27-05-2025
What is a bow echo, how did it signal Delhi's fierce storm?
Storms may be triggered by moisture influx, winds changing with height, and rapid cooling aloft. But when the winds that drive these storms can become increasingly dangerous, it can be detected by a special radar observed feature known as the 'bow echo'.
In the early hours of Sunday (May 25), Delhi-NCR was hit by squalls or sudden storms accompanied by strong winds, with speeds around 100 kmph. Behind the scenes, the IMD Radar imagery clearly showed a chain of thunderstorms with a bulge in the centre — the shape of an archer's bow.
A bow echo is a radar signature of a high wind-driven storm front. It forms due to strong winds pushing the centre of the storm line forward and indicates intense straight-line winds. This is often a precursor to more destructive windstorms.
The term 'bow echo' comes from the way bands of rain showers or thunderstorms curve outward, or 'bow out,' as strong winds within the storm reach the surface and spread horizontally, as per the National Oceanic and Atmospheric Administration (NOAA). Ted Fujita, a Japanese American meteorologist known for developing the scale to classify tornadoes, coined this term in the 1970s.
Like bow echo, the Met Office also monitors other special-radar observed features such as the 'hook echo' — which is generally associated with tornadoes.
While not heavily discussed in the Indian meteorological landscape, the bow echo is an important radar feature commonly associated with derechos, fast-moving swaths of widespread windstorms most frequently observed in the Midwestern United States during late spring and summer.
How does the wind mechanics work?
Behind a bow echo are powerful winds that continuously spread out after downdrafts. Umasankar Das, senior scientist, National Weather Forecasting Centre at the IMD, said, 'A 'bow echo' is a radar signature of a convective system (CS), typically a squall line, that appears bowed or arched when viewed on radar. This bowing shape results from strong winds spreading out from the storm's downdrafts, creating a gust front at the surface. Bow echoes are often associated with severe weather, including damaging straight-line winds.'
A bow echo forms when a group of thunderstorms pushes strong, cool winds from the storm down to the ground. This cool air spreads along the surface, pushing the warm, moist air ahead of it upward, creating new thunderstorm cells. As more storms form and rain falls, more cool air builds up behind the storm, strengthening this push forward, called the gust front. This makes the storm line bend or 'bow' outward, like an archer's bow. The cycle keeps going as long as new storms keep forming at the front, helping the system grow and move forward with strong winds.
What was its role during the May 25 storm over Delhi-NCR?
Sunday's storm was driven by favourable conditions, including the presence of low-pressure zones due to a western disturbance that lies as an upper-air cyclonic circulation over north Punjab and adjoining Jammu and Kashmir, and two more cyclonic circulations over West Rajasthan and North Haryana, as per the IMD. At Delhi's primary weather station, winds reached up to 82 kmph.
'A slightly slow-moving severe storm compared to earlier estimates now attained proper 'Bow Echo' visible on radar imagery…' wrote weatherman Navdeep Dahiya on X on Saturday night, after sharing a Doppler radar image. As the storm progressed from Punjab and Haryana towards Delhi and Noida, the bow formation as part of the radar signature became increasingly evident, indicating that the storm continued with higher intensity. The higher the convexity, the more intense the system is.
'This is nothing but the movement of thunderclouds observed by Doppler radar. The winds at 15,000 to 20,000 feet usually carry or push the thunder clouds in their direction. In this case, the winds were blowing from the Northwest. Thunderclouds moved from Punjab to Haryana and then reached up to Delhi,' said Mahesh Palawat, vice president of Skymet Weather Services.
'..Such a squall line was observed during thunderstorm activity in Odisha too, this month', said Rahul Saxena, senior IMD scientist. He added that these have appeared often in India during intense thunderstorms.
While not unknown to India, the IMD observed a similar feature during a 100 km/h squall on May 31, 2022, across Delhi and Noida, which was, however, 'short-lived', lasting for an hour and recorded at 100 kmph over Safdarjung, whereas 70 kmph at Palam. Typically, a bow echo of far-destructive nature lasts 1 to 3 hours, depending on factors like moisture availability.
