Squall-Lines

SQUALL-LINES

Moving lines of thunderstorms

Squall-Lines

Squall-lines is a term used for describing moving lines of thunderstorms. They are hundreds of kilometers in length and have a life span of several hours, which is considerably longer than its component thunderstorms. Squall-lines generate gusty winds and heavy rains which are more intense and extensive than individual thunderstorms.

Squall-lines are often observed during the southwest monsoon along the west coast of Peninsular Malaysia and coastal areas of East Malaysia facing the South China Sea.

In Peninsular Malaysia, squall-lines referred to as "Sumatras" are formed along the Straits of Malacca during the night and pre-dawn. Sumatras tend to move eastwards in the morning affecting mainly the west coast states of Peninsular Malaysia.

Squall-lines in East Malaysia appear to exhibit the same characteristics, developing off-shore in the night before sweeping landwards in the morning.

What happens when a squall-line approaches

If you live along the coastal regions of western Peninsular Malaysia....
	You observe a characteristic cloud formation, a thick band of cumulonimbus clouds which rises to great heights, approaching from the sea.
	Wind speed rapidly accelerates and becomes gusty as the squall-line approaches.
	There is a sudden drop in temperature.
	The passage of the squall-line is often accompanied by thunder, lightning and heavy rain.
	On reaching land, deprived of its latent heat of condensation, the squall-line rapidly dissipates.
	Rainfall decrease in intensity, and conditions return to normal within about an hour.

When are squall-lines most likely ?

Squall-lines usually occur during pre-dawn and early morning, and are most frequent between the months of April and November. However, squall-lines can occasionally form in the afternoons and move landwards along with the prevailing winds.

Origin of "Sumatras"

The Sumatras is a phenomenon unique to the west coast of Peninsular Malaysia.
	Its formation is due to the location of the Indonesian island of Sumatra on the west, and Peninsular Malaysia on the east, separated by the narrow Straits of Malacca.
	During the Southwest Monsoon, southwesterly winds blowing over the mountain range of Sumatra creates mountain waves on the leeward side, resulting in an unstable atmosphere.
	Relatively warmer waters of the Malacca Straits provide a source of moisture for convective development.
	In a highly unstable atmosphere, thunderclouds grow in-situ and merge into line squalls.
	Squall-lines move northeastwards steered by the southwesterly wind towards the west coast areas of Peninsular Malaysia.

Squall-lines are dangerous

Squall-lines indicate areas of extreme instability and severe turbulence. Embedded in squall-lines are thunderstorm cells, which can produce lightning, heavy rain leading to flash floods, and strong winds. In severe cases, they can cause injuries and loss of lives as well as damage to property.

Squall-lines are extremely dangerous to aviation as.....
	Strong wind shears within clouds are the cause of many fatal crashes.
	Downdrafts and updrafts in cloud are dangerous to light aircraft which are not equipped with sophisticated instrumentation for their detection.
	Downbursts from below cloud may affect flights at low levels particularly as aircraft approach airfields.

Flash floods can claim victims....

Due to the high intensity rainfall, low lying and poorly drained areas may be subjected to heavy flooding as the squall-line passes.

In late 1996 Penang and Prai was affected by flash flood caused by Sumatras which developed in the northern Malacca Straits moving rapidly inland in the early morning. These floods resulted in considerable damage of property and public facilities. The rainfall was the highest ever recorded.

Role of the Malaysia Meteorological DEPARTment

All meteorological forecast offices at principal airports in the country are equipped with weather radars which can detect development of squall-lines and monitor their movement. The meteorological office at the Kuala Lumpur International Airport at Sepang has a Doppler Radar capable of detecting wind shear in squalls. With this latest radar technology to detect hazardous weather conditions, timely warnings are issued by the Malaysia Meteorological Department.