Embedded Files
Imagine a south-facing coastline. Because of solar heating, a pressure gradient has developed, with the pressure gradient force directed toward the north, toward land.
Because of this force, the low-level air is accelerating northward, producing a wind blowing from sea to land. As the air passes over land, it feels the retarding effect of friction. This takes the form of a force directed in the direction opposite to the way the wind is blowing. (To slow something down, you have to push against it.)
At first, friction is much weaker than the pressure gradient force, and the wind keeps increasing.
But the force of friction, just like wind resistance, increases at a rate proportional to the square of the wind speed. As the wind speed increases, the strength of friction gets closer to the strength of the pressure gradient force.
As these two forces become similar in strength, the acceleration of the wind tapers off. (Remember, acceleration (times mass) is equal to the sum of the forces.) Eventually, the wind gets so strong that the friction force equals the pressure gradient force. And that's it! The wind speed reaches its highest value, and stays there.
With the net force equal to zero, the air can neither speed up nor slow down. This is called a steady state.
Common experience might dictate, incorrectly, that if there was no force on the air, it would slow down. In daily life, when things left alone slow down, it's not because there's no force acting on them, it's because there's no force other than friction. In this sea breeze example, we've got two forces: the friction trying to slow the air down, and the pressure gradient force trying to speed the air up. This is why the air's velocity can be maintained.
Basic concept: why do strong pressure gradients correspond to strong winds?
Report abuse