 We can use a very simple demonstration to show how differences in velocity from one end of an air parcel to the other can cause changes in area. Let's let this be our air parcel here, outlined in the dark blue. Several things can happen to this air parcel. One, it can translate, so it can just move with a certain velocity across from left to right. The second thing it can do is it can have a zero velocity here and have a higher velocity here on this end, and it can then grow, and so you see that the area is increasing as time goes on. So we can combine these two motions to see what happens, and so we have some velocity at the parcel, but we have a greater velocity on the right hand side, and we see that as it moves, it grows. It is also possible that as it moves, it shrinks because if the velocity on this side is less than the velocity on this side, then as it moves along, you will see that actually the area is contracting. We can do the same sort of analysis in the y direction, and from this we can show that in fact the difference in velocity from here to here can result in the growth or the shrinking of the area of the parcel.