A brief spell of showers in Delhi on Monday helped push the quantum of rain this August past the long-period average for the month. Or did it?
On Monday, the India Meteorological Department (IMD) said Delhi had received 234.5mm of rain in the first 10 days of August- more than the month’s long-period average of 233.1mm. By Tuesday, however, the figure had changed as IMD cut Delhi’s cumulative August rainfall to 197.2mm in its bulletin, leaving the Capital around 36mm short of its expected rainfall for the month.
The weather agency later admitted that the earlier figure was incorrect. So how does IMD collect and compile rainfall data and where can such an error creep in?
How Does IMD Measure Rainfall?
Rainfall is measured through a network of rain
gauges installed at different weather stations. These include manual rain gauges, where readings are recorded by observers, as well as Automatic Rain Gauges (ARGs) and Automatic Weather Stations (AWS), which record and transmit observations electronically.
A rain gauge essentially measures the depth of water collected from rainfall. The measurement is expressed in millimetres. So, 20mm of rain means enough water has accumulated to form a 20mm-deep layer on a flat surface, assuming there is no runoff, evaporation or seepage.
Does IMD Measure Rain From Just One Place In Delhi?
No. Delhi has multiple rainfall-monitoring stations, and rainfall can vary significantly across the city. During a localised shower, one part of Delhi can receive heavy rain while another gets little or none. IMD therefore collects observations from several stations, including Safdarjung, Palam, Lodhi Road, Ridge, Ayanagar and other locations. The readings from these stations help the weather agency monitor rainfall across the Capital and produce its rainfall reports.
What Does 8:30 Am Cut-Off Mean?
This is an important part of understanding IMD’s rainfall figures. The daily rainfall figure used by IMD is generally based on the 24-hour period ending at 8:30am. In other words, a day’s reported rainfall does not necessarily mean the rain that fell between midnight and midnight. For example, rainfall recorded between 8:30am on August 5 and 8:30am on August 6 is counted in the rainfall observation for August 6. This reporting window is why the timing of rainfall and the timing of IMD’s daily bulletin matter when comparing numbers.
Does A Rainfall Error Mean Weather Forecast Was Also Wrong?
Not necessarily as a rainfall observation tells us what has already happened while a forecast predicts what is expected to happen. They are separate processes, even though observed rainfall is used to monitor and verify weather conditions.
In Delhi’s case, the IMD also issued an orange alert on Tuesday, predicting light or moderate rain across most parts of the city and heavy showers in some pockets. By 5:30pm, however, Safdarjung had recorded only 4mm of rain during the relevant period, with other stations also reporting relatively small amounts.
So How Much Rain Delhi Actually Received This August?
After correcting the erroneous entry, IMD’s cumulative figure stood at 197.2mm for the first 10 days of August. That is around 36mm below the month’s long-period average of 233.1mm. The correction means that even though Delhi had initially appeared to have already crossed its normal August rainfall mark, the data showed that the Capital was still short of it.
