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Union University Department of Physics

The Science Guys



Science Guys > October 2001

October 2001

How does gravity affect the wind?

One doesn’t ordinarily think about our Earth’s gravitational field when a cooling wind helps to dry a tired brow. But the Earth’s gravity does indeed play an important role in the behavior of wind, albeit an indirect one.

The most basic answer to our question is that gravity is necessary in order to hold our atmosphere to the planet. Air molecules are in constant motion at the molecular level, like a room full of bugs flying in every direction. The typical air molecule moves about 500m/s (1100mph) in air at 27°C (about 80°F), but only for a very short distance before bumping into something or another air molecule. At the outer edge of our atmosphere those molecules moving outward, without anything with which to collide, could escape from the Earth. Some molecules move faster and some move slower, but the point is that without our gravity air molecules would wander away from us and out into space. Thus gravity is important just to keep the air around us.

You might think that air molecules moving at over 1000 mph in your home would constitute a wind beyond the ferocity of the most vicious hurricane, but that’s obviously not the case. This is because as many air molecules move upward as downward, and to the right as to the left, and they only move a tiny distance before colliding with something. Air molecules race about in random directions, careening off the walls and each other. There is no net migration of air molecules in one direction, which is a fancy way of saying their molecular motion creates no wind.

As gravity hugs the blanket of air to the Earth’s surface, what physicists call a density gradient is set up in the air. The air near the ground is pulled on by gravity and compressed by the air higher in the sky. This causes the air near the ground to be denser and at a greater pressure than air at higher elevations. You may have heard that mountain climbers must wear oxygen tanks if they ascend to the peaks of Mt. Everest in order to aid their breathing. The air becomes very thin at those extreme elevations and the strenuous climbing demands an abundance of oxygen- hence the need for oxygen tanks.

Now, consider yourself at the beach (perhaps a delightful prospect!) The sun is out and the ground is getting hot. The air at the ground thus warms up and expands. As air expands the density decreases so it has a tendency to rise. As it rises it does not leave a void. Cooler air over the cooler ocean moves inland to replace the rising air. Thus a sea breeze flows inland from the ocean, driven by a density gradient caused by the combination of gravity, pressure, and temperature effects.

At night the reverse happens. The land cools quickly relative to the ocean, and air over the ocean is warmer and rises. Air moves away from the land and over the ocean to replace the rising warm ocean air and we have a land breeze flowing from the land to the sea.

Of course, there are other factors that drive our major winds. Different areas of the Earth possess different temperatures and the winds interact with ground features. The winds on our Earth are the result of a complex interplay between temperature, pressure, and density gradients, and the Earth's rotation. But at its root, the density gradient that gravity helps set up is an important factor in making our winds possible.