User's Guide _______________________________________________________________________
152 __________________________________________________________________ M211107EN-C
Thinner
Fog is reported to get thinner (32) if the last 20-minute visibility average
minus the first 20-minute visibility average is more than zero (>0), and
the difference must be greater than 30% of the first
20-minute visibility average.
Stable
Fog has been stable (33) if the last 20-minute visibility average minus the
first 20-minute visibility average is less than 20% of the lower average
visibility.
Thicker
Fog is reported to get thicker (34) if the last 20-minute visibility average
minus the first 20-minute visibility average is less than zero
(<0) and the difference is more than 30% of the new 20-minute visibility
average.
Table 22 lists the limits used for METAR code reporting:
Table 23 WMO METAR Code Limits
10-Minute Visibility Average
Higher than or equal to 1 km but lower than or
equal to 5 km.
Weather Classes
The weather classes are continuous, and showers or intermittent.
The weather class is continuous if there are fewer than two clear (no
precipitation) periods during the preceding hour.
The weather class is showers or intermittent if there are two or more than
two clear periods during the preceding hour in precipitation.
However, if more than 30 minutes have elapsed from the last detected
precipitation, the PRECIPITATION DURING THE PRECEDING
HOUR code is used.