clump together to form a complex mixture. Various types of aerosol, classified according to physical form and how they were generated, include dust, fume, mist, smoke and fog. There are several measures of aerosol concentration. Environmental science and environmental health often use

6237-469: The velocity of the gas at the surface of the particle is zero. For small particles (< 1 μm) that characterize aerosols, however, this assumption fails. To account for this failure, one can introduce the Cunningham correction factor , always greater than 1. Including this factor, one finds the relation between the resisting force on a particle and its velocity: where This allows us to calculate

6318-426: The visibility, in some cases to under 600 m (2,000 ft). The depth of the diamond dust layer can vary substantially from as little as 20 to 30 m (66 to 98 ft) to 300 metres (980 ft). Because diamond dust does not always reduce visibility it is often first noticed by the brief flashes caused when the tiny crystals, tumbling through the air, reflect sunlight to the eye. This glittering effect gives

6399-609: The world include 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 are often found in lowlands and valleys, such as the lower part of the Po Valley and the Arno and Tiber valleys in Italy; Ebro Valley in northeastern Spain; as well as on the Swiss plateau , especially in

6480-476: The world that has cold winters, it is most frequent in the interior of Antarctica, where it is common year-round. Schwerdtfeger (1970) shows that diamond dust was observed on average 316 days a year at Plateau Station in Antarctica, and Radok and Lile (1977) estimate that over 70% of the precipitation that fell at Plateau Station in 1967 fell in the form of diamond dust. Once melted, the total precipitation for

6561-465: The year was only 25 mm (0.98 in). Diamond dust may sometimes cause a problem for automated airport weather stations . The ceilometer and visibility sensor do not always correctly interpret the falling diamond dust and report the visibility and ceiling as zero (overcast skies). However, a human observer would correctly notice clear skies and unrestricted visibility. The METAR identifier for diamond dust within international hourly weather reports

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