Featured Post

Tuesday, January 25, 2011

Local breezes and gusts of wind

WAVY SEA: The wind blows onshore from the sea by day. — Photo by Amanda Choo

Alan Rogers

ONE night a couple of weeks ago, when on a balcony of a village valley house deep in the French Pyrenees Mountains, a gust of downslope mountain air blew my newspaper out of my hands.
The following bright sunny day, I visited a French fishing port on the Mediterranean Sea and sat on the beach under cloudless skies to find that I was windburnt and not sunburnt.
These natural life experiences triggered my thoughts of Sabah where I climbed Mt Kinabalu and stayed in Keningau, as well as the numerous times I visited Santubong and Damai beaches in Sarawak.
Why do winds descend onto Damai and hit Keningau in the evening? Why by day do onshore breezes, even under a cloudy sky, turn my ‘Mat Salleh’ skin red?
Why is it dangerous to swim in the South China Sea at Damai at night? The notices tell us not to and we should heed the direction in which the red flags are flying.
“Mountains make their own climates! Discuss.”
This was a question I first encountered in the 1960s in my Oxford University entrance examination.
Air moves from localised high pressure to low pressure areas and thus winds from high to low pressure areas are created.
By day, both on Mt Santubong and Mt Kinabalu, the mountain slopes are warmed more quickly
owing to direct radiation from the sun, than the air in the valleys and thus lower pressure exists on the mountain and higher pressure in the valleys.
This generates an up valley or anabatic wind during the afternoon.
This is illustrated in Diagram 1.

At night air cools faster at the top of the mountains and then moves down slope to the warmer air in the valley bottoms creating a katabatic wind.
The greater the difference in the density of air (air pressure) between the summit and the valley floor, the steeper the pressure gradient and the faster the wind speeds — hence the loss of my newspaper in the Pyrenees.
I recall years ago in the Derbyshire Peak District in the UK, when ascending Mam Tor (hill) from the Edale Valley by day the up valley wind got under my raincoat.
I felt like a spaceman walking in thin air as I was lifted physically up the mountainside by the incredible up draught.
At Pine Lodge Resort, Keningau the refreshing and cooling night-time down draughts are welcomed after sweaty ascents to the summit of Mt Kinabalu earlier in the day.
Pressure differences also create local winds at sea level.
At Damai during the day, the land heats up more quickly than the sea.
(Land has a nine times faster capacity to gain and loose heat than water.) Out at sea during the
day, the cooler sea produces an area of high pressure whereas the warmer land is an area of low pressure.

This is shown in Diagram 2.
Remember air blows from high to low pressure thus onshore breezes from the sea by day.
Take a canoe by day into Damai Bay and you have to paddle very hard to prevent the onshore breeze taking you back to the beach.
No wonder I was wind burnt there even on a cloudy day.
As the land loses its heat faster than the sea, which releases the heat accumulated from the sun’s radiation more slowly, a reversal in local pressure conditions 
ensues. The land is now an area of 
cooler temperatures and higher pressure whereas the sea is warmer and inevitably an area of relatively low pressure.
Now these are land to sea breezes or offshore breezes. Take a canoe at night and 
certainly you will be blown out deep into the South China Sea and will certainly struggle to get back to the beach.
While local fishermen may not be aware of the mechanics of air movement, they have through valuable experiences understood the vagaries of these winds much as mountain climbers always respect the traditional adage that 
a wise climber starts to climb no later than day break.

Visit Santubong and Damai beaches and put this article to test.
If at Santubong look at the directions the flags are flying at the adjacent golf course holes and at Damai the direction of the flags flying there.
Are there land or sea breezes by day or night and is the wind descending from Mt Santubong at
night? Why does the cloud creep down that mountain at night?

This article is merely ‘a rule of thumb’ for global air pressure conditions affecting our daily weather in East Malaysia will also influence the strengths of local mountain/valley winds and land/ sea breezes.
Look vertically and horizontally and feel the strength of our local winds on your back or on your face.

No comments:

Post a Comment