The Atmosphere and the Weather

The energy state of a gas depends on its

1. Temperature
2. Pressure
3. Density

We will first describe these macroscopic properties of gases in terms of what is going on at the molecular or microscopic level using the kinetic model. In the kinetic model, the individual molecules that make up a gas are treated like tiny spheres, all moving in random directions. 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.

We will also describe how each of the three variables change in the vertical, i.e., as one moves up and down in the real atmosphere.

Temperature is determined by the average speed of the molecules making up a substance. The higher the temperature, the faster they move. For gases, this is the random motion of the individual molecules that make up the gas. Using the concept of energy, the higher the temperature, the more energy that is possessed by the gas. It should make sense, then, that the higher the temperature, the higher the energy, and the faster the speed at which the molecules are moving.

Often, the Earth's atmosphere is divided into several different layers that can be defined according to air temperature.

Layers of the atmosphere based on temperature

Troposphere:

-- Located from the surface of the Earth to approximately 11 kilometers in altitude.

-- Contains about 75 % of the total mass of the atmosphere.

-- Maximum air temperature occurs near the Earth's surface and typically drops with increasing height at an average rate of 6.5 degrees Celsius per 1000 meters (or 3.6 degrees Fahrenheit per 1000 feet). The reason the temperature decreases with increasing height is because the atmosphere is mainly heated from below by the underlying ground surface.

-- Essentially all weather occurs in this layer.

Tropopause:

-- Isothermal (constant temperature) layer that exists somewhere from 11 to 20 kilometers.

-- Separates the troposphere from the stratosphere.

-- Acts like a "lid" on rising air motion.

-- Layer in the atmosphere where the jet stream exists.

Stratosphere:

-- Extends from 20 to 48 kilometers above the surface.

-- Temperature increases with altitude because ozone gas molecules, present in this layer, absorb ultraviolet sunlight creating heat energy.

-- This layer of ozone is also called the ozone layer (depicted in yellow in the Figure on the left) which protects life from the harmful effects of the sun's ultraviolet radiation. NOTE: Even though we refer to the ozone layer, keep in mind that ozone molecules account for a very small percentage of all air molecules in the stratosphere.

A brief look at air density and air pressure

Air density can be defined as the number of air molecules per unit volume. Near sea level there are about 2.7x10¹⁹ molecules per cm³(cubic centimeter) or 4.4x10²⁰ molecules per inch³. While this may seem like a lot of molecules in a small area, molecules are very small. By comparison, the number density for solids and liquids is much higher. In a gas there is lots of empty space between the individual molecules.

Air molecules are held near the earth by gravity. In other words, air has weight. Weigh an empty bag, then fill it with air, it now weighs more.

This strong, invisible force pulling down on the air (gravity) squeezes air molecules closer together, which causes their numbers in a given volume to increase (increase in number density). The more air above a level, the greater the squeezing effect (or compression).

Since air density is the number of air molecules in a given space (volume), air density is typically greatest at the surface and decreases as we move up in the atmosphere.

From a microscopic point of view, gas pressure is caused by the collisions of gas molecules on a surface. The physical units for pressure is force per area.

The weight of the air molecules acts as a force upon the earth. The amount of force excerted over an area of surface is called atmospheric pressure or air pressure. Since the air (a gas) is a fluid, the pressure force acts in all directions, not just downward. The pressure force pushing downward due to the weight of the air is the same as the pressure force acting sideways and even upward. If you are having trouble understanding this, make an analogy with another fluid liquid water. Consider a tall glass of water. The water pressure anywhere in the glass depends on the weight of the water above (that is the deeper you move downward in the glass, the stronger the water pressure. The pressure force is not just downward because if you make a hole in the side of the glass water is forced out by the water pressure.

In the atmosphere, the air pressure at any point depends on the weight per area of the air above that point. As we climb in elevation, fewer air molecules are above us; hence, atmospheric pressure always decreases as you move upward in the atmosphere. Another way to look at it is that the air pressure at any point in the atmosphere is exactly enough to support the weight of the column of air above it. A balance exists between the gravitational force pushing air downward and the pressure force.

In-class demonstration for air pressure