Why might clouds move in different directions to the wind?

Thanks to Paul Williams for the answer!

James - It's usual for the wind speed and even direction to be different at the altitude of the clouds than at ground level. This is known as wind shear. Before we get into that though, what actually causes wind in the first place? Paul Williams is Professor of Atmospheric Science at the University of Reading.

Paul - Winds in the atmosphere are generally generated by pressure differences, and I think we're used to seeing these on weather maps. In general, you might have high pressure in one area, low pressure in another area, and of course, the wind will try to blow from high to low pressure, except there's a complicating factor, which is that we're on a rotating planet, and so as it tries to blow from high to low pressure, it gets deflected to the right in the northern hemisphere by something called the Coriolis force, which is a mysterious, actually fictitious force, doesn't really exist, but we have to include it in our equations to account for the fact that we're on a rotating planet.

James - This means that in general, in the atmosphere, wind does not travel in the direction of high to low pressure, but in the perpendicular direction, bizarrely. But what's causing vertical wind shear, i.e. the difference in wind velocity and direction for neighbouring bits of the atmosphere, one on top of the other?

Paul - At the level of the clouds, when the air is moving, there is a balance of forces. We have the force resulting from the pressure difference being balanced by the Coriolis force, and that's nice and simple, and that's what sets the wind direction at those altitudes, but when we get close to the ground, there's a third force that comes into the equations, and that's associated with friction. It's called the turbulent drag, resulting from the interaction between the atmospheric flow and the surface of the earth, and that reduces the speed, and it puts this third force into the equation, and that gives us a different wind direction. So that's the basic reason why the wind direction can be different at the level of the clouds compared to at ground level. And interestingly, we have the same effect in the ocean as well, not just the atmosphere. And in fact, this phenomenon was first discovered in the ocean by a Norwegian scientist called Ritjof Nansen, who in the 1890s was on an expedition of the Arctic, and he noticed that the icebergs were not moving in the same direction as the winds and the surface ocean currents, and that sparked his interest, and eventually a Swedish oceanographer called Ekman studied the mathematics of this, and so this effect, the fact that the wind direction changes with height both in the ocean and the atmosphere, is today called the Ekman spiral, after him.

James - So Tony, your observations are correct. Wind does in fact change velocity and direction at the altitude of the clouds and at ground level, and this is known as wind shear. The main contributing factor is friction, caused by the wind bumping into obstacles like the terrain, trees, buildings and the like down here on the ground. Variations in pressure do influence the winds and can vary with altitude, though they are generally related. Thank you to Professor Paul Williams from the University of Reading for helping us clear the air.