Seventy three people have been confirmed dead and hundreds more are still missing after the heaviest rains in over twenty years have brought widespread flooding and numerous landslides. The worst hit area is in the Kegalle District in the central part of the country, where a series of huge landslides have engulfed at least three villages, and driven back repeated rescue missions by the Sri Lankan military. Landslides
are a common problem after severe weather events, as excess pore water
pressure can overcome cohesion in soil and sediments, allowing them to
flow like liquids. Approximately 90% of all landslides are caused by
heavy rainfall. Other areas of the country, including the capital, Colombo, have suffered extensive flooding, with close to a quater of a million people in temporary shelters.
The aftermath of a landslide in Kegalle District, Sri Lanka, on Wednesday 18 May 2016. Eranga Jayawardina/AP.
The rains are associated with the onset of the Sri Lankan summer monsoon, which lasts from May to October, typically brining around 400 mm of rain to many parts of the country in an average year. Monsoons
are tropical sea breezes triggered by heating of the land during the
warmer part of the year (summer). Both the land and sea are warmed by
the Sun, but the land has a lower ability to absorb heat, radiating it
back so that the air above landmasses becomes significantly warmer than
that over the sea, causing the air above the land to rise and drawing in
water from over the sea; since this has also been warmed it carries a
high evaporated water content, and brings with it heavy rainfall. In the
tropical dry season the situation is reversed, as the air over the land
cools more rapidly with the seasons, leading to warmer air over the
sea, and thus breezes moving from the shore to the sea (where air is
rising more rapidly) and a drying of the climate.
Diagrammatic representation of wind and rainfall patterns in a tropical monsoon climate. Geosciences/University of Arizona.
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