The Atmosphere and the Weather

The Atmosphere

The Earth's atmosphere is a relatively thin layer of gases that surounds the planet. The atmosphere is retained by Earth's gravity. The mixture of gases that is found on Earth today is commonly called air. Beside gases, the atmosphere also contains very minute quantities of microscopically small suspended particles of solid and liquid (called aerosols), which includes things like dust, pollen, and cloud droplets. Gases are composed of individual molecules that are not chemically bonded together. The gas molecules and suspended aerosols are very tiny and have very little mass. Thus, they have very little weight on Earth compared with the solid and liquid substances that compose the Earth's ground surface. In essence, the larger and heavier solids and liquids sink to the bottom, and the gases (the atmosphere) float to the top and becomes the outer layer surrounding the Earth. Not all planets have atmospheres of gas surrounding them; yet many other planets have much more massive atmospheres than the Earth. Living on the surface of the Earth (at the bottom of the atmospere), we have become so adapted to our particular atmosphere that we cannot survive more than a few minutes without it. Just as fish need to be surrounded in an environment of water to survive, we must be surrounded in an environment of air. And anywhere we go, e.g., under water, outer space, etc., we must have air with us for long-term survival.

Meteorology is the study of the atmosphere and the processes (such as cloud formation, lightning, and the movement of the wind) that cause what we refer to as the "weather". Most of the world's weather systems and their related features, including clouds and rain, develop in the lowest layer of the atmosphere, called the troposphere. Such weather systems, or patterns of air movement, develop as a result of the flow of heat from warmer regions of the Earth near the equator to colder regions nearer the poles. The air, and the heat it carries however, does not flow in a straight line, because of the Earth's rotation. As a consequence, the air flow is deflected, forming the swirling patterns of air circulation we observe on Earth.

If the earth were the size of a basketball, its atmosphere would be thinner than a piece of paper. The atmosphere does not have an abrupt, absolutely definable top. It is most dense just above the Earth's solid surface and becomes thinner and thinner as one moves upward, eventually fading into outer space. For humans, the habitable atmosphere (where the air is dense enough to breathe) extends only to about 20,000 ft (~4 miles) above sea level.

Despite its relative thinness, our existance is completely dependent upon the atmosphere. Essential functions of the atmosphere include:

1. Allows breathing, we cannot live without sufficient oxygen
2. Acts as a shield, protecting the surface from harmful ultraviolet (uv) radiation and other high energy particles from the Sun and space, and even meteors on a collision course with the Earth.
3. The atmosphere transports heat and moisture creating more uniform conditions around the globe than if there were no atmosphere. Without the atmosphere, very much larger extremes in temperature would exist between the Equatorial and Polar regions than are observed. Without the atmosphere, much larger extremes in temperature would occur between day and night. The atmosphere transports water (in the form of water vapor) from oceans to land. This is essential for life on land to exist.
4. The atmosphere keeps the average surface temperature of the Earth much warmer than if there were no atmosphere through the Greenhouse effect. With no atmosphere, the average surface temperature of the Earth would be about 0 degrees Fahrenheit. The presence of the atmosphere keeps the average surface temperature at 59 degrees Fahrenheit.

The atmosphere also influences our existance in several non-life essential ways:

1. Propagation of sound. Sound does not travel through empty space (e.g., outer space)
2. Provides visual sights such as blue skies, clouds, rainbows, and colorful sunsets
3. Sense of smell is also influenced by the atmosphere

The material that makes up the atmosphere is mostly in the form of a gas. A gas is one of three basic forms or "states" of matter.

1. Gas - substance in which individual molecules are not chemically bonded together. Even though many gases are not visible, it does not mean there is nothing there. Gases are composed of the same molecules that make up liquids and solids. Water vapor is an example of a type of gas.
   We will use the kinetic model concept to help us to understand and visualize how gases behave. In the kinetic model, the individual molecules that make up a gas are treated like tiny spheres, all moving in random directions. As in the atmosphere of Earth, the gas molecules (spheres) are quite small compared to the average distance between molecules (spheres). The spheres collide with each other and any solid or liquid that happens to be in the way, but they remain separate, i.e., they do not stick together. More on the kinetic model for gases will follow shortly.
2. Liquid - substance in which individual molecules are chemically bonded together, but the bonds are continuously breaking and reforming, so the substance flows. Liquid water is an example of a liquid.
3. Solid - substance in which individual molecules are tightly bonded togther in an orderly fashion. Bonds are difficult to break and the substance does not flow. Ice is an example of a solid.

Look at slides 1-8 in this Kinetic Model Slide Show for a more visual explanation of the Kinetic Theroy of Matter.

As mentioned above, the atmosphere also contains trace amounts of tiny liquid or solid particles called aerosols that are suspended above the surface of the Earth. Examples of aerosols are dust, smoke, cloud droplets, and pollen. Most aerosols are much too small to see individually. However, when concentrations of aerosols are high, such as the microscopic liquid droplets that make up clouds, they can be seen.