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Pressure gradients: Highs and Lows
   Perhaps because we usually can't see it, we take air for granted, and ignore some of its physical characteristics. But air is as real as water just much less dense and is another key player in creating Earth's weather. Wind Diagram

   Our atmosphere is composed of 78% nitrogen and 21% oxygen, with the balance made up of smaller amounts of what are called "trace gases." If you weighed all the molecules piled high in a stack over a one inch square stretching from Earth's surface to the edge of our atmosphere, you'd find 14.7 pounds worth of atoms pressing down on our planet. All those molecules exert their downward force and create what's called "air pressure". At Earth's surface, this pressure is called one "1 atmosphere." Experiments by Evangilista Toricelli in the mid-seventeeth century found that this pressure, weighing down on water surrounding an empty tube, forced the liquid up a certain number of inches. Contemporary barometers (the name for the instruments we use to measure pressure) record pressure as how many inches of mercury (Hg) are forced up the tube. Meteorologists use a measure known as "millibars" (mb), which is what you'll see on weather maps. One atmosphere equals 29.92 inches of mercury (29.92 Hg) equals 1013.25 millibars.

   Farther up there are less molecules of air between you and the outer edges of the atmosphere. So as you might expect, there's less air weighing down, and so air pressure is less no matter how you record it! The higher the altitude, the lower the pressure: the lower the altitude, the higher the pressure. At 12 km above Earth, the pressure might be 100 mb. At 50 km, only 1 mb.


High and Low pressure
   There's an old saying, "Nature abhors a vacuum." In the case of weather, you can apply that to what causes winds and helps determine how strong they will blow. As the Sun heats the Earth, layers of air close to the surface also warm up. As the molecules of air heat up, they move around more. That's how a physicist or chemist defines heat, atoms in faster motions spread out, and become less dense. Air that's warmer than the surrounding air rises: that's why hot air balloons fly. Something that's less dense than the surrounding fluid and you can think of air as a kind of fluid, which is how meteorologists think of our atmosphere also rises to the surface. That's why a Ping-Pong ball will float on water. So, for both these reasons warmer, less dense air rises, which creates a region of lower pressure. Colder, denser air wants to flow into such a region, which is the birth of wind.

   But the exact character of the wind, how fast it's blowing and variations in direction is also affected by local conditions: the lie of the land, whether there are hills and mountains, the "roughness" of the surface, etc. Winds blow more easily over the smooth sea, and encounter more friction over hills or a forest, for example, and therefore are slower. In general, winds close to Earth's surface blow slower than those high up in the atmosphere.

multi-cell cloud   In areas of low pressure, the rising air carries water vapor upward with it. If the air is wet enough, and rises high enough, clouds form, and sometimes precipitation. Low-pressure regions bring cloud and rainy weather.

   Contrariwise, high-pressure regions have air descending, and there's no water vapor rising and condensing out into clouds and rain, making bad weather less likely.

   In the Northern Hemisphere, air flows counterclockwise around a region of low pressure, and clockwise around a high-pressure zone. Contrariwise, winds tend to flow clockwise around low-pressure areas in the Southern Hemisphere, and counterclockwise around highs.