1.5.1 Quantity and Intensity of Rainfall in the Forecast

Based on the rainfall recorded within 24 hours, IMD has divided the rainfall into 6 categories.

● Very light rainfall – 0.1 mm to 2.4 mm

● Light rainfall – 2.5 mm to 15.5 mm

● Moderate - 15.6 mm to 64.4 mm

● Heavy rainfall - 64.5 mm to 115.5 mm

●Very heavy rainfall - 115.6 mm to 204.4 mm

● Extremely heavy rainfall – 204.4 mm and above

The rainfall intensity is determined by how much rain is received per hour. If the area is forecasted with moderate rainfall and if it rains 50mm in an hour, then the rain is considered to be high-intensity rainfall but if it rains by 24 hours it will be considered as low-intensity rainfall. Kerala still does not have automated weather instruments in adequate amounts to receive this information locally.

In general, heavy rainfall what we call based on the intensity of rainfall we experience and the forecasted heavy rainfall is different. In prediction, the heavy rainfall. The amount of rainfall recorded during the twenty hours is the heavy rainfall in forecasting which is different from rainfall we experience. This is what confuses rain forecasting.

Extremely heavy rainfall / High-intensity rainfall in the form of rain which can cause flash flooding. Many times during the days when the 2018 flood hits Kerala, some areas the rainfall intensity was from 300mm to 400mm in 24 hrs period. There is no information on how long it rained as there are no instruments available for determining the intensity of rainfall.