It’s been 41 years since Delhi endured a single-day rainfall in July as torrential as was recorded in July 1982. The Safdarjung observatory of Delhi recorded 153 mm rainfall for July 10, making it fall under the category ‘very heavy’.
There are five categories within which a day’s rainfall may be classified based upon its intensity. A drizzle below 15 mm is classified as light; moderate is between 11 mm and 64.5 mm; 64.5 mm to 115.5 mm is considered heavy; 115.6 mm to 204.4 mm is very heavy; and rainfall beyond 204.4 mm is extremely heavy.
In 36 hours starting 8.30am on Saturday, Delhi recorded an unprecedented 260mm of rainfall — over 30 per cent more than July’s quota of 195.8mm — prompting the government to issue a flood warning and shut schools in the Delhi NCR region on Monday.
The water level of the Yamuna River in Delhi is on the rise, with the recorded level reaching 203.62 m at the old railway bridge on Sunday. The warning level stands at 204.50 m.
Why is Delhi experiencing such heavy rainfalls?
“There has been a strong interplay between an active western disturbance over the Himalayas and a vigorous monsoon leading to extremely heavy rains over major parts of the region,” said the India Meteorological Department (IMD).
July marks the beginning of monsoon in India, which is a result of a seasonal wind system known as the Southwest Monsoon. During summer, the landmass of India heats up faster than the neighbouring seas, creating a low-pressure system over the region. At the same time, high-pressure conditions prevail over the Indian Ocean. This contrast in pressure sets up a pressure gradient that drives moist air from the ocean towards the land. As the warm and moist air from the Indian Ocean moves towards the land, it encounters the Western Ghats and the Himalayas. These geographic barriers force the air to rise, leading to the formation of clouds and subsequent condensation. This process results in heavy rainfall across various parts of the country. Now, the Southwest Monsoon is a normal occurrence but it’s made worse when it collides with the Western Disturbance, as is happening at present.
What Are Western Disturbances?
The Western Disturbance is a meteorological phenomenon that affects the weather patterns in North India, particularly during the winter months. It is a weather system that originates from the Mediterranean region and moves eastwards towards the Indian subcontinent. As cold air from the polar region encounters warm and moist air from the Mediterranean, it creates an atmospheric disturbance. This disturbance forms a low-pressure system known as ‘trough’ in the upper atmosphere, which brings changes in weather conditions. It is due to the interaction of these two phenomena, along with a cyclonic circulation over central Rajasthan, that the northern Indian belt is experiencing such heavy rainfalls.
Having seen the unit of length ‘mm’ (millimetre) being used to categorise rainfalls, it might seem a bit confusing as to why volume is being measured using a unit of length.
Rainfall is typically measured through a cylindrical instrument called the rain gauge which has a length of 8 inches (204.4 mm). To measure rainfall, the rain gauge is placed in an open and unobstructed area, away from trees, buildings, or other objects that could interfere with the collection of rainwater. Now, as the rain keeps pouring, the water keeps collecting within the cylinder. The collected water level can then be read on the measuring scale of the rain gauge. The scale indicates the depth of water collected, which represents the amount of rainfall.
Numerous states in Northern India have experienced an intense spell of monsoon rain, resulting in significant damage, loss of life, and property across the country.The rising water levels in the Beas River caused landslides and flash floods in various districts, leading to tragic loss of lives. In Himachal Pradesh, heavy rainfall has triggered landslides and flash floods. The meteorological department issued a red alert for Monday for certain parts of the Jammu region and Ladakh, while residents in dozens of lower catchment areas in the Jammu region were evacuated to safety as a precautionary measure.
Uttarakhand is on heavy rain alert for Monday, while the Solani river in Haridwar crossed the danger mark of 231 m.
As per a 2021 study published in the European Geosciences Union’s Earth System Dynamics journal, rising temperatures will lead to Indian monsoons becoming more intense as well as unpredictable. “For every degree Celsius of warming, monsoon rainfalls will likely increase by about 5%”, said Anja Katzenberger, the lead author of the study.
A report published by the Indian Ministry of Earth Sciences, India’s surface air temperatures have risen by 0.7°Celsius from 1971 to 2018 which has also caused the air to become more moist. The Indian ocean’s surface also experienced an overall increase in temperature by 1°C from 1951 to 2015. These changes have been constantly building up and altering the region’s rainfall patterns, which has now resulted in an orange alert for the capital along with red alerts for hilly states like Himachal Pradesh and Uttarakhand.