Wind
All the answers so far are true!
However most students find it so difficult to understand the air at first. Why - because we cannot use the knowledge of our five natural senses; sight, smell, taste, hearing and touch. The air is invisible to us and like electricity strange.
Water is much more tangible and therefore easier to understand. The Ocean currents are 'wind' just the same and caused by temperature changes and the rotation of the earth (Coriolis effect). Sudden local changes in direction and speed are caused by channels and protrusions on the sea bed and above it. Ships are crashed against rocks and people are sucked under to drown because of these sudden changes. If we wish to cross a river direct then we have to head upstream or into the flow or current to reach a point on the other side.
I find with my students that if we can understand the effects in water first then it is much easier to move on to the air and Coriolis, gradiant winds and the like.