At least in Delhi, the declaration of monsoon onset may not have appeared to square with reality since the rain hardly had any intensity The India Meteorological Department (IMD) declared the onset of monsoon over Delhi on July 2, as well as parts of Uttar Pradesh and Madhya Pradesh it had not covered by July 1, apart from making further progress in Punjab, Haryana, Gujarat, and Rajasthan. At least in Delhi, the declaration of monsoon onset may not have appeared to square with reality since the rain hardly had any intensity. How did the IMD declare monsoon onset over Delhi then? Here are three charts that try to make sense of this.HT ImageThat Delhi did not receive much rain on the day IMD declared monsoon onset over Delhi can be seen by looking at the rainfall different weather stations in Delhi received in the 24 hours ending 08:30 AM on July 2, the standard 24-hour period for which daily rain is calculated in India. Of the 11 weather stations, four received less than 2mm rain, three received 2-3 mm rain, another three received 4-5 mm rain, and only one -- Chattarpur – received 18.5 mm rain. 24-hour rain in the 0.1-2.4 mm range is called “very light rain”. Similarly, “light rain” is the category for rain in the 2.5-7.5 mm range and “moderate rain” is the category for 7.6-35.5 mm rain. Clearly, most of Delhi got light to very light rain.Chart 1HT graphicIs such low intensity rain expected when monsoon has arrived? Station-wise data is not publicly available from the IMD for a long enough period to verify this. However, the gridded data of the IMD – it averages rain over boxes of two latitudes and longitudes 0.25 degrees apart – can help us in answering this question. This shows that Delhi averaged rain of 2.2 mm in the 24 hours ending 08:30 AM on July 2 (otherwise stated, this simply reported as the rain on July 2 by IMD). As the accompanying chart shows, there have been several years since 2001 -- the first year for which onset data for Delhi is available – when Delhi has averaged rain around this level or under 7.5 mm, which would be categorised as light rain.Also Read: Weather Bee: How cool is this summer?To be sure, it is also possible that the IMD declared onset after observing the rain after 08:30 AM. For example, in some years when rain up to 08:30 AM was very light, the period after got more rain. However, there are some years when the rain on both days was in the very light or light category.Chart 2HT graphicThe chart above suggests that intensity of rain is not all that important for declaring monsoon onset in Delhi. So how does the IMD get to know that the monsoon has arrived in Delhi? This is because while monsoon is Delhi’s (and India’s) main rainy season, rain is not the only aspect that characterises this season. Another characteristic of the season is an elongated anti-clockwise pattern of wind at the altitude of around 1-1.5 kilometres from surface (or at the altitude of 850 millibar pressure), with winds over Delhi blowing from the east. This happens because of a pressure gradient being established around a low-pressure region, which is called a trough because of its elongated shape. The physics of this wind movement is similar to the anti-clockwise rotation of wind around a low-pressure region in a cyclone. When this pattern is established along with widespread rain and towering clouds, one can say that the monsoon has arrived in Delhi. As the accompanying map shows, models predicted – predictions made very close to the event largely hold true -- that this wind pattern was established by July 2.MapHT graphicTo be sure, rains not being intense enough does mean that the onset of monsoon was not strong. As HT reported on July 3, the low-pressure region shifted somewhat southward of Delhi soon after it had arrived over Delhi, taking rain away from it. Nonetheless, it does look like the bare minimum conditions required for declaring monsoon onset were established by July 2.Unlock a world of Benefits with HT! From insightful newsletters to real-time news alerts and a personalized news feed – it's all here, just a click away! -Login Now!See Less