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AIM

10/12/17

7−1−40

Meteorology

5. 50−meter  increments for products between

150 meters and 800 meters.

6. 100−meter increments for products between

800 meters and 1,200 meters.

7. 200−meter increments for products between

1,200 meters and 2,000 meters.

7−1−16. Reporting of Cloud Heights

a. Ceiling, by definition in the CFRs and as used

in aviation weather reports and forecasts, is the height

above ground (or water) level of the lowest layer of

clouds or obscuring phenomenon that is reported as

“broken,” “overcast,” or “obscuration,” e.g., an

aerodrome forecast (TAF) which reads “BKN030”

refers to height above ground level. An area forecast

which reads “BKN030” indicates that the height is

above mean sea level.

REFERENCE−

AIM, Paragraph 7−1−30 , Key to Aerodrome Forecast (TAF) and Aviation

Routine Weather Report (METAR), defines “broken,” “overcast,” and

“obscuration.”

b. Pilots usually report height values above MSL,

since they determine heights by the altimeter. This is

taken in account when disseminating and otherwise

applying information received from pilots. (“Ceil-

ing” heights are always above ground level.) In

reports disseminated as PIREPs, height references

are given the same as received from pilots, that is,

above MSL.

c. 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 MSL, it is so indicated by the contraction

“MSL” or “ASL” following the height value. The

heights of clouds tops, freezing level, icing, and

turbulence are always given in heights above ASL or

MSL.

7−1−17. Reporting Prevailing Visibility

a. Surface (horizontal) visibility is reported in

METAR reports in terms of statute miles and

increments thereof; e.g.,

 1

/

16

1

/

8

3/

16

,

 1

/

4

5/

16

,

 3/8

,

 1/2

,

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/

2, 

3/

4, 

2, 2 

1/

2,

 3, 4, 5

, etc., AWOS:

M

1

/

4

1

/

4

1/

2

3

/

4

, 1, 1 

1

/

4

, 1 

1/

2, 

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 condi-

tion; e.g., fog, haze, smoke, etc., or combinations

thereof.

b. Prevailing visibility is the greatest visibility

equaled or exceeded throughout at least one half of

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.

c. 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.

7−1−18. Estimating Intensity of Rain and

Ice Pellets

a. Rain

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.

2. Moderate. Individual drops are not clearly

identifiable; spray is observable just above pave-

ments and other hard surfaces.

3. Heavy. Rain seemingly falls in sheets;

individual drops are not identifiable; heavy spray to

height of several inches is observed over hard

surfaces.

b. Ice Pellets

1. Light. Scattered pellets that do not com-

pletely cover an exposed surface regardless of

duration. Visibility is not affected.

2. Moderate. Slow accumulation on ground.

Visibility reduced by ice pellets to less than 7 statute

miles.