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!

Next Post: The Mycenaeans of Ancient Greece
How Global Warming Works