Heavy rainfall has been occurring across different parts of the country as the Southwest Monsoon takes hold and one may have come across reports of weather stations issuing colour-coded alerts " yellow, orange or red " in their area, state, etc.
So, what are these colour codes, what do they signify and how are they decided?
Why are colours used?
According to the Indian Meteorological Department (IMD), "colour codes are used in weather warnings for bringing out the severity of the weather phenomena expected".
The key idea is to forewarn relevant officials and the disaster management authority "about the impact of the weather expected so as to keep them ready for necessary action related to disaster risk reduction".
At a basic level, the message in the four different colours are to be read thus: Green - No Action needed; Yellow - Watch and stay updated; Orange - Be prepared; Red - Take action.
But while that is the general interpretation, specific weather events like rainfall, thunderstorm, lightning, etc, have more pointed warnings matched with these colours.
How is the colour forecast decided?
The IMD says that to decide the colour to be assigned to a given weather situation under a five-day forecast scheme, a specific matrix is followed, "giving thrust on the probability of occurrence of the event as well as its impact assessment".
Having said that, the assessment regarding the colour code for "impact-based warning", includes meteorological factors, hydrological factors, geophysical factors, etc, "which may interact with each other to determine the impact and risk".
Thus, the weather office takes into account all these factors to decide the appropriate colour code for warning.
Who does it apply to?
IMD points out that "even though colour code criteria used by all the centres are the same, it is not necessary that the colour code used for subdivisional warning will be the same as the colour code used for any district in that subdivision".
That is because the criteria laid out is of a general nature while, considering the location, the weather activity and impact may be different.
The chart above has a sample of the colour codes for some weather situations.
So, what do the colours say?
Take, for example, the colour code for rainfall warning. Green is the colour when there is no forecast for any heavy rainfall. The colour can be Orange or Red "if it is already a flood situation and heavy rainfall is expected".
Scattered or isolated heavy is to be designated Yellow. When there is isolated heavy to very heavy rainfall consecutively for three days, then the colour is "Orange for Day 1 and 2 and Red for Day 3".
For isolated extremely heavy rainfall or scattered heavy to very heavy rainfall, the colour is Red.