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The Importance of Fog

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T el:(+34) 968 162 005 The importance of fog . and how to measure it At some time in our lives most of us The method used to determine night Visibility can be measured by several will be aected by reduced visibility time visibility is based on the methods although all are partially from fog; when our ight has been contrast between a bright point light limited to some degree. The most grounded or diculty when driving source against a dark or black common and readily available on roads or navigating waterways. background and the mathematical methods are by: trained human Poor visibility conditions not only representation of this is known as visual observers, transmissometers require greater concentration on the Allard’s Law. and forward scatter / backscatter task at hand, which can lead to meters. There are also instruments increased stress levels, but also have Visibility can be reduced by heavy that take samples of the air volume a cost implication. The amount of rainfall, dust or even smoke but the and look at the scattering capability E-mail:[email protected] lost annually around the largest cause is from the naturally of the chemical constituents of that world due to time delays or damage occurring weather phenomenon of air sample to determine visibility. caused by reduced visibility are fog or haze. However, for the purpose of this beyond normal calculation. article we will ignore these. There are generally accepted to be Visibility, in terms of general ve types of naturally occurring fog: - the trained human observer meteorological applications, is the radiation fog, frontal fog, orographic The human observation of visibility is horizontal distance that an average fog, advection fog and steam fog. naturally subjective and can be costly human eye, at 2 m height above the For a full denition of each of these if this method is used for more than ground, can distinguish the contrast and how they are formed you may a few hours and in more than one between a dark object in the distance like to look at American location. However, humans do have against a bright background. This Meteorological Society’s Online an incredible ability to process distance is then considered to be the Glossary of Meteorology at: information and evaluate the day time visibility at that location http://amsglossary.allenpress.com continuously changing face of the and a simple formula to determine /glossary. weather as it passes any location and this is known as Koschmieder’s Law. are therefore still the standard to For our purposes here, fog is dened which all other visibility This contrast of a dark object against as the visible portion of small water measurements are compared. a light background will vary from droplets suspended in the one location to another depending atmosphere on or near the earth’s However, no two human eyes are the on the topography and conditions, surface. It is caused by the reduction same and even the ability to even over a few miles not to mention of the air temperature down to or determine contrast, which is known around the world. Because of this the near the dewpoint and reduces as the minimal liminal contrast m ‘standard’ environment is dened as: visibility to 1 km or less. Haze can be threshold, can vary from values of a cloudless day at 12:00 noon, during described as fog that has a lower 0.018 to over 0.042 and is one of the high summer, at the equator in a relative humidity and does not factors that has a major impact on location with a very at topography reduce visibility as much, generally visibility calculations. and a light, consistent coloured from 10 km down to a minimum of surface with a strongly contrasting 1 km. Beyond 10 km the visibility is large forest covered hillside in the considered to be good. distance. 10 T el:(+34) 968 162 005 - transmissometers However, for short term projects or Taking into consideration all three transmissometers are the instrument where consideration of visibility methods human observation still has reference equivalent to humans as measurements is only a small factor the edge because of the ability to they look at the total extinction of this price is prohibitive. adapt to changing conditions, but light over a relatively short when time and costs are restricted predetermined opposed path using a - forward and backscatter meters measurements by automated, visual light source such as halogen or Forward and backscatter meter objective equipment, with their ever xenon. They do not determinewhat technology has come a signicant increasing capabilities, can provide a is scattering light away from the way towards a suitable compromise better solution. receiver but can determinehow of acceptable performance at low muchis being scattered away and cost. Both types of scattering meters E-mail:[email protected] then relate this information to use the relationship of the amount of visibility for that particular path light received at a known given angle There are several sources for more information: length or a short multiple thereof. from a transmitted light source (mostly infra-red) to determine the Book: As with any instrument of reference extinction coecient (EXCO) of the Vision through the atmosphere quality, there needs to be atmosphere which is then used to by W. E. Knowles Middleton University of Toronto Press considerable care taken with calculate visibility (using Allard’s or installation and calibration as well as Koschmieder’s law). These sensors are Websites: compact and reliable as well as being ensuring that conditions remain Met glossary constant. For example if the very economical at only a few http://amsglossary.allenpress.com measurement path is not exactly thousand pounds per unit. They have /glossary. opposed or there are electrical the limitation that they are still only Royal Met Society deviations of components due to able to measure a relatively small www.royal-met-soc.org.uk temperature changes, the accuracy of sample volume and are assuming the instrument will be aected. that this is representative of the area UK Met Oce www.met-oce.gov.uk Additionally, transmissometers tend around that sample volume. For most to be calibrated to perform best in cases this is ne but when there are All these links are also available dry conditions and have dierent fast moving fog banks rolling from our own site: characteristics in precipitation, which through the sample volume the www.sensovant.com are hard to factor into the measurements will not be truly measurement values. representative of the actual conditions. Nevertheless for the level If you would like to nd out more m Cost is also an important point when of technology available today these about automated visibility considering this technology. The cost instruments provide an increasingly measurements contact Sensovant. of a transmissometer is over a good price and performance hundred thousand pounds sterling, capability that work in most email: [email protected] which is justied for a reference class environmental conditions. tel: (+34) 968 162 005 instrument used on large airports with important safety implications. 11.
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