From
2-6-6. REPORTING WEATHER CONDITIONS
a. When the prevailing visibility at the usual point
of observation, or at the tower level, is less than
4miles, tower personnel shall take prevailing
visibility observations and apply the observations as
follows:
1. Use the lower of the two observations (tower
or surface) for aircraft operations.
2. Forward tower visibility observations to the
weather observer.
3. Notify the weather observer when the tower
observes the prevailing visibility decrease to less than 4 miles or increase to 4 miles or more.
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2-6-7. DISSEMINATING WEATHER
INFORMATION
TERMINAL. Observed elements of weather information shall be disseminated as follows:
a. General weather information, such as “large
breaks in the overcast,” “visibility lowering to the
south,” or similar statements which do not include
specific values, and any elements derived directly
from instruments, pilots, or radar may be transmitted to pilots or other ATC facilities without consulting the weather reporting station.
b. Specific values, such as ceiling and visibility,
may be transmitted if obtained by one of the
following means:
1. You are properly certificated and acting as
official weather observer for the elements being reported.
NOTE
USAF controllers do not serve as official weather
observers.
2. You have obtained the information from the
official observer for the elements being reported.
3. The weather report was composed or verified
by the weather station.
4. The information is obtained from an official
Automated Weather Observation System (AWOS) or
an Automated Surface Observation System (ASOS).
These two sections seem to contradict each other, however it seems that if they state the visibility, then they have been certificated, and are acting as the official observer for the elements being reported.
Source: http://www.faa.gov/airports_airtraffic/air_traffic/publications/at_orders/media/ATC.pdf
