Heating the Earth                                                                                   Name___________________________

                                                                                                               Per_______Date__________________

Read the passage below and fill in the blanks.

1) Weather ______________, ______________, and other kinds of _______________ instruments provide ______________ information about _______________ conditions.

2) The ______________ is a ______________ of _____________ that surround the _______________.

3) The _____________ factors that ____________ to cause weather are ___________ energy, _______ pressure, ___________, and ______________.

4) The ______________ also helps warm the _________ by _____________, ____________, and ____________ the sun's radiant energy.

5) In the upper atmosphere, a ___________ of ____________ gas (_____) absorbs one form of _______________ energy, called _____________________ ________________.

6) ____________ energy that is neither _____________ nor _____________ by the atmosphere reaches the Earth's ____________.

7) __________ is the direct __________ of _______ energy from one substance to another.

8) _____________ is the ____________ of _________ energy in a __________(gas or liquid).

9) _____________ is the transfer of _________ ___________ through ____________ ___________.

10) You _____________ see ______________ rays, but you can feel them as ___________.

11) This process is called the ________________ __________ because the _________ ___________ acts like the ___________ in a _________________ to trap __________.

12) Because most of the ___________ _________ are ______________ by ____________ ___________ the amount of this ________ in the atmosphere is ________ important.

13) The _________ at which the _________ _________ strike the surface is ______ the same everywhere on __________.

14) The same ___________ of ___________ __________ is ___________ over a _____________ area.

15) Air ____________ is measured with a ________________.

Heating the Earth

When you woke up this morning, did you stop to think about the weather? Was the sun shining? Was it warm enough for a picnic? Did you take your umbrella with you?

Weather affects your daily life and influences you and the world around you. The type of homes people build, the clothes they wear, the crops they grow, the jobs they perform, and the ways in which they spend their leisure time are all determined by the weather.

Today, people have a good understanding of the weather. Weather satellites, computers, and other kinds of weather instruments provide accurate information about weather conditions. Meteorologists (meet-ee-uh-RAHL-uh-jihsts), people who study the weather, use this information to predict the weather. Their forecasts help you plan your daily activities. But what exactly is weather and what causes it?

You can think of weather as the daily condition of the Earth's atmosphere (AT-muhs-feer). The atmosphere is a mixture of gases that surround the Earth. Weather is caused by the interaction of several factors in the atmosphere. The atmospheric factors that interact to cause weather are heat energy, air pressure, winds, and moisture.

Heat Energy and the Atmosphere

Almost all of the Earth's energy comes from the sun. This energy is called radiant energy. The sun's radiant energy warms the Earth. The atmosphere also helps warm the Earth by absorbing, storing, and recycling the sun's radiant energy. Let's see how this happens.

As the sun's energy reaches the atmosphere, part of it is reflected (bounced back) into space and part is scattered throughout the atmosphere. This happens when incoming rays of sunlight strike water droplets and dust particles in the atmosphere.

Much of the sun's energy that is scattered throughout the atmosphere is absorbed by the atmosphere. In the upper atmosphere, a layer of ozone gas (0₃) absorbs one form of radiant energy called ultraviolet radiation. You have probably heard that ultraviolet radiation, which causes sunburn, can be dangerous to people. Too much ultraviolet radiation can cause skin cancer. That is why the ozone layer, which absorbs much of the ultraviolet radiation from the sun, is so important to life on Earth. Ultraviolet radiation does, however, have some beneficial uses. Ultraviolet lamps are used to kill bacteria in hospitals and in food-processing plants, where bacteria could cause packaged foods to spoil.

Radiant energy that is neither reflected nor absorbed by the atmosphere reaches the Earth's surface. Here it is absorbed by the Earth and changed into heat. The sun's energy that is absorbed by the Earth is spread throughout the atmosphere in three basic ways: conduction, convection, and radiation.

Heat Transfer in the Atmosphere

Conduction is the direct transfer of heat energy from one substance to another. As air above the Earth's surface comes into contact with the warm ground, the air is warmed. So temperatures close to the ground are usually higher than temperatures a few meters above the ground. However, soil, water, and air are poor conductors of heat. So conduction plays only a minor role in heating the land, ocean, and atmosphere.

Convection is the transfer of heat energy in a fluid (gas or liquid). Air is a fluid. When air near the Earth's surface is heated, it becomes less dense and rises. Cooler, denser air from above sinks. As the cool air sinks, it is heated by the ground and begins to rise. This process of warm air rising and cool air sinking forms convection currents. Convection currents are caused by the unequal heating of the atmosphere. Most of the heat energy in the atmosphere is transferred by convection currents.

Radiation is the transfer of heat energy through empty space. Heat energy that is transferred by radiation does not need the presence of a solid, liquid, or gas. It can travel through a vacuum, or empty space. Heat from the sun reaches the Earth by radiation. When radiant energy from the sun is absorbed by the Earth, it is changed into heat.

The Greenhouse Effect

As you have just read, some of the sun's radiant energy (in the form of ultraviolet rays) is absorbed by the Earth and changed into heat. Ultraviolet rays pass easily through the atmosphere and reach the Earth. Later, this energy is radiated back from the Earth to the atmosphere in the form of infrared rays. You cannot see infrared rays, but you can feel them as heat. (Although humans cannot see infrared rays, rattlesnakes and some other snakes have heat-sensitive pits on their head that "see" the heat given off by small animals.)

You may actually be more familiar with infrared rays than you realize. The heat lamps often used in restaurants to keep food warm make use of infrared radiation. If you hold your hand near a light bulb or stove, you can feel the heat given off as infrared rays.

Infrared rays are not like ultraviolet rays, however. Infrared rays cannot pass through the atmosphere and out into space. Carbon dioxide (CO₂) and other gases in the atmosphere absorb the infrared rays, forming a kind of "heat blanket" around the Earth. This process is called the greenhouse effect because the carbon dioxide acts like the glass in a greenhouse to trap heat. The greenhouse effect makes the Earth a comfortable place to live. What do you think would happen to the temperature at the Earth's surface if there were no greenhouse effect?

Because most of the infrared rays are absorbed by carbon dioxide, the amount of this gas in the atmosphere is very important. Carbon dioxide is produced by burning fossil fuels, such as coal, oil, and natural gas. As the amount of carbon dioxide in the atmosphere increases, more infrared rays will be absorbed. The greenhouse effect will increase and temperatures at the Earth's surface will go up.

Higher temperatures might result in altered weather patterns, including warmer winters and changes in rainfall. You will learn more about the results of the greenhouse effect in Chapter 2.

Temperature Variations

If the Earth's atmosphere is warmed by heat rising from the surface, how can the air temperature vary so much from place to place? To help you answer this question, look at Figure 1-7.

The angle at which the sun's rays strike the surface is not the same everywhere on Earth. At the equator (the imaginary line that separates the Earth into two halves), the sun is nearly overhead. The sun's rays strike the Earth at a 90 degrees angle all year long. The greatest heating occurs where the sun's rays are most direct; that is, at or near an angle of 90 degrees. So areas at or near the equator receive the most radiant energy and have the highest temperatures.

The farther away from the equator an area is, the less radiant energy it receives. Why is this so? In these areas, the angle at which the sun's rays strike the Earth is less than 90 degrees. As the angle of the sun's rays becomes smaller, the rays become less direct. The same amount of radiant energy is spread over a wider area. The result is less heat and lower temperatures.

Measuring Temperature

Air temperature is measured with a thermometer. Most thermometers consist of a thin glass tube with a bulb at one end. The bulb is filled with a liquid, usually either mercury or alcohol that is colored with dye.

Thermometers make use of the ability of a liquid to expand and contract. When a liquid is heated, it expands, or takes up more space. When a liquid is cooled, it contracts, or takes up less space. What happens to the liquid in a thermometer when the air temperature rises? What happens to the liquid when the air temperature falls?

Temperature is measured in units called degrees. The temperature scale used by scientists is the Celsius (SERL-see-uhs) scale. On the Celsius scale, the freezing point of water is 0 degrees. The boiling point of water is 100 Celsius. Normal human body temperature is 37 Celsius.