Obviously, not all methods may be used at all times,
and some methods work better than others in different
situations. When a cloud layer height value falls
halfway between two reportable valves, round down to
the nearest reportable increment given in table 1-3.
Table 1-3.Reportable Values for Cloud Layer Height and
HEIGHT IN FEET
5,000 or less
5,001 to 10,000
Nearest 1,000 feet
NAVMETOCCOMINST 3141.2 and NAVMET-
OCCOMINST 3144.1 discuss in detail the various
methods and procedures used to determine cloud height
and ceiling height.
How is the amount of cloud layer coverage and
total sky coverage measured?
Define cloud layer.
Define cloud ceiling.
Define total obscuration.
Define summation sky coverage.
Given: Layer No.1 is 1/8 fog (SFC)
Layer No.2 is 2/8 cumulus 3,000 ft
Layer No.3 is 2/8 altocumulus 12,000 ft
Layer No.4 is 5/8 cirrus 20,000 ft
What is the ceiling height?
A cloud height of 7,550 feet would be reported as
LEARNING OBJECTIVES: Describe
prevailing visibility, sector visibility, and
differing level visibility. Define runway visual
Visibility, as well as ceiling height, aids in
decisions involving air traffic control. For this reason,
the observation of visibility must be timely, accurate,
and representative. There are four types of visibility that
you must observe: (1) prevailing visibility, (2) sector
visibility, (3) differing level visibility, and (4) runway
visual range. Both NAVMETOCCOMINST 3141.2
and NAVMETOCCOMINST 3144.1 provide thorough
and detailed guidance on visibility observations.
Ashore, visibility is observed in statute miles. Aboard
ship, visibility is observed in nautical miles. Observing
stations located outside the United States report
visibility in meters.
When observing visibility, you should note the
distance as follows:
To the nearest 1/16 mile when visibility is less
than 3/8 mile
To the nearest 1/8 mile when it is between 3/8
and 2 miles
To the nearest 1/4 mile, between 2 and 3 miles
To the nearest mile, from 3 to 15 miles
In 5-mile increments, above 15 miles
When the visibility falls between two values, the lower
value is always used.
For example, a measured
visibility of 3 3/4 miles is called "3 miles." See
Appendix II of this module for a visibility conversion
Prevailing visibility is the greatest distance that
known objects can be seen and identified throughout
half or more of the horizon circle. The most reliable
method for determining prevailing visibility is by the
eye of a trained observer. The sensors provided with
automatic observing systems provide only an
approximation of prevailing visibility based on the
sampling of obstructions-to-vision present in only a
small area around the sensor. To aid in the
determination of prevailing visibility, observation
stations are required to maintain a visibility chart. The
visibility chart identifies each daytime and nighttime
visibility marker with direction and distance to the
marker. Daytime markers are generally dark, prominent
objects that stand out when viewed against the lighter
sky. Nighttime markers are usually unfocused lights of
moderate intensity, such as radio tower lights or channel
At sea, since the ship is usually moving, fixed
visibility markers are not available. The Combat
Information Center (CIC), however, maintains tracks