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 Convective significant meteorological information (convective SIGMET/WST)

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Urgent pilot weather reports (urgent PIREP/UUA)

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 Center weather advisories (CWA)

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 Airmen’s meteorological information (AIRMET/WA)

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Isolated thunderstorms that are rapidly developing and increasing in intensity

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 Low ceilings and visibilities that are becoming widespread

haze (meteorology). An obstruction to visibility caused by fine dust or salt particles dispensed through a portion of the atmosphere. The particles are so small they cannot be felt or individually seen with the naked eye. Haze is distinguished from fog by its bluish or yellowish tinge.
head angle (countersunk fastener dimension). The included angle formed between the sides of the head of a countersunk rivet or screw.

head eccentricity (rivet dimension). The amount of difference between the center of the shank of a rivet and the center of its head.
heading indicator. A gyroscopic flight instrument that gives the pilot an indication of the heading of the aircraft.
headless fastener. A type of friction-held fastener used to join mechanical parts. Neither end of the fastener is enlarged to form a head. A roll pin is an example of a headless fastener. See roll pin.
head of pressure. The pressure exerted by a column of fluid that is caused by the height of the column.
headphone (audio system). A small receiver, or earphone, mounted on a band worn over the head. Most headphones have two receivers mounted in soft cups so one can be comfortably worn over each ear.
head pressure of a fluid. The pressure exerted by a column of fluid caused by the height of the column.
head-up display. See HUD.