Chapter 7: The Atmosphere

Chapter 7: The Atmosphere

1. The Air Around You

   • Composition of the Atmosphere
     • Nitrogen
       • Nitrogen is the most common gas in the atmosphere. It makes up a little more than three-fourths of the air we breathe. Each nitrogen molecule consists of two nitrogen atoms.
     • Oxygen
       • Oxygen is the second most common gas in the atmosphere, and it makes up less than one-fourth of it's volume. Oxygen is required for fire to burn fuel. Most oxygen molecules have two oxygen atoms, but ozone, a form of oxygen, has three oxygen atoms per molecule.
     • Carbon Dioxide
       • A carbon dioxide molecule has two oxygen atoms and one carbon atom. Organisms give off carbon dioxide as a waste product, and burning fossil fuels  also releases carbon dioxide.
     • Other Gases
       • Nitrogen and oxygen makes up ninety-nine percent of the atmosphere, while carbon dioxide and argon make up most of the remaining one percent. The remaining gases are called trace gases because only small amounts of them are present.
     • Water Vapor
       • The amount of water vapor in air varies greatly from place to place and time to time. Water vapor also plays an important part in Earth's weather. Clouds form when water vapor condenses and forms water droplets or crystals of ice. If these droplets or crystals become heavy enough, they fall as rain or snow.
     • Particles
       • Pure air only contains gases, and it only exists in laboratories. The air we breathe contains tiny particles of dust, smoke, salt, and other chemicals. You can see some of these particles, but most are too small to see.
   • Importance of the Atmosphere
     • Earth's atmosphere makes conditions suitable for life. The Atmosphere contains gases that are essential for life. It also keeps Earth's surface warm, enough for water to stay a liquid. The atmosphere also protects Earth from ultraviolet waves and meteoroids.
   • Air Quality
     • Sources of Pollution
       • Some air pollution is caused by natural processes, such as forest fires and soil erosion. Most air pollution is caused by burning fossil fuels, such as gasoline and coal.
     • Smog and Acid Rain
       • The burning of fossil fuels can cause smog and acid rain. Photochemical smog, a type of smog, is caused by the action of sunlight on pollutants. Photochemical smog can irritate the face and harm living organisms. Acid rain, caused by burning high-sulfur coal, is rain with more acid than normal. Acid rain can contaminate water and destroy human-made structures. 
   • Improving Air Quality
     • In the United States, federal and state governments have made laws and regulations in order to reduce air pollution. Voluntary measures, such as using public transportation instead of driving, could also reduce air pollution. Many people think stricter regulations are needed to reduce air pollution while others think it is expensive and not worth the cost.

2. Air Pressure

   • Properties of Air
     • Density
       • The amount of mass in a given volume of air is its density. If there are more molecules in a given volume, the density is greater. If there are less molecules in a given volume, the density is less.
     • Pressure
       • The force pushing on an area of surface is known as pressure. Air pressure is the result of the weight of a column of air pushing down on an area. The column of air extends upwards through the entire atmosphere. 
   • Measuring Air Pressure
     • Mercury Barometers
       • A barometer is an insrument used to measure air pressure. A mercury barometer consists of a glass tube, open at the bottom end, resting in a dish of mercury. When air pressure increases, it presses down more on the surface of the mercury. 
     • Aneroid Barometers
       • A barometer is an insrument used to measure air pressure. An aneroid barometer is an airtight metal chamber. When air pressure increases, the walls of the chamber push in. When air pressure decreases, the walls bulge out. When the shape of the chamber changes, the needle in the barometer moves, measuring the air pressure.
     • Units of Air Pressure
       • Most weather reports use inches of mercury to measure air pressure, but the National Weather Service measures air pressure using millibars.
   • Altitude and the Properties of Air
     • Altitude Affects Air Pressure
       • As altitude increases, air pressure decreases because there is less weight pressing on it. As altitude decreases, air pressure increases because there is more air pressing on it.
     • Altitude Also Affects Density
       • As altitude increases, air density decreases. This is because the higher up in the atmosphere you are, there farther apart air molecules are. This makes it difficult to breathe.

3. Layers of the Atmosphere

   • The Troposphere
     • The troposphere is the lowest layer of the atmosphere, where all life lives. It is the layer in which all weather occurs. The depth of the troposphere varies from less than nine kilometers to sixteen kilometers. Though it is the shallowest layer of the atmosphere, it contains almost all of its mass. As altitude increases in the troposphere, temperature decreases.
   • The Mesosphere
     • The Mesosphere is the third layer of the atmosphere, the layer that protects Earth from most meteoroids. The mesosphere starts from the top of the stratosphere, than stops at about eighty kilometers above Earth's surface.
   • The Stratosphere
     • The stratosphere is the second layer of the atmosphere, and it contains the ozone layer. The stratosphere extends from the top of the troposphere to about fifty kilometers about Earth's surface. The upper stratosphere is warmer than the lower stratosphere. This is because the ozone layer in the middle of the stratosphere absorbs the energy from the sun and converts it into heat. The ozone layer also protects Earth from dangerous ultraviolet waves. 
   • The Thermosphere
     • The outermost layer of Earth's atmosphere is the thermosphere. The thermosphere has no definite outer limit, but it blends gradually with space. Although the air of the thermosphere is very thin, it is very hot. This is because sunlight hits the thermosphere first. Even though the thermosphere is very hot, the air molecules are too far apart to collide with anything to warm it. The thermosphere is also divided into two layers.
     • The Ionosphere
       • The ionosphere is the lower layer of the thermosphere. The sun's energy cause gas molecules in the ionosphere to become electrically charged, ions. Radio waves bounce off these ions and create brilliant light displays, called aurora borealis, in the sky. 
     • The Exosphere
       • The exosphere is the upper layer of the thermosphere. Communications satellites orbit earth in the exosphere, 

4. Energy in Earth's Atmosphere

   • Energy From the Sun
     • Visible Light
       • Visible light includes all the colors you see in a rainbow. The different colors result in different wavelengths. Red and orange light have the longest wavelengths while blue and violet light have the shortest wavelengths.
     • Non-Visible Radiation
       • One form of electromagnetic energy, infrared radiation, has wavelengths that are longer than red light. It is not visible, but it can be felt as heat. The sun also gives off ultraviolet radiation, which is an invisible form of energy with wavelengths shorter than violet light. It can cause sunburns, skin cancer, and eye damage. Solar radiation includes a full spectrum of wavelengths, from below infrared to above ultraviolet.
   • Energy in the Atmosphere
     • Before reaching Earth's surface, the sun must pass through the atmosphere. Some solar radiation is absorbed by the atmosphere, mostly by ozone layer. Some sunlight is reflected by clouds, dust particles, and gases. This process is called scattering. The sky looks blue because blue and violet light is scattered more than red and orange light. The sky looks red when the sun sets because the light has to pass through a greater thickness of the atmosphere. Therefore, more light from the red end of the spectrum reaches your eyes because most of the blue light is scattered.
   • Energy at Earth's Surface
     • A lot of the sun's energy is absorbed by the atmosphere and converted into heat. When Earth's surface is heated, it radiates most of the energy back into the atmosphere as infrared radiation. The process in which gases hold heat in the air is the greenhouse effect. The greenhouse effect keeps Earth's atmosphere at a temperature that is comfortable for most living things. Over time, the amount of energy absorbed by the atmosphere and Earth's surface is nearly in balance with the amount of energy radiated into space.  Therefore, Earth's average temperatures remain fairly constant.

5. Heat Transfer in the Atmosphere

   • Thermal Energy and Temperature
     • Measuring Temperature
       • Air temperature is typically measured using a thermometer. Thermometers work because the liquid inside of them expand and contract according to the temperature.
     • Temperature Scales
       • Temperature is measured in units called degrees. Two scales are commonly used: the Fahrenheit and Celsius scales. On the Fahrenheit scale, the freezing point of water is 32° F and the boiling point of water is 212° F. On the Celsius scale, the freezing point is 0° C and the boiling point is 100° C.
   • How Heat is Transferred
     • Radiation
       • Radiation is the direct transfer of energy by electromagnetic waves. The energy coming from the sun is  radiation.
     • Conduction
       • The direct transfer of energy from one substance to another that is touching is called conduction. The closer together the particles in a substance are, the more effectively they can conduct heat. Conduction works well on some solids, such as metals, but not as well in liquids and gases.
     • Convection
       • In fluids, particles can move easily from one place to another. As the particles move, their energy goes along with them. The transfer of heat by the movement of a fluid is called convection.
     • Heating the Troposphere
       • Radiation, conduction, and convection work together to heat the troposphere. During the day, the sun's radiation heats Earth's surface. The land becomes warmer than the air. The air near Earth's surface is warmed by both conduction and radiation. Only the first few meters of the troposphere is heated by conduction. Thus, the air close to the ground is generally warmer than the air a few meters up. The warm air near the ground rises, the cooler air from above sinking. This cycle creates convection currents, warming the rest of the troposphere. 

6. Winds

   • What is Wind?
     • A wind is a horizontal movement of air from an area of high pressure to an area of low pressure.
     • Measuring Wind
       • Wind speed can be measured with an anemometer. Anemometers mounted on spokes that spin on an axle. The force of the wind against the cups turns the axle. A meter on the axle shows the wind speed.
     • Wind-Chill Factor
       • When wind blows on your skin, it is removing body heat. The stronger the wind, the colder you feel. The increased cooling a wind can cause is called the wind-chill factor.
   • Local Winds
     • Local winds are winds that blow over short distances.
     • Sea Breeze
       • A sea breeze is a local wind that blows from a body of water to the land.
     • Land Breeze
       • A land breeze is a local wind that blows from the land to a body of water.
   • Global Winds
     • Global winds are winds that blow steadily from specific directions over long distances.
     • Global Convection Currents
       • Temperature differences between the equator and the poles create giant convection currents in the atmosphere. These air movements create global winds. 
     • The Coriolis Effect
       • If the earth did not rotate, global winds would blow in a straight line from the poles to the equator. Because the earth spins, global winds do not follow a straight path. As the wind blows, Earth rotates from west to east underneath them, making it seem as if the winds have curved. The way the earth's rotation makes winds curve is called the Coriolis effect. 
   • Global Wind Belts
     • Doldrums
       • Regions near the equator with little or no wind are called doldrums. 
     • Horse Latitudes
       • Horse latitudes are calm areas of falling air about 30° north and south of the equator. 
     • Trade Winds
       • Trade winds blow from the horse latitudes toward the equator. 
     • Prevailing Westerlies
       • Prevailing westerlies blow away from the horse latitudes. The prevailing westerlies play an important part in weather in the United States.
     • Polar Easterlies
       • Polar easterlies blow cold air away from the poles. The polar easterlies play an important part in weather in the United States. 
     • Jet Streams
       • About ten kilometers above Earth's surface are bands of high-speed winds called jet streams. These winds are hundreds of kilometers wide but only a few kilometers deep. Jet streams generally blow from west to east at speeds of two hundred to four hundred kilometers per hour. As they travel around Earth, they wander north and south along a wavy path.