Special observation

From Glossary of Meteorology

special observation

A category of aviation weather observation taken to report significant changes in one or more of the observed elements since the last record observation.

The specific criteria demanding a special observation change from time to time. Basically they cover 1) significant changes in ceiling and visibility, particularly in the low ranges; 2) the appearance of low clouds; 3) the appearance and disappearance of a tornado or thunderstorm; 4) the beginning and ending of frozen or freezing precipitation; 5) pressure jumps; and 6) significant changes in the wind.