Unit 9 Science Notes

Ozone

Another important variable gas is ozone. Ozone is a rare form of oxygen (O2). An ozone molecule has three oxygen atoms (O3) rather than two. Ozone exists in small amounts in the lower atmosphere. It is a variable gas because its amount changes with the altitude in the atmosphere. Ultraviolet rays act upon oxygen molecules to form the ozone layer high in the atmosphere. The ozone layer acts as a shield that protects the earth from most of the Sun's ultraviolet rays.

Troposphere

The layer closest to the earth is the troposphere. When you play basketball, ride your bike, or read a book, the air you are breathing is part of the troposphere. The troposphere is the lowest and most dense layer of the atmosphere. It extends from the earth's surface to an average height of 7 miles. In comparison with the other atmospheric layers, the troposphere is the thinnest. The temperature within the troposphere decreases with height to about -55°C (-70°F). That is why as you climb a mountain or ascend in an airplane, it continues to get colder. The warm surface of the earth heats the troposphere. Air near the bottom is the warmest. Pay attention to the weather report tomorrow. You will notice that temperatures for cities in the mountains are usually lower than temperatures of cities at lower elevations. As warm air rises, cooler air sinks to replace it, creating wind currents and weather. Almost all clouds and storms occur in the troposphere.

Stratosphere

About 15 minutes after takeoff, the plane levels out and the captain announces that you have reached cruising altitude. Most of the time, passengers are allowed to unbuckle their seat belts and walk around the cabin. The cruising altitude for most planes is in the stratosphere. The stratosphere is the second layer from Earth's surface, extending to a height of about 30 miles. The strong, steady, horizontal winds provide the perfect conditions for long-distance flights. In the lower part of the stratosphere, temperatures are cold. Little vertical air movement is possible because of the cold, heavy air. This layer has strong steady winds and few sudden changes in weather. Unlike the troposphere, the temperature in the stratosphere gets warmer with altitude. The increase in temperature is mostly due to the ozonosphere, or ozone layer, located at the upper end of the stratosphere. The ozone layer absorbs most of the Sun's harmful ultraviolet (UV) radiation. The absorption of UV radiation heats the upper stratosphere. Temperatures rise steadily in the upper part to about 70°C (150°F).

Trace gases

Besides the primary gases in the atmosphere, there are many other gases in very small amounts. This is why they are called trace gases. These gases make up the remaining 1% of the atmosphere. The most abundant trace gas is argon. Other trace gases include neon and hydrogen.

Mesosphere

Despite their name, shooting stars, or falling stars, are not really stars. When meteors enter the earth's atmosphere, they burn up and look like stars shooting across the sky. Most meteors burn up in the third layer of the atmosphere, called the mesosphere. The mesosphere is the layer above the stratosphere. This layer reaches to about 50 miles high. The air in the mesosphere is very cold. It's the coldest layer in the atmosphere, decreasing to about -90°C (-130°F). As in the troposphere, temperatures in the mesosphere get colder with altitude. It is heated from the ozone layer below it.

Primary gases

If you analyzed a sample of air, you would find two main gases. Nitrogen would make up 78% of the sample and oxygen would make up 21%. Most of the remaining 1% is trace gases.

Variable gases

Some gases are variable gases, because their concentrations in the atmosphere can change. For instance, carbon dioxide levels change due to industrial processes that add it to the air. Also, water vapor can account for as much as 3% of the total volume of very moist air. In very dry air, water vapor may make up as little as 0.1%. Let's discuss water vapor further, due to its important role in the atmosphere.

Thermosphere

The thermosphere starts just above the mesosphere. It extends to over 350 miles above the earth's surface. The thermosphere differs from the lower layers in two main ways. First, layers of single gases replace the gas mixture of the lower atmosphere. Gravity separates the gases so that the heaviest elements are at the bottom and the lightest are on top. Nitrogen molecules are at the lowest layer of the thermosphere, called the ionosphere. Energy from the Sun is very strong in this layer. The energy breaks apart molecules and atoms of air, leaving positively charged particles, called ions, and free-floating electrons. The ions and electrons cause the beautiful auroras, also known as Northern and Southern Lights. The free electrons also create electromagnetic waves, which people use when sending radio signals over long distances. Radio waves traveling in straight lines could not travel around the curvature of the earth. The ionosphere reflects radio waves, allowing them to be sent around Earth's sphere. For this reason, the ionosphere is important in communications. A second difference between the thermosphere and other layers of the atmosphere is temperature. Within the thermosphere, temperatures increase with height and may exceed 2,000°C (3,600°F). Since this layer is the first to receive the Sun's energy, air in the thermosphere can get extremely hot. It is easily the hottest of all layers in the atmosphere. The exosphere is at the upper end of the thermosphere, where the earth's atmosphere merges into space. Hydrogen and helium are the main gases in this layer. Most satellites orbit the earth in the exosphere. Beyond the thermosphere, the atmosphere blends into space, where no air exists.

Water vapor

Water vapor is an important gas in our atmosphere. The amount of water vapor in the air is called humidity. The most humid areas are near the equator and over oceans and tropical rain forests. Cold regions, like Antarctica, and very dry places, like the Sahara Desert, have low humidity. Water vapor is important for two main reasons. First, it traps heat and helps keep the earth warm. Second, water vapor creates precipitation, which provides the earth with fresh water. Water vapor is also responsible for most of the weather conditions in our atmosphere.

Review

•The atmosphere consists primarily of nitrogen (78%) and oxygen (21%) gases. Together they make up 99% of the atmosphere. •Trace gases make up the remaining 1% of the atmosphere. Trace gases include argon, neon, and hydrogen. •Variable gases, as their name suggests, vary in concentration. Important variable gases are carbon dioxide, water vapor, and ozone. •The layers of the atmosphere have distinguishing features. Here's a summary: ◦Troposphere—This is the layer we most interact with, whether we are breathing or hang gliding. Temperature decreases with altitude. Almost all weather occurs here. ◦Stratosphere—This layer has steady winds, good for long-distance flights. Temperature increases with altitude due to the ozonosphere in the upper part of the layer. ◦Mesosphere—This layer burns up meteors, seen as falling or shooting stars. Temperature decreases with altitude. It is the coldest layer of the atmosphere. ◦Thermosphere—In this layer, gases are layered, not mixed. It consists of two sub-layers: ionosphere—where auroras occur and radio waves are reflected—and exosphere—where satellites hover. It is the hottest layer of the atmosphere, until it blends into space.