Consider the atmosphere. Better yet, genuflect. The oxygen content of the air keeps us alive, and the movement of air, though occasionally destructive, is, on the whole, remarkably tame.
The Scale of the Atmosphere
It's easy enough to see that the atmosphere envelops the entire earth, which is about 25,000 miles around. Applying the formula we all learned in high school (and since forgot), the atmosphere covers the approximately 300 million square miles of the surface of the earth. Since most of our planet's surface is relatively near sea level, the gravitational pull of the earth and the weight of the air above don't vary all that much from place to place, unless you climb a mountain.
The pressure at sea level hardly ever varies more than 10% from the average of about 1000 millibars. The lowest pressure ever recorded is 880 millibars. The temperature can be 130 degrees in the desert and fifty below zero at the poles, but for the most part stays roughly between zero and a hundred on the Fahrenheit scale.
Looking in the vertical dimension presents a much different picture. As you lower the pressure on a gas, its temperature falls and it expands. If you climb a ten thousand foot mountain, you will experience a temperature drop of between 30 and 50 degrees, and the pressure will decrease by one-third. If you climb to the top of Mount Everest, the pressure will have decreased by two-thirds and you'll need assistance breathing, and the temperature will be colder than at the south pole. Mount Everest is less than six miles high.
Think about that! Go six miles in one direction (up), and things change drastically. Go six miles in another direction (horizontally) and the change is imperceptible. It's because we normally travel horizontally that we don't realize just how thin the earth's atmospheric skin is.
What Makes the Air Move?
Meteorologists talk about a mythical "parcel" of air, a quantity of maybe a cubic foot that can be characterized as having a certain movement. We call the movement wind.
A parcel of air cannot just pick up and go where it wants, the way you can just decide to go to the grocery store. Air is constrained in its horizontal movement by two main factors: the shape of the earth's surface, and the pressure gradient (gradient is a scientist's way of saying change over distance).
On the flat surface of the earth, the air is pushed from the higher pressure to the lower. However, once a parcel of air starts to move, it is subject to another force that arises from the rotation of the earth. This Coriolis force is complicated and will not be discussed here. The result is that the air moves with low pressure on its left in the northern hemisphere, and with low pressure on its right in the southern hemisphere.
The Strength of the Wind
The wind speed is determined by the strength of the pressure gradient, and the latitude. The first is not very mysterious; the stronger the pressure gradient, the stronger the force on a parcel of air, and so the stronger the wind. The latitude enters through the Coriolis force, which varies inversely with latitude. For a given pressure gradient, the higher the latitude the weaker the wind.
Strong Winds of the World
A strong and destructive wind arises when the pressure gradient is very pronounced. This happens over a large area (many hundreds of miles) in a tropical cyclone. It happens on a somewhat smaller scale in extra-tropical cyclones. And it occurs on a small scale (a mile or less) in tornadoes. Winds in a hurricane can approach 200 miles per hour, and in extra-tropical cyclones can occasionally reach triple digits.
Tornadoes produce the strongest winds on earth, measured in extreme cases in excess of 300 miles per hour. The tornado has a very large pressure gradient over a relatively small distance, and the power of the wind is concentrated and intense.
Give Thanks
Step outdoors on a fine spring day, or almost any time for that matter, and be grateful that our earth and atmosphere are as agreeable as they are. Consider what it's like in an F5 tornado or at the top of Mount Everest and be happy that conditions at the surface of the earth are, on the whole, quite benign.
Further reading:
- Forces and Winds, University of Illinois
Jon Plotkin - The author was a math major at Cornell and has a master's degree in meteorology from MIT.
