 Let's see how the gradient wind flow aloft can affect surface weather. Look at how the velocity changes as air flows around the ridge and then a trough aloft. Initially, the velocity is about geostrophic and straight line flow. As it rounds the ridge, it speeds up and then it slows down again to geostrophic in the straight line section. As it goes through the trough around the low pressure aloft, it slows down to sub-geostrophic. And then speeds up to geostrophic in the next straight section. The speeding up causes divergence aloft, and the slowing down causes convergence aloft, just as you learned in lesson 9. You also saw how convergence aloft can lead to divergence at the surface, which contributes to a surface high, and how divergence aloft can lead to convergence at the surface, which contributes to surface low. Thus, gradient flow contributes to surface weather. We often see a surface low forming on the downwind side of the trough.