BOW ECHO
Context:
The intense storm that hit Delhi recently appeared in an unusual crescent or archer’s bow shape on the India Meteorological Department’s (IMD’s) weather radar imagery, technically termed a “bow echo.”
WHAT IS BOW ECHO?
- A bow echo is essentially a line of storms, also called a squall line, that appears in a distinctive archer’s bow or crescent shape on weather radar imagery.
- Scale and Duration: A bow echo can extend from 20 km to 100 km in length and typically lasts between 3 and 6 hours.
- Significance for Meteorologists: Meteorologists specifically track bow echoes because they are often a precursor to more destructive windstorms.
- They are associated with severe straight-line winds.
- Origin of Term: The term “bow echo” was coined in the 1970s by Ted Fujita, a Japanese American meteorologist renowned for developing the Fujita scale for classifying tornadoes.
HOW DOES A BOW ECHO FORM?
The formation of a bow echo involves a self-sustaining cycle of atmospheric processes:
- Rain-Cooled Air Descent: When precipitation from thunderstorms falls, it cools the air.
- This rain-cooled air descends to the ground due to its higher density.
- Horizontal Spread and Gust Front: As this cool air reaches the surface, it spreads out horizontally.
- This creates a leading edge or boundary known as a gust front, which separates the rain-cooled air from the surrounding warm, moist air at the surface.
- Lift and New Thunderstorms: The gust front acts like a mini cold front, pushing up the warm, moist air into the atmosphere.
- This forced ascent of warm, moist air leads to the formation of new thunderstorms along the gust front.
- Reinforcing Cycle: These new thunderstorms produce more rain, which in turn creates more rain-cooled air.
- This continuous production of rain-cooled air helps the gust front to maintain and even intensify its strength.
- Bow Formation: As this process repeats and intensifies, a point is reached where there is a strong inflow of air on the trailing side of the line of storms.
- This inflow, combined with the strong winds at the leading edge, causes the entire squall line to bend outwards, forming the characteristic archer’s bow shape.
- Sustained Strong Winds: The cycle continues as long as new thunderstorms keep forming at the front, helping the system to grow and move forward, generating and sustaining strong, damaging straight-line winds.
BOW ECHOES IN INDIA
- Not a New Phenomenon: Bow echoes are not a new phenomenon in India.
- Previous Occurrences:
- On May 31, 2022, a bow echo formed over Delhi and Noida. Although short-lived (lasting about an hour), it produced winds of up to 100 kmph.
- Such squall lines have also been observed during thunderstorm activity in Odisha in recent times.
- Common Occurrence: According to senior IMD scientists, these phenomena have appeared often in India during intense thunderstorms.
- Delhi Storm Example: The recent Delhi storm (May 2025), with winds reaching up to 100 kmph, was a clear example of a bow echo’s destructive potential.
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