HOW ARE WEATHER ALERTS ISSUED? When talking about rainfall alerts, the Green code is for less than 64 mm of rain in 24 hours. If the expected rainfall is between 64.5 mm and 115. 5 mm, then a yellow alert is issued. An Orange alert is for a rainfall range between 115.6 to 204.4 mm in a day, while a red alert is issued when a rainfall above 204.5 mm is expected for a 24 hour duration. These alerts are mainly for the people residing in the affected areas and how they should react in that particular situation. While during thunderstorms, the wind speed decides the alert issued, in the case of foggy conditions, the visibility range becomes the deciding factor. In the case of a dust storm, both the speed of the wind along with visibility are considered while issuing an alert.