By convention, raingauges in India are opened every morning at 08:30 IST and the amount of water collected since 08:30 IST of the previous morning is measured and converted to rainfall amount. This 24-hour rainfall amount is used for all climatological and statistical purposes. However, it does not give any indication of when it rained during the 24-hour period or with what intensity. This can be found out by opening the raingauge at more frequent intervals and by employing automatic raingauges which provide a graph of rainfall against time.

Meteorologists call it a ‘rainy day’ when at least 2.5 mm of rain has fallen at a place in 24 hours. Any amount less than 2.5 mm does not really matter and is called just a ‘trace’. If the rainfall is between 6.5 and 12.4 cm, it is called ‘heavy rain’. When it is between 12.5 and 24.4 cm, it is described as ‘very heavy rain’, and when it is more then 25 cm, as ‘exceptionally heavy rain’.

The same definitions are used while making forecasts. Thus a forecast of heavy rain implies that 6.5 to 12.4 cm of rain may fall in the next 24 hours and a forecast of very heavy rain means that 12.5 to 24.4 cm of rain is likely to fall during the coming 24 hours. Heavy or very heavy rain implies very intense showers of short duration, or a less severe but incessant downpour, or both.

People get used to the rainfall pattern of their region. Thus, heavy rain is very common for Mumbai or Goa during the monsoon, while for west Rajasthan it may be a rare event. It is therefore a meteorological practice to compare the actual rainfall with its normal value, which is the value obtained by averaging the rainfall over several years. Rainfall normals are derived typically over periods of 30 or 50 years.

When the actual rainfall exceeds the normal rainfall by 20 % or more, meteorologists call it ‘excess’. On the other hand, if it is below the normal rainfall by 20 to 59 %, it is called ‘deficient’. When the deficit is more than 60 %, the rainfall is termed as ‘scanty’. If the rainfall is between -19 and +19 %, it is described as normal. This categorization applies to rainfall over all durations like day, week, month or season, and over all regions like city, taluk, district and sub-division. However, an exception is made for the entire country’s monsoon rainfall, which is considered to be normal when it is within -10 and +10 % of its long period average.

A basic characteristic of the monsoon rain is that it does not keep on falling continuously, nor does it keep falling uniformly over all parts of India. So, at any given time during the monsoon season, some parts of the country may be receiving good rains and other parts may be without rains. After a few days of rain, which is called a ‘wet spell’, generally there is a lull, or a ‘dry spell’. It is good to have alternating short wet and dry spells. A long wet spell could result in local flooding of low-lying areas having poor drainage, and it is bad for agriculture as crops need sunshine too for their growth. On the contrary, crops begin to wither under a long dry spell, and a really prolonged dry spell may lead to drought conditions.

When meteorologists describe the state of the monsoon over the country, they usually speak about which meteorological sub-divisions have a vigorous monsoon, or which sub-divisions have an active monsoon. Although these terms appear to be rather qualitative, it is not really so, and they are defined quite precisely.

The monsoon is said to be ‘active’ when the rainfall averaged over the area of a sub-division is 1.5 to 4 times its normal rainfall for the sub-division for that day. However, there is a condition that a minimum of two stations in the sub-division should have received at least 3 cm rain in the previous 24 hours. For sub-divisions on the west coast, which generally get heavy rains, the requirement is for 5 cm rain instead of 3 cm over at least two stations. These conditions help to ensure that sporadic instances of heavy rain do not create a false impression of a strong monsoon.

The monsoon is said to be ‘vigorous’ when the rainfall averaged over the area of a sub-division is more than 4 times its normal rainfall for the sub-division for that day. Here again, there is a condition that a minimum of two stations in the sub-division should have received at least 5 cm rain (7 cm for west coast) in the previous 24 hours.

Besides the scale of monsoon activity, it is also important to know how the rainfall is distributed across a sub-division. When as many as three-fourths of the stations, or more, within a sub-division have reported rain, meteorologists say that rain has occurred ‘at most places’ in that sub-division. When 51 to 74 % of the stations have reported rain, the term ‘at many places’ is used instead. When 26 to 50 % of the stations have reported rain, the term ‘at a few places’ is used. When only one-fourth of the stations, or less, have recorded rainfall, it is said to have occurred ‘at isolated places’. In this categorization, the amount of rainfall is not a matter of consideration, it should just be 2.5 mm or more, but it is the distribution across an area that is described. The same nomenclature is also adopted for rainfall forecasts.