How Does Weather Work?

First, the earth is rotating (You knew that, right?)

Second, warm molecules move away from each other faster than cooler molecules. This means that warm air rises – the force of gravity is less on warm air because it is less dense than cold air. As it rises it creates an area of low air pressure - there are fewer molecules in a given volume of warm air that cold air. However, as it rises, it also cools and that means it can hold less water vapor. The water vapor turns back into liquid water and….it rains! The heat that is lost as water vapor returns to its liquid state can precipitate thunder storms and sometimes tornadoes and hurricanes.

Third, cold air sinks. Sinking air makes an area of high pressure - more molecules in a given volume of air. In general, air from an area of high pressure flows towards an area of low air pressure. The movement of air from an area of high pressure to an area of low pressure is what we call wind. The greater the difference in pressure is, the stronger the wind.

The greatest pressure difference on the earth is where the warm equatorial air hits the cold polar air. The cold air pushes under the warm air. The rising air that results from the clash of the cold air from the poles and warmer air originating from the equator produces a narrow band of strong winds that blow 20,000- 40,000 feet in the upper atmosphere. These upper atmosphere winds circle the Earth's north and south poles at speeds of 80-190 mile an hour. These ribbons of fast moving air are called the Jet Streams.

Because of the rotation of the earth, the general flow of jet stream in the Northern Hemisphere is from the west to the east. But it often dips southward or rises northward as it flows around the earth. During the summer in North America, the jet steam is located around the Canadian/US border and is relatively weak (the temperature difference between the polar and equatorial air is not as great). In the winter when the temperature difference between the polar and equatorial air is greatest, the jet stream picks up in strength and dips down into the United States.

Because the most violent and variable weather is produced when cold air clashes with warm air, storms occur frequently along the path of both the Northern and Southern Hemisphere jet streams. The major agricultural lands of the world are located under the storm path of the jet streams. It is where many of the world's first civilizations began and is where we grow most of our crops today.

What do you suppose happens when overall global temperatures rise or fall? The path of the jets streams change, therefore where it rains also changes and that means our ability to grow food will change. Not enough rain, there will be droughts. Too much rain and there will be flooding and even more importantly, as we will see later, there will be an increase in plant disease. Either way, crops don’t grow and produce well and there will be less food to go around. If the climate change lasts for a long period of time, it can disrupt rain patterns enough to bring down a civilization.

It is the long term availability of food that makes or breaks a civilization!

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How Global Warming Works

The Sun

Because of the relative sizes of the earth and its sun (the sun being much bigger), the rays of the sun travel in parallel lines to the earth and through the earth's atmosphere. Only about half of the incoming rays get to the earth's surface because air molecules, clouds, and dust either absorb some of the rays or reflect them back into space. The sun's rays that do penetrate the earth's atmosphere warm the earth's surface but despite the fact that they all begin with the same potential energy, the incoming rays have different heating effects at different places on the earth.

The Rays of the Sun as They Travel Through the Earth’s Atmosphere at the Equator and at the North Pole (Winter in the Northern Hemisphere – Summer in the Southern Hemisphere)


If you look at figure above, you will see one of the reasons why. Because of the curvature of the earth, the sun's rays travel through more atmosphere and therefore through more air molecules and dust to reach the poles than they do at the equator. (And it hits a bigger area at the poles.) So although the energy coming from the sun is the same, less of the sun's energy reaches the poles than the Equator. In addition, the snow and ice at the poles reflects much of the sunlight back into the atmosphere.

Consequently, the Earth is most intensely heated by the sun at the equator and least at the poles. The exact same phenomenon of uneven heating occurs in the oceans as well. The oceans at the equator are warmer than that closer to the poles.

Once the land and oceans of the earth absorb the sun’s rays, the energy is transferred into heat and this heat is then reradiated back out into the atmosphere. Much of that heat is absorbed by the molecules in the air (such as carbon dioxide). Since there is less energy reaching the poles there is less heat radiating from at the poles and the air is cold. Conversely, the intense energy reaching the equator means that the air at the equator is hotter.

In addition, the energy from the sun is also used in the process of evaporation (water changing from a liquid to a gas). The hotter the air, the more evaporation can occur. Warm air evaporates more water than cold air and it can hold more water molecules in the form of vapor than cool air. Consequently not only is the air at the equator warmer it is also “wetter”.

These differences in the heat and water vapor in the air prime the global weather machine because weather is really nothing more than an attempt of air molecules to homogenize these differences.

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How Weather Works