Context: Recently, a massive Shelf Cloud formation has been spotted in Haridwar, Uttarakhand.

What is Shelf Cloud?

• A shelf cloud is a low, horizontal wedge-shaped arcus cloud, resembling a shelf hanging from the sky.

• The shelf cloud is attached to the base of the parent cloud above it (usually a thunderstorm).

• It is usually associated with a solid line of storms. The wind will come first with rain following behind it. It may appear to rotate on a horizontal axis.

• Rising cloud motion often can be seen in the leading (outer) part of the shelf cloud, while the underside often appears turbulent, boiling, and wind-torn.

Formation

• The formation of shelf clouds occurs when a mass of cold and dense air is forcefully driven by the wind into a warmer air mass.

• This phenomenon takes place during the downdraft of a thunderstorm, where the cold air rapidly descends and disperses, creating a strong gust front.

Distinctive Features of Shelf Cloud

• Shelf clouds exhibit several distinct features that contribute to their dramatic appearance.The underside of the shelf cloud is often dark and menacing, featuring turbulent and rapidly rising air.

• This area is characterized by a swirling mass of clouds, indicating the intense updrafts and strong winds associated with the thunderstorm.

• The leading edge of the cloud is typically smooth and well-defined, appearing as a rolling arc or shelf. This edge may have a greenish or bluish tint due to the scattering of sunlight caused by water droplets within the cloud.

Significance and implications of Shelf Clouds

• As shelf clouds are usually associated with thunderstorms, their presence serves as a visual warning sign of impending severe weather. However, they are typically harmless.

Are they dangerous?

• Shelf clouds usually don’t cause tornadoes or extremely dangerous weather on their own.

• They often appear ahead of powerful thunderstorms with heavy rain, strong winds, and occasionally hail or tornadoes.

• These clouds usually form with thunderclouds and downdrafts, they are associated with strong wind gusts, heavy rain or hail.

What is Arcus Cloud?

• Arcus clouds are spectacular low-level, long, and thin clouds associated with powerful thunderstorms.

• They are sometimes seen beneath Cumulonimbus clouds.

• There are two types of Arcus clouds: Shelf clouds and Roll clouds.

• Shelf clouds are attached to the storm cloud, whereas Roll clouds are a horizontal column separated from the storm cloud.

Formation

• When a cold downdraft from a cumulonimbus cloud reaches the ground, the cold air may spread rapidly along the ground, pushing existing warm moist air upwards.

• As this air rises, water vapor condenses into the patterns associated with Arcus clouds. The new cloud may roll if it experiences different wind directions above and below.

• As Arcus clouds form with cumulonimbus clouds and downdrafts, they are associated with strong gusty winds, heavy rain or hail showers as well as thunder and lightning.

• Occurrence: The clouds have been seen in Kolkata, Madurai, Mumbai, Chennai, and several other Indian cities in the monsoon months.

• They are also seen around the world routinely in places such as Australia, Alaska, California, etc., during powerful thunderstorms.