14.2 Information on cloud base height is obtained by use of ceilometers (rotating or fixed beam), ceiling lights, ceiling balloons, pilot reports, and observer estimations. The systems in use by most reporting stations are either the observer estimation or the rotating beam ceilometer.
14.3 Pilots usually report height values above mean sea level, since they determine heights by the altimeter. This is taken into account when disseminat-ing and otherwise applying information received from pilots. (“Ceiling” heights are always above ground level.) In reports disseminated as pilot reports, height references are given the same as received from pilots; that is, above mean sea level.
14.4
In area forecasts or inflight Advisories, ceilings are denoted by the contraction “CIG” when used with sky cover symbols as in “LWRG TO CIG OVC005,” or the contraction “AGL” after the forecast cloud height value. When the cloud base is given in height above mean sea level, it is so indicated by the contraction “MSL” or “ASL” following the height value. The heights of cloud tops, freezing level, icing, and turbulence are always given in heights above mean sea level (ASL or MSL).

15.
Reporting Prevailing Visibility

15.1 Surface (horizontal) visibility is reported in
METAR reports in terms of statute miles and 3/5/3/1/
increments thereof; e.g., 1/16, 1/8, 16, 1/4, 16, 8, 2, 5/
8, 3/4, 7/8, 1, 1 1/8, etc. (Visibility reported by an unaugmented automated site is reported differently than in a manual report; i.e., ASOS/AWSS: 0, 1/16, 1/8, 1/4, 1/2, 3/4, 1, 1 1/4, 1 1/2, 1 3/4, 2, 2 1/2, 3, 4, 5, etc., AWOS: M1/4, 1/4, 1/2, 3/4, 1, 1 1/4, 1 1/2, 1 3/4, 2, 2 1/2, 3, 4, 5, etc.) Visibility is determined through the ability to see and identify preselected and prominent objects at a known distance from the usual point of observation. Visibilities which are determined to be less than 7 miles, identify the obscuring atmospheric condition; e.g., fog, haze, smoke, etc., or combina-tions thereof.
15.2 Prevailing visibility is the greatest visibility equalled or exceeded throughout at least one.half the horizon circle, not necessarily contiguous. Segments of the horizon circle which may have a significantly different visibility may be reported in the remarks section of the weather report; i.e., the southeastern quadrant of the horizon circle may be determined to be 2 miles in mist while the remaining quadrants are determined to be 3 miles in mist.
15.3
When the prevailing visibility at the usual point of observation, or at the tower level, is less than 4 miles, certificated tower personnel will take visibility observations in addition to those taken at the usual point of observation. The lower of these two values will be used as the prevailing visibility for aircraft operations.

16.
Estimating Intensity of Rain and Ice Pellets

16.1 Rain
16.1.1 Light. From scattered drops that, regardless of duration, do not completely wet an exposed surface up to a condition where individual drops are easily seen.
16.1.2 Moderate. Individual drops are not clearly identifiable; spray is observable just above pave-ments and other hard surfaces.
16.1.3 Heavy. Rain seemingly falls in sheets; individual drops are not identifiable; heavy spray to a height of several inches is observed over hard surfaces.
16.2 Ice Pellets
16.2.1 Light. Scattered pellets that do not com-pletely cover an exposed surface regardless of duration. Visibility is not affected.
16.2.2 Moderate. Slow accumulation on the ground. Visibility is reduced by ice pellets to less than 7 statute miles.
16.2.3
Heavy. Rapid accumulation on the ground. Visibility is reduced by ice pellets to less than 3 statute miles.

17.
Estimating the Intensity of Snow or Drizzle (Based on Visibility)

17.1 Light. Visibility more than 1/2 statute mile.
17.2 Moderate. Visibility from more than 1/4 statute mile to 1/2 statute mile.
17.3 Heavy. Visibility 1/4 statute mile or less.

18. Pilot Weather Reports (PIREPs)
18.1 FAA air traffic facilities are required to solicit PIREPs when the following conditions are reported or forecast: ceilings at or below 5,000 feet, visibility at or below 5 miles (surface or aloft), thunderstorms and related phenomena, icing of a light degree or greater, turbulence of a moderate degree or greater, wind shear, and reported or forecast volcanic ash clouds.
18.2 Pilots are urged to cooperate and promptly volunteer reports of these conditions and other atmospheric data, such as cloud bases, tops and layers, flight visibility, precipitation, visibility restrictions (haze, smoke, and dust), wind at altitude, and temperature aloft.
18.3 PIREPs should be given to the ground facility with which communications are established; i.e., EFAS, AFSS/FSS, ARTCC, or terminal ATC. Radio call “FLIGHT WATCH,” which serves as a collection point for the exchange of PIREPs with en route aircraft, is one of the primary duties of EFAS facilities.
　
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