Weather Bee: Why Meerut, Delhi rained like Mumbai this week

DElhi and districts nearby received heavily rainfall this hebdomad, in particular since midweek. As HT reported on fri, it was not a record-breaking spell in Delhi, but it was still a lot of anomalous rain. To be sure, Mumbai got more rain than Delhi over the four days ending July 10, and most of Meerut got as much rain as Mumbai. In fact, a large part of the western coast got about the same amount of rain as Delhi and neighbouring districts. However, HT’s analysis of the gridded data from the India Meteorological Department (IMD) shows that this quantity of rain is anomalous in the northern districts, not on the western coast. This highlights some important features of the monsoon.Here are three maps that explain that.That Meerut got as much rain as Mumbai can be seen from the accompanying map of rainfall rates for the four days ending July 10 (IMD counts the 24-hour period ending 8:30 am on the day as the rainfall for that date). Delhi averaged around 30 mm/day in this period, which the IMD classifies as “moderate” rain. Some districts to the east and north-east of Delhi, such as Meerut and Muzaffarnagar, however, received more than 64.4 mm of rain, which the IMD classifies as different categories of heavy rain. Large parts of the western coast received moderate rain like Delhi, with Mumbai and neighbouring districts getting different levels of heavy intensity rain. While both the west coast and the pocket of northern India described averaged moderate to heavy rain in the past four days, it was more unusual in the former than the latter. This can be seen by comparing the rain in the past four days with the 1971-2020 average for these four days. IMD currently considers the 1971-2020 average as the Long Period Average (LPA) for checking how unusual rain is. The comparison with the LPA shows that rain in Delhi was around five times the LPA for Delhi and around 11-12 times the LPA in Meerut and Muzaffarnagar. On the other hand, the rain was relatively more business-as-usual on the west coast: only around 1.5-2 times the LPA in places that averaged moderate rain. In Mumbai, it was only around 2.5 times the LPA. Even in Thane and Pune, where rain was more intense, the ratio was 6-8 times, not 11-12 times like in Meerut. Why is it more normal for the western coast to get heavy rain than for Delhi or western Uttar Pradesh? Because of the physics of rainfall. Rainfall first needs moisture in the air. This is far more readily available on the western coast because of its proximity to the sea than for landlocked places such as Delhi and western Uttar Pradesh, where local moisture supply is lower and external moisture arrives from faraway Bay of Bengal. This is one reason why moderate to heavy intensity rain is far more usual for places on the western coast than in places near Delhi.To be sure, moisture in the air is a necessary, but not a sufficient condition for rainfall. For example, there are many days in the monsoon when the air is clearly humid or saturated with water vapour (commonly called moisture), but it does not rain. This is because rain is water (the liquid state of water) and not water vapour (the gaseous state of water). For rain to happen, water vapour must cool down to water’s liquid state. A common example of this is beads of water appearing on a refrigerated bottle of cold drink kept outside on a humid day. Since there are no refrigerated bottles in the atmosphere, moisture is cooled down differently: by rising up in the air. When air pressure decreases, moisture rises up, where temperatures are cooler, and condenses to form rain. Even this lift-up has support on the west coast. The hills of the Western Ghats act as an obstruction to the moist wind blowing in from the sea, leaving the wind no option but to rise. This is another reason why moderate to heavy intensity rain is far more usual for places on the western coast than in places near Delhi. This also explains why the hilly regions in the north are somewhat rainier than places near Delhi.Clearly, Meerut and Delhi have no business being as rainy as Mumbai and Pune. How did this happen then? Meerut and Delhi may not have hills like the Western Ghats, but that is no bar on low pressure. That is exactly what made these places rainy this week. As the accompanying map shows, the region around Delhi and Meerut had some of the lowest pressures around July 9, when it was raining the most. This is why it rained down like the western coast here around that time.