Assistenza Al Volo - ANNO XLV - NUMERO 4/2020
AVIATION ENGLISH BITE a cura di Michael Ferrario
“Tower, How foggy is it?” ……. “Say Again”. It is not phrased properly for aviation purposes but it is a good question since fog is defined differently for aviation, meteorology and the general public. It is rather simple to remember that in aviation it is defined by visibility less than 1,000 m except when followed by “MI”, “BC”, “PR” or “VC” since each descriptor qualifier has a specific definition. Since fog and mist can seem to be the same thing, mist is when visibility is at least 1,000 m but not more than 5,000 m with no descriptor qualifiers. The FAA likes to use fractions so, fog is when visibility is less than 5/8 of a statue mile with “MI”, “BC”, “PR” or “VC” as descriptor qualifiers. While mist is greater than or equal to 5/8 of a statue mile to 6 statute miles (9.65 Km) with no descriptor qualifiers. So, how about the meteorological definition? You will be surprised to find that fog is just a cloud that touches the ground for most meteorological purposes but the WMO (World Metrology Organization) defines fog as “A suspension of very small, usually microscopic water droplets in the air, reducing visibility at the Earth’s surface.” and
they go on to say, in various documents, “The term ‘’fog” is used when microscopic droplets reduce horizontal visibility at the Earth’s surface to less than one kilometer, while the term “mist” is used when the droplets do not reduce horizontal visibility to less than one kilometer”. When it comes to the general public, definitions become very diverse. It largely depends on what purpose the definition is being made for. Driving handbooks between countries and even between states in the U.S. offer different definitions. They seem to vary between 100 and 500 meters of visibility when describing fog. In practice, mist is considered synonymous with “light fog” as a way of describing it. Generally, by far the most common definition among many handbooks that I could find was that fog reduces visibility due to cloud touching the ground. The common thread between all these definitions is visibility, which can be defined simply as the distance at which a given known object can be seen and identified with the unaided eye but given that visibility can be observed at night and measured by instruments, we need to turn to a
25
