When should thunderstorms occurring at a station be reported?

Thunderstorms should be reported when very dark cumulonimbus (CB) clouds and lightning are observed because these are key indicators of the presence of a thunderstorm. Cumulonimbus clouds are the result of vigorous convection and are specifically associated with severe weather conditions, including thunderstorms. Their dark appearance indicates an extensive vertical development typical of storm activity, suggesting the likelihood of severe weather phenomena such as heavy rain, strong winds, and the possibility of hail. Lightning is another strong indicator of a thunderstorm, affirming that the conditions for a storm are present, and thus, it becomes crucial information for pilots and meteorological services.

While heavy rain, hail, and previous thunderstorm records can be relevant to weather observations, they are not as definitive on their own. Heavy rain may occur without thunderstorms, and hail is a specific phenomenon rather than a general indicator of active thunderstorm conditions. Moreover, having no recent thunderstorm events does not provide information about the current weather at a station, making it less relevant for reporting ongoing conditions. Therefore, reporting the presence of dark CB clouds and lightning is the most direct and informative way to communicate the occurrence of a thunderstorm.