Forces in the Atmosphere, Page 6 of 11

Gravity is the most obvious force, one that everybody feels. (You don't think about it until you accelerate in response to it, like on a roller coaster or a bungee jump.) Gravity always points "down", toward the center of the Earth (or to the center of mass of whatever object is supplying the force). By itself, gravity is strong enough to cause a change in velocity (an acceleration) of 9.8 meters per second per second at sea level (or about 20 miles per hour per second), independent of the mass of the object being pulled downward.

To see if gravity is presently acting, drop something. Presumably, it accelerates downward. When it hit the ground, it was going much faster than just after you released it. If the object started high enough above the ground or the floor, it might have been falling for an entire second. If so, it was traveling at approximately 9.8 meters per second when it hit the ground. (No cheating please; this doesn't apply if you use an object such as a feather or a piece of paper that is significantly affected by some other force such as wind resistance or aerodynamics.)

Right now you're sitting on a rapidly-spinning Earth. At this very moment you're hurtling around the axis of the Earth at roughly 750 miles per hour. Because of this, everything on Earth feels an apparent force called the Centrifugal Force that, if you turned off gravity, would cause us to fly off the spinning Earth into space! In fact, because of the centrifugal force, the Earth is slightly bulged at the equator and slightly flattened at the poles.

We are not capable of distinguishing between the Earth's gravity and centrifugal force. What you think of as the downward pull of gravity is actually the combination of gravity and the much weaker upward/sideward pull of the centrifugal force. "Down" is the direction that the two combined forces pull us, not quite directly toward the center of the Earth.

From now on, we'll keep gravity and the centrifugal force together, and just refer to the combination of them as "gravity".

Notice something obvious about a falling object: as it falls, it passes through the air. But all objects experience gravity; what's holding the air up? In other words, why doesn't air fall too?

The force that counteracts gravity and keeps the air up in the atmosphere is called the vertical pressure gradient force. (Technically, it's actually the vertical component of the pressure gradient force, fundamentally no different from the horizontal pressure gradient force which we'll get to later.) The air pressure is essentially zero at the top of the atmosphere and increases rapidly as you go down: every 3-4 miles, the air pressure doubles.

Imagine an air parcel again, floating in the air. The air pressure at the top of the parcel, pushing it down, is just a little bit weaker than the air pressure at the bottom, pushing it up.

This slight excess upward force is just enough to counter the force of gravity, that is, the weight of the air. So the air floats in equilibrium, just like the air around it.

The main driving force of motions in the atmosphere is the horizontal pressure gradient force. Horizontal variations of air pressure are much weaker than vertical pressure variations, only about one ten-thousandth as large. But there's no horizontal gravity force to counterbalance them. So the horizontal pressure gradient force generally forces the air to move, producing wind.

Horizontal pressure gradients work just like vertical pressure gradients: a stronger pressure on one side of an air parcel than the other causes a net force (push) in a particular direction.

Horizontal pressure gradients are typically the only force mentioned on the nightly weather report. Forecasters are always talking about the positions of high and low pressure systems. Between the highs and the lows, there are horizontal pressure gradients and winds.

Friction is one force that only exists if air is in motion. Although there can be frictional forces within the upper atmosphere, they are usually very weak. The only place friction becomes a significant force for the atmosphere is near the ground. There, objects as small as blades of grass or waves in the ocean, or as large as trees or buildings, slow down the air. The effect of friction is felt as high as 1-2 km (1 mile) up in the atmosphere during the daytime, as turbulence extends upward from the ground.

The coriolis force is the final atmospheric force of fundamental importance. Like the centrifugal force, it is caused by the rotation of the Earth. Also like the centrifugal force, it is an "apparent" force, one that wouldn't exist if you weren't rotating along with the Earth. And for its strangest characteristic, it only exists when you're moving on the Earth.

The coriolis force results from the fact that the Earth is constantly changing its orientation beneath you. You don't notice it much, unless you look at the sun or stars and find them in a different position than they were in an hour ago.

Imagine the following experiment: you are standing at the end of a 10,000 foot runway facing south, at noon. You take a ball and roll it down the center of the runway, in the direction of the sun. What happens?

Questions or Comments

Technical: E-mail John Fulton < jdfult@nimbus.met.tamu.edu >
Scientific: E-mail Dr. John Nielsen-Gammon. < nielsen@ariel.met.tamu.edu >