Introduction to the Atmosphere

This section provides a brief overview of the properties associated with the atmosphere. The general concepts found in this section are:

  • The earth's atmosphere is a very thin layer wrapped around a very large planet.

  • Two gases make up the bulk of the earth's atmosphere: nitrogen (), which comprises 78% of the atmosphere, and oxygen (), which accounts for 21%. Various trace gases make up the remainder.

  • Based on temperature, the atmosphere is divided into four layers: the troposphere, stratosphere, mesosphere, and thermosphere.

  • Energy is transferred between the earth's surface and the atmosphere via conduction, convection, and radiation.

  • Ocean currents play a significant role in transferring this heat poleward. Major currents, such as the northward flowing Gulf Stream, transport tremendous amounts of heat poleward and contribute to the development of many types of weather phenomena.

This section includes seven classroom activities.

Atmospheric Properties

The thin envelope of air that surrounds our planet is a mixture of gases, each with its own physical properties. The mixture is far from evenly divided. Two elements, nitrogen and oxygen, make up 99% of the volume of air. The other 1% is composed of "trace" gases, the most prevalent of which is the inert gaseous element argon. The rest of the trace gases, although present in only minute amounts, are very important to life on earth. Two in particular, carbon dioxide and ozone, can have a large impact on atmospheric processes.

Another gas, water vapor, also exists in small amounts. It varies in concentration from being almost non-existent over desert regions to about 4% over the oceans. Water vapor is important to weather production since it exists in gaseous, liquid, and solid phases and absorbs radiant energy from the earth.

Structure of the Atmosphere

The atmosphere is divided vertically into four layers based on temperature: the troposphere, stratosphere, mesosphere, and thermosphere. Throughout the Cycles unit, we'll focus primarily on the layer in which we live - the troposphere.

Troposphere

The word troposphere comes from tropein, meaning to turn or change. All of the earth's weather occurs in the troposphere.

The troposphere has the following characteristics.

  • It extends from the earth's surface to an average of 12 km (7 miles).
  • The pressure ranges from 1000 to 200 millibars (29.92 in. to 5.92 in.).
  • The temperature generally decreases with increasing height up to the tropopause (top of the troposphere); this is near 200 millibars or 36,000 ft.
    • The temperature averages 15°C (59°F) near the surface and -57°C (-71°F) at the tropopause.
    • The layer ends at the point where temperature no longer varies with height. This area, known as the tropopause, marks the transition to the stratosphere.
  • Winds increase with height up to the jet stream.
  • The moisture concentration decreases with height up to the tropopause.
    • The air is much drier above the tropopause, in the stratosphere.
    • The sun's heat that warms the earth's surface is transported upwards largely by convection and is mixed by updrafts and downdrafts.
  • The troposphere is 70% and 21% . The lower density of molecules higher up would not give us enough to survive.

Atmospheric Processes

Interactions - Atmosphere and Ocean

In the Cycles overview, we learned that water is an essential part of the earth's system. The oceans cover nearly three-quarters of the earth's surface and play an important role in exchanging and transporting heat and moisture in the atmosphere.

You may have figured out by now that the oceans and atmosphere interact extensively. Oceans not only act as an abundant moisture source for the atmosphere but also as a heat source and sink (storage).

The exchange of heat and moisture has profound effects on atmospheric processes near and over the oceans. Ocean currents play a significant role in transferring this heat poleward. Major currents, such as the northward flowing Gulf Stream, transport tremendous amounts of heat poleward and contribute to the development of many types of weather phenomena. They also warm the climate of nearby locations. Conversely, cold southward flowing currents, such as the California current, cool the climate of nearby locations.

Energy Heat Transfer

Practically all of the energy that reaches the earth comes from the sun. Intercepted first by the atmosphere, a small part is directly absorbed, particularly by certain gases such as ozone and water vapor. Some energy is also reflected back to space by clouds and the earth's surface.


Energy is transferred between the earth's surface and the atmosphere via conduction, convection, and radiation.

Conduction is the process by which heat energy is transmitted through contact with neighboring molecules.

Some solids, such as metals, are good conductors of heat while others, such as wood, are poor conductors. Air and water are relatively poor conductors.

Since air is a poor conductor, most energy transfer by conduction occurs right at the earth's surface. At night, the ground cools and the cold ground conducts heat away from the adjacent air. During the day, solar radiation heats the ground, which heats the air next to it by conduction.

Convection transmits heat by transporting groups of molecules from place to place within a substance. Convection occurs in fluids such as water and air, which move freely.

In the atmosphere, convection includes large- and small-scale rising and sinking of air masses and smaller air parcels. These vertical motions effectively distribute heat and moisture throughout the atmospheric column and contribute to cloud and storm development (where rising motion occurs) and dissipation (where sinking motion occurs).

To understand the convection cells that distribute heat over the whole earth, let's consider a simplified, smooth earth with no land/sea interactions and a slow rotation. Under these conditions, the equator is warmed by the sun more than the poles. The warm, light air at the equator rises and spreads northward and southward, and the cool dense air at the poles sinks and spreads toward the equator. As a result, two convection cells are formed.

Meanwhile, the slow rotation of the earth toward the east causes the air to be deflected toward the right in the northern hemisphere and toward the left in the southern hemisphere. This deflection of the wind by the earth's rotation is known as the Coriolis effect.

Radiation is the transfer of heat energy without the involvement of a physical substance in the transmission. Radiation can transmit heat through a vacuum.

Energy travels from the sun to the earth by means of electromagnetic waves. The shorter the wavelength, the higher the energy associated with it. This is demonstrated in the animation below. As the drill's revolutions per minute (RPMs) increase, the number of waves generated on the string increases, as does the oscillation rate. The same principle applies to electromagnetic waves from the sun, where shorter wavelength radiation has higher energy than longer wavelength radiation.


Most of the sun's radiant energy is concentrated in the visible and near-visible portions of the spectrum. Shorter-than-visible wavelengths account for a small percentage of the total but are extremely important because they have much higher energy. These are known as ultraviolet wavelengths.

Concluding Thoughts

The physical and chemical structure of the atmosphere, the way that the gases interact with solar energy, and the physical and chemical interactions between the atmosphere, land, and oceans all combine to make the atmosphere an integral part of the global biosphere. For students to truly understand the nature and importance of the atmosphere, they should understand the answers to these questions:

  1. What is the structure and composition of the atmosphere?

  2. How does solar energy influence the atmosphere?

  3. How does the atmosphere interact with land and oceans?

  4. How is heat transferred throughout the earth system?

Activities

The following activities will help your students better understand the concepts covered in this section.

To proceed, either click on Activities in the menu at the top or click on another unit (for example, Introduction to Climate) to switch units.