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Answer 3 to the last question argued that air must sink to replace the air which moved from sea to land at low levels. This is not an especially satisfying answer because it doesn't involve a cause and effect - it would be equally correct to say that air must move from sea to land at low levels to get out of the way of the air that's sinking!
This is an example of a "kinematic" explanation: one which simply treats air motions as a consequence of other air motions, without addressing the underlying forces which must be there to cause the air to change speed or direction - to accelerate. But sometimes kinematic explanations are unavoidable.
In this case, what would happen if we made the hydrostatic approximation? Remember that the hydrostatic approximation assumes that gravity and the vertical pressure gradient force are always so close to balance that they can be assumed to be equal and opposite. For our sea breeze, this makes certain things simpler. For example, if we know the change in temperature (density) over land, we could compute the change in pressure directly from the hydrostatic equation, and thereby estimate exactly the horizontal acceleration.
But the vertical motion doesn't go away completely just because we declare the vertical acceleration to be very small. We can still determine the vertical motion kinematically by applying another law: the Law of Mass Conservation.
The Law of Mass Conservation means just what it sounds like: air cannot vanish or magically appear. Together with the fact that air is pretty much incompressible at a given pressure, this can be restated simply as "if air comes in, air must go out".
You'll see what I mean when I apply this principle to our sea breeze. Over land, air is moving in from the sea. We call this "horizontal convergence", or convergence for short. This makes for extra air over land; the excess air has to go somewhere, and we've already accounted for all the horizontal winds by considering the horizontal pressure gradients that drive them. The only other place the excess air can go is up, so there must be upward motion over land. This is nice, because this provides a source of air to feed the wind aloft blowing from land to sea.
Meanwhile, over water, air is leaving at low levels and blowing onto the land. We call this "horizontal divergence", or divergence for short. With horizontal winds already accounted for, the only source for replacement air is aloft. So there must be downward motion over water to counteract the horizontal divergence.
This is not very satisfying as a physical explanation, but it does lead directly to a nice simple picture of the sea breeze flow: a complete circulation with air blowing from sea to land at low levels, air rising over land, air blowing from land to sea aloft, and air sinking over water. In other words, it's great for figuring out what will happen, but it doesn't really explain the why. For that, you have to work directly with the forces, like we did in the previous page of this module. Use the principle of mass conservation to answer the following questions:
1. Strong divergence (outflow) is typically observed in the upper troposphere above a hurricane. Therefore the interior of the hurricane must be dominated by:
2. Convergence is typically observed at low levels within low pressure systems over land. Therefore the weather near low pressure systems will tend to be:
3. Observing a severe thunderstorm, a Doppler radar suddenly detects an area of apparent strong convergence in the upper portions of the storm. What warning should be issued?
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