With our low-pressure-area-the-size-of-Australia still very much in residence, I’m beginning to forget what a still, sunny day is like. I have to say that in my experience (six years) Hamilton is a pretty wind-free place. It’s certainly a lot quieter than where I lived near Portsmouth in the UK, which would be frequently blasted by south-west winds. That was one of the places where the trees tended to grow on a lean, denoting the direction of the prevailing wind. You could use them as a compass. So this week’s wind (which took down part of our fence at home) doesn’t feel all that unusual to me, though it probably does to long-term Hamiltonians.
So why is it so windy? When I first learned about high and low pressure areas, I had in mind that the wind should blow from high pressure to low pressure. That seemed perfectly logical, and couldn’t understand why the weathermen talked about winds blowing anticlockwise around a low-pressure system (northern hemisphere). Why not towards the centre of the system, to even out the pressure?
The reasoning is actually quite subtle and involves balancing the effects of the pressure gradient with that of the Coriolis force. Imagine you are at the north pole, and throw a ball southwards. OK, you don’t have much of a choice of direction, so let’s say you throw it southwards down the Greenwich Meridian. As the ball travels, the earth rotates underneath it. The rotation of the earth is from west to east, so, the path the ball appears to take to an observer on the earth is a curved one, bending towards the right. The faster the ball goes, the greater the force that appears to be acting. That’s the coriolis effect – in a rotating frame, there is an effective sideways force on something that is moving. In the southern hemisphere, the sideways force is in the opposite direction.
Go back to the (southern-hemisphere) low pressure system and imagine a lump of air moving from high pressure to low pressure. As it moves it will experience a sideways coriolis force, which will bend it towards the left (anti-clockwise). When it has bent through 90 degrees, the coriolis force now is in the opposite direction (towards the high pressure) to that due to the pressure gradient. At the right velocity, the two forces balance, and the air continues to move at a constant velocity (Newton’s first law) which will be along the isobars, perpendicular to the gradient. In the southern hemisphere, this will be clockwise about a low pressure area, and in the northern hemisphere is will be anticlockwise.
Where the isobars are close together, as we have had for the last week, we have a strong pressure gradient and we would expect a strong wind in order to give enough coriolis force to balance the pressure force.
That’s quite a tricky explanation to do in words – I hope I’ve got it right. I think after writing this there was good reason to be a little confused back at school as to what was happening.