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Sun 11 Jul 2004
Category : Commentary/wind.txt
I have the answer to my own question. It takes longer to fly back to Singapore than to fly to San Francisco because there is a constant headwind that reduces the speed of the aircraft by about 15-20%. (When flying to San Francisco, the plane was aided by a slight tailwind.) That's why it takes about 20% longer to fly back, increasing the journey by more than 2 hours.
So the next question is: why is there a headwind? And I believe it's like that the whole year round because it always takes longer to fly back. Why does the wind always blow from west to east, at least at high altitudes? What makes this puzzling is because I know, from sailing dinghies off Changi, that the wind usually blow in-shore from the east?
A quick search through Google turns up these two articles : What makes the wind? and Weather Systems from West to East.
So the earth's rotation is indeed the cause. But it's counter-intuitive. We should expect to reach Singapore faster when coming back from San Francisco because the earth's rotation swings Singapore back towards us.
But the earth's rotation also causes the hot air that rises from the equator, on its way towards the poles, to move east. This is because, due to something called the Coriolis effect, they maintain the speed of rotation at the equator.
"So, as these winds travel away from the equator, they move eastward relative to the ground beneath them - since the winds have a greater rotational speed than the ground. This explains why high altitude winds blow from west to east. And it is these high altitude winds that, to a large extent, control the weather."
So, mystery solved. But the point I'm getting at is that we can learn almost anything we want from the world-wide web, without going through formal school. Here's to Tim Berners-Lee.