When the prevailing visibility at the usual point of observation, or at the tower level, is less than 4 miles, tower personnel must take prevailing visibility observations and apply the observations as follows:
Use the lower of the two observations (tower or surface) for aircraft operations.
Forward tower visibility observations to the weather observer.
Notify the weather observer when the tower observes the prevailing visibility decrease to less than 4 miles or increase to 4 miles or more.
Describe the wind as calm when the wind velocity is less than three knots.
Forward current weather changes to the appropriate control facility as follows:
When the official weather changes to a condition:
(a) Less than a 1,000-foot ceiling or below the highest circling minimum, whichever is greater.
(b) Where the visibility is less than 3 miles.
(c) Where conditions improve to values greater than those listed in (a) and (b).
When changes which are classified as special weather observations during the time that weather conditions are below 1,000-foot ceiling or the highest circling minimum, whichever is greater, or less than 3 miles visibility.
Towers at airports where military turbo-jet en route descents are routinely conducted must also report the conditions to the ARTCC even if it is not the controlling facility.
If the receiving facility informs you that weather reports are not required for a specific time period, discontinue the reports.
EN ROUTE. When you determine that weather reports for an airport will not be required for a specific time period, inform the FSS or tower of this determination.