What Is Air Temperature?

Transcription

1 2.2 Read What Is Air Temperature? In Learning Set 1, you used a thermometer to measure air temperature. But what exactly was the thermometer measuring? What is different about cold air and warm air that a thermometer can measure? What makes the temperature of air, or any other matter, change? You need to know the answers to these questions to explain the temperature patterns you have looked at on weather maps. To understand what air temperature is, you need to know that air, like all matter, is made up of tiny particles called atoms and molecules. You also need to know that the tiny particles that make up matter are always moving. The more energy the particles have, the faster they move. Scientists use special microscopes that scan a surface to make images of individual atoms. The photo above shows atoms arranged in a pattern. Each atom looks like a small sphere. The energy of motion is called kinetic energy. The idea that matter is made up of tiny, constantly moving particles is called the kinetic theory of matter. You will have to know this to understand air temperature and what happens when warmer and cooler air masses meet up. Scientists have a name for this special form of kinetic energy that results from the motions of atoms and molecules. They call this form of energy thermal energy. Thermal energy is the energy of motion of the particles of a substance. You recognize thermal energy as heat when it is transferred from one substance to another. Identify Main Ideas After reading this section on air temperature, prepare an outline highlighting its main ideas. Be sure to include important details and facts as well. atom: the basic unit of matter. molecule: the combination of two or more atoms. kinetic energy: the energy associated with an object in motion. kinetic theory of matter: the theory that states that all matter is made up of tiny, constantly moving particles. theory: a set of statements or principles developed to explain a group of facts or phenomena, especially one that has been repeatedly tested or is widely accepted and can be used to make predictions about natural phenomena. WW 67 WEATHER WATCH

2 Learning Set 2 Why Are There Differences in Temperature? thermal energy: the energy of motion of the particles of matter in a substance. heat: the transfer of thermal energy from a warmer substance to a cooler one. How Is Thermal Energy Transferred? Before you think about how thermal energy is transferred in air, you need to understand how warm air is different from cold air. You can use the kinetic theory to describe the difference. Both warm and cold air are made up of moving particles. Some particles in air move quickly, and some move slowly. There are more particles moving quickly in warm air than in cold air. What happens when warm air comes into contact with cold air? Thinking about a number of marbles on a fl at surface can help you picture the difference between cold and warm air. Imagine that the marbles are air particles. Suppose you shoot a marble slowly onto the surface. The slowly moving marble hits another marble. That marble begins to move, but it does not move very fast. It may strike another marble on the surface, but it may not have enough energy to make that marble move. Now imagine shooting a marble at a high speed. Something very different happens. When a high-speed marble hits another marble, that marble begins to move fast. That marble hits another marble, which hits another marble, and so on. When a moving marble hits another marble, it transfers some of its kinetic energy to that marble. The marble that is hit begins to move, or speeds up, and the marble that hit it slows down. The more energy the moving marble has when it hits another marble, the more energy can be transferred to the Project-Based Inquiry Science WW 68

3 2.2 Read other marble. If enough energy is transferred to the second marble, it moves away with enough speed to hit another marble, and so on. Something similar happens with air particles. Unlike the marbles, all the air particles are moving. When they collide, they also transfer kinetic energy from one particle to another. In warm air, more particles are moving faster than in cold air. The more energy a particle has, the more energy it can transfer to other particles it collides with. Consider two containers of air. One container has warm air, and therefore the particles are moving fast. One container has cold air, and the particles are moving slowly. Think about what will happen when you put the air in the two containers into contact with each other. You would expect the fastmoving particles to collide often enough with the slow-moving particles that the fast-moving particles would slow down, and the slow-moving particles would speed up. This would happen until all the particles are moving at the same average speed. warm air cold air warm and cold air Air temperature is a measurement of the average speed at which the particles in air are moving. In the previous example, the warm air was at a higher temperature than the cold air because the average speed of the particles of the warm air was greater than the average speed of the particles of cold air. The particles of warm air had more speed, and so, they had more energy. This energy was transferred from the higher-energy particles to the lowerenergy particles until all the particles had the same average energy. As this transfer took place, the temperature of the warm air dropped and the temperature of the cold air rose, until the temperature of the two containers of air was the same. WW 69 WEATHER WATCH

4 Learning Set 2 Why Are There Differences in Temperature? When the two containers of air reach the same temperature, collisions will still continue to occur among the particles. Some particles will still be moving faster than other particles. However, the average speed of all the particles in both containers is now the same. Stop and Think 1. What is the relationship between temperature and kinetic energy of particles? 2. How is thermal energy transferred between particles of matter? 3. Use what you know about energy transfer to describe why your hands feel cold if you go outside without gloves when it is 32 F outside? Assume that your body temperature is 98.6 F. How Do Thermometers Measure Temperature? A common type of thermometer is a bulb thermometer. It consists of a glass, ball-shaped bulb and a narrow tube, as pictured on the next page. The glass bulb is filled with a liquid, usually colored alcohol. You place the thermometer where you want to record the temperature, and then you compare the level of the liquid in the tube to the scale on the thermometer. As with air, when the liquid in the bulb heats up, the atoms and molecules move faster. The particles push each other farther apart, and the liquid, which is made up of particles, expands. Because the liquid is trapped inside the narrow tube of the thermometer, the liquid must rise as it expands. The more the liquid heats up, the more it will expand and the higher it will go in the tube. The marks on the tube allow you to measure the height of the liquid. The thermometer is calibrated, specially marked so that the level of the liquid in the tube corresponds to the accepted standard of degrees. Think about what happens when you place a thermometer in a location hotter than the thermometer s bulb. Because the air is warmer than the glass bulb of the thermometer, the glass gets warmer. The warmer air transfers heat to the cooler glass bulb. As the glass bulb becomes warmer, heat is transferred from the thermometer s glass bulb to the liquid inside the thermometer, because the glass is now hotter than the liquid. This heat Project-Based Inquiry Science WW 70

5 2.2 Read transfer continues until the temperature of the liquid in the thermometer, the glass bulb, and the air are all the same. Now think about what happens if you take the thermometer to a cooler location. The liquid particles in the bulb are now at a higher temperature than the surrounding air, and the glass of the thermometer bulb is also warmer than the surrounding air. As the air comes in contact with the glass of the thermometer bulb, energy is transferred from the liquid in the bulb to the glass, and then from the glass to the air. The air warms a tiny bit, and as it does, the glass bulb and the liquid end up a bit cooler than they were. As the liquid in the bulb transfers some of its energy to the glass and to the air, the particles that make up the liquid slow down and move closer together. The liquid in the tube now takes up less space, and its level inside the tube drops. Cooling stops when the temperature of the liquid, the glass bulb, and the surrounding air are all the same. What about the temperature of the air? Does it get colder if a cool thermometer is brought into the room, or warmer if a warm thermometer is brought in? Yes, but only by a tiny amount. Because there are so many more particles of air in a room than in a thermometer, the air around a thermometer does not warm or cool very much. On the other hand, if you brought a very large block of ice into a room, the ice would melt (warm up) and the air in the room would cool. You might remember that there are different scales used to measure temperature. The thermometer in the picture shows the Fahrenheit and Celsius scales. When you write a temperature, you should always include the unit and scale, such as 0 C or 32 F. WW 71 WEATHER WATCH

6 Learning Set 2 Why Are There Differences in Temperature? Stop and Think 1. What two energy transfers occur when the liquid in a thermometer becomes warmer in warm air? 2. Why does the height of the liquid in the tube of a thermometer give you a measure of the liquid s temperature? 3. Why is the temperature of a thermometer the same as the temperature of the surroundings? 4. Why do you think the marks on a thermometer are evenly spaced? conduction: the transfer of heat through direct contact. convection: the transfer of heat in a fluid (gas or liquid) by currents of warm, rising fluid and cool, sinking fluid. radiation: the transfer of energy by electromagnetic waves. 5. Why do you have to leave a thermometer in a substance for some amount of time to get an accurate measurement of the substance s temperature? How Can Air Become Warmer? You know that the temperature of a substance is a measure of the kinetic energy, or speed, of the atoms and molecules of the substance. You also know that to raise the temperature of the substance, or heat it, you need to make its molecules move faster. You need to transfer thermal energy to the substance. Transfer of thermal energy can happen through three different processes: conduction, convection, and radiation. You already know about one way to raise the temperature of air and other substances. This is through the process of conduction. When fastermoving particles come in contact with slower-moving particles, the faster (warmer) particles collide with the slower (cooler) particles, and energy is transferred to the slower particles. The slower-moving particles then have more energy then they did before, and move faster. When a substance that is warmer is near a substance that is cooler, the faster-moving atoms and molecules in the warmer substance transfer energy to the atoms and molecules in the cooler substance through collisions. The kinetic energy of the cooler substance increases and its temperature increases. This process of conduction continues until both substances are the same temperature. Conduction is one way the air around you becomes warmer. During the day, the surface of Earth absorbs energy from sunlight, and becomes warmer than the air touching it. As molecules in the cooler air collide with molecules Project-Based Inquiry Science WW 72

7 1.2 Investigate 2.2 Read in the warmer ground, the slower moving, cooler molecules gain energy. The air becomes warmer and the ground becomes cooler. If the air and ground reach the same temperature, conduction will stop. Another way air is heated is by convection. Groups of atoms and molecules tend to move together through a liquid or gas. Convection is the transfer of energy by the motion of groups of atoms or molecules in a liquid or gas. When atoms and molecules with more energy are all moving in the same direction, they take their energy in that direction. Consider this: Air particles with higher energy are moving faster. When faster-moving molecules that make up air all move in some direction, the region they move into becomes warmer. Convection happens naturally in the atmosphere and is another way that the air around you is heated. As air becomes warmer (gains kinetic energy), it tends to rise, and as it becomes cooler (loses kinetic energy) it tends to fall. As air near the surface is heated by Earth s surface, convection carries the warmer air away from Earth s surface and cooler air from higher above the surface moves down. Air in the atmosphere is constantly cycling in this way. gas: matter that has no definite shape or volume. A gas takes the shape and volume of its container. The particles are far apart and move rapidly and randomly. WW 73 WEATHER WATCH

8 Learning Set 2 Why Are There Differences in Temperature? electromagnetic waves: a form of energy, such as light, ultraviolet rays, or infrared rays, that can travel through a vacuum. solar energy: energy given off by the Sun that travels through space as electromagnetic waves and strikes Earth. Convection occurs in gases and liquids. Convection causes air masses high in the atmosphere to move from one place to another, resulting in weather changes. Convection also causes masses of water molecules in the oceans to move around, This also effects the weather. You will investigate and read about convection several times in this Unit. Radiation is the third way air can be heated. Radiation transfers energy from electromagnetic waves to the substances those waves come in contact with. You probably are familiar with several types of electromagnetic waves. Visible light moves in electromagnetic waves. So do ultraviolet light and infrared light. X-rays are also a type of electromagnetic wave. Electromagnetic waves can travel through a vacuum (empty space), so they do not need moving particles to transfer energy. This is good news for people on Earth, because almost all energy on Earth comes from the Sun. Because most of the distance between the Sun and Earth is empty space, radiation is the only way solar energy can reach you. When solar energy reaches the ground, atoms and molecules on Earth s surface absorb some of the energy. This makes the particles vibrate faster. The particles on Earth s surface collide with particles above them (in the air) and below them, heating the air above and the ground below by conduction. Project-Based Inquiry Science WW 74

9 2.2 Read Stop and Think 1. Describe the processes of conduction and convection. Both processes help transfer energy. How are the two processes alike? How are they different? 2. Describe the process through which Earth s surface is warmed by sunlight. 3. How is the air near Earth s surface warmed on a sunny day? Reflect 1. Why does energy always transfer from a warmer object to a cooler object? 2. When you place a thermometer in ice-cold water, the level of liquid in the thermometer starts to drop. Why should you wait until the thermometer reading is steady before recording the temperature? 3. If two areas of Earth s surface receive different amounts of sunlight, what will be the effect on the temperature of the atmosphere in those areas? 4. What areas of Earth do you think have the highest average temperatures? What areas do you think have the lowest average temperatures? Give your reasoning. 5. How do you think a strong wind might affect air temperature? 6. In most places, the average temperature in summer is higher than the average winter temperature. From this observation, what can you conclude about the amount of the Sun s solar energy that strikes Earth s surface in summer and winter? Update the Project Board Record what you now understand about the transfer of thermal energy onto the What are we learning? column of the Project Board. Be sure to describe how all three processes conduction, convection, and radiation affect the temperature of Earth s surface and atmosphere. Connect this information to the evidence that supports your understanding to the What is our evidence? column. WW 75 WEATHER WATCH

10 Learning Set 2 Why Are There Differences in Temperature? What s the Point? All matter is made of particles (atoms and molecules) in motion. Scientists call this the kinetic theory of matter. As particles gain energy, they move faster. When the average speed of the particles increases, the temperature of the substance increases. Temperature is a measure of the average speed of the particles in matter. Temperature is measured using a thermometer. Thermal energy is the total kinetic energy of the particles of matter. Heat is thermal energy that is transferred from one object to another. Thermal energy is always transferred from matter at a higher temperature to a substance with a lower temperature. This transfer continues until both substances are at the same temperature. Transfer of thermal energy in matter occurs in three different ways: conduction, convection, and radiation. Conduction occurs when substances in contact transfer heat through particle collisions. Convection is the transfer of thermal energy by the motion of groups of atoms or molecules in a liquid or gas due to differences in density. Radiation is the transfer of energy by electromagnetic waves. In how many different ways can you identify the transfer of thermal energy in this picture? These three processes work together to warm the atmosphere. The Sun radiates solar energy toward Earth. When the energy waves strike Earth s surface, energy is transferred to the surface. The atoms and molecules on Earth s surface gain energy and begin to move faster. When these particles collide with particles of air touching Earth s surface, energy is transferred to the air particles, which then move faster, increasing the air temperature. As air particles begin to move more, they collide with one another more, causing them to spread out. Thermal energy is always transferred from matter at a higher temperature to matter with a lower temperature. This transfer continues until both objects are at the same temperature. Project-Based Inquiry Science WW 76