Anvil of air
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An "anvil" of white clouds scatters late-afternoon sunlight at low angles over Kampong Trail, between Potong Pasir Avenue 1 and Woodleigh Park, at 6.30pm on Sunday.
The anvil refers to the flat, spreading top of a cumulonimbus cloud - a towering mass of turbulent rising air often reaching 10km high or more - and is named for its resemblance to the metalworking tool.
According to Associate Professor Koh Tieh Yong, weather and climate scientist at Singapore University of Social Sciences' School of Science and Technology, this occurs when atmospheric pressure at the top of the cloud is low. As a result, the air mass turns cold, loses its buoyancy and starts spreading sideways instead of rising further.
In Singapore's tropical climate, clouds form mainly through convection, which involves heated air at the planet's surface rising rapidly in the form of thermal currents. This typically produces cumulus clouds, which often appear in low, close clusters on sunny days.
However, cumulus clouds can combine to form tall towers known as cumulus congestus, or towering cumulus. These in turn may grow into cumulonimbus clouds, which can produce showers.
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