Describe the different forms of energy and heat in the atmosphere: kinetic energy, potential energy, latent heat, and sensible heat.

The universe is made up of a combination of matter and energy. The concept of matter is easy to grasp because it is the “stuff” we can see, smell, and touch. Energy, by contrast, is abstract and therefore more difficult to describe and understand. Energy comes to Earth from the Sun in the form of radiation, which we see as light and feel as heat. This energy is then transformed and transported by the Earth system.

Forms of Energy

Energy can be thought of simply as having the capacity for doing work, such as making an object move. Common examples include the chemical energy from gasoline that powers automobiles, the heat energy from stoves that excites water molecules (boils water), and the gravitational energy that has the capacity to move snow down a mountain slope in the form of an avalanche. These examples illustrate that energy takes many forms and can also change from one form to another. For example, the chemical energy in gasoline is first converted to thermal energy (which we commonly refer to as heat) in the engine of an automobile, which is then converted to mechanical energy that moves the automobile along.

You are undoubtedly familiar with some of the common forms of energy, such as thermal, chemical, nuclear, radiant (light), and gravitational energy. Energy can be placed into one of two major categories: kinetic energy and potential energy.

Kinetic Energy

Energy associated with an object by virtue of its motion is described as kinetic energy. A simple example of kinetic energy is the motion of a hammer when driving a nail. The swinging hammer can move another object (do work). The faster the hammer is swung, the greater its kinetic energy (energy of motion). Similarly, a larger (more massive) hammer possesses more kinetic energy than a smaller one, provided that both are swung at the same velocity. Likewise, the winds associated with a hurricane possess much more kinetic energy than do light, localized breezes because hurricane winds are larger in scale (cover a larger area) and travel at higher velocities.

Kinetic energy is also significant at the atomic level. All matter is composed of atoms and molecules that are continually vibrating and, by virtue of this motion, have kinetic energy. For example, when a pan of water is heated on a stove, the water molecules begin to vibrate faster. Thus, when a solid, liquid, or gas is heated, its atoms or molecules move faster, and the material possess more kinetic energy.

Potential Energy

As the term implies, potential energy has the potential or capacity to do work. For example, large hailstones suspended by an updraft in a towering cloud have gravitational potential energy. If the updraft weakens, gravity will pull these hailstones to Earth to do destructive work on roofs and vehicles. Many substances, including wood, gasoline, and the food you eat, contain potential energy, which is capable of doing work, given the right circumstances.

Temperature

In everyday use, temperature is used to describe how warm or cold an object is, using a standard measure. In the United States, the Fahrenheit scale is used most often to express temperature. For example, the Weather Channel may forecast tomorrow’s high temperature to be 88°F. However, scientists and most other countries use the Celsius and Kelvin temperature scales. A discussion of all three scales is provided in Chapter 3.

Temperature is formally defined as a measure of the average kinetic energy of the atoms or molecules in a substance. When a substance is heated, its molecules and atoms move faster, and its temperature rises. By contrast, when an object cools, the atoms and molecules vibrate more slowly, and its temperature drops.

It is important to note that temperature is not a measure of the total kinetic energy of an object. For example, a cup of boiling water has a much higher temperature than a bathtub of lukewarm water. However, the quantity of water in the cup is small, so it contains far less total kinetic energy than the water in the tub. Much more ice would melt in the tub of lukewarm water than in the cup of boiling water. The temperature of the water in the cup is higher because the atoms and molecules are vibrating faster, but the total amount of kinetic energy (also referred to as heat, or thermal energy) is much smaller because there are far fewer atoms and molecules.

Heat

Heat is defined as energy transferred into or out of an object because of temperature differences between that object and its surroundings. If you hold a mug of hot coffee, your hand will begin to feel warm or even hot. By contrast, when you hold an ice cube, heat is transferred from your hand to the ice cube. Heat flows from a region of higher temperature to a region of lower temperature. Once the temperatures become equal, heat flow stops.

It is also common to use the word heat to describe thermal energy, which is the energy contained in a substance as a result of its temperature. A hot object is described as containing more thermal energy or heat than a cold object of equal mass and composition. Meteorologists subdivide heat into two categories: latent heat and sensible heat.

Latent Heat

Heat is released or absorbed when water changes from one state of matter to another, a process called a phase change. For example, a phase change occurs when liquid water evaporates and becomes water vapor. During the process of evaporation, heat from the surroundings (environment) is absorbed by liquid water, causing its molecules to vibrate more rapidly. When the rate of vibration is great enough to overcome the surface tension holding together the water molecules, some of the molecules escape (evaporate) and become water vapor. Because the most energetic (fastest-moving) molecules escape, the average kinetic energy (temperature) of the remaining liquid water decreases. Therefore, evaporation is considered a cooling process because it removes heat from the environment. The cooling effect of evaporation is something you certainly have experienced upon stepping out, dripping wet, from a swimming pool or shower.

During evaporation, the energy absorbed by the escaping water vapor molecules is called latent heat (latent = to lie hidden). The term latent is used to describe this phenomenon because the thermal energy (heat) required to evaporate the water is stored, or “hidden,” within the escaping water vapor. The latent heat stored in water vapor is eventually released, usually to the atmosphere, during condensation—when water vapor returns to its liquid state during cloud formation. Therefore, condensation, the opposite of evaporation, returns energy to the environment and is considered a warming process.

Through the combined processes of evaporation and condensation, latent heat transports large amounts of energy from Earth’s surface, mainly the oceans, to the atmosphere. The importance of latent heat in atmosphere processes will be explored in Chapter 4.

Keep in mind that latent heat is exchanged between the environment and water molecules anytime water undergoes a phase change. Figure 2.10 illustrates the phase changes water undergoes; some involve the absorption and storage of latent heat, while the opposite processes release latent heat back to the environment. For example, when water vapor condenses and releases heat to the atmosphere, this is referred to as latent heat of condensation.

Sensible Heat

In contrast to latent heat, sensible heat is the heat that we can feel and measure with a thermometer but that does not involve a phase change. It is called sensible heat because it can be “sensed.” On a clear summer day, the sunlight that is absorbed by the atmosphere, or by your exposed skin, will cause an increase in temperature. Like latent heat, sensible heat can be transported from one location to another. Warm air that originates over the Gulf of Mexico and flows into the Great Plains in the winter is one example.

• energy: The capacity to do work, such as making an object move.

• heat: The kinetic energy of random molecular motion.

• kinetic energy: Energy associated with an object in motion.

• latent heat: The energy absorbed or released during a change of state.

• potential energy: Energy that exists by virtue of a body’s position with respect to gravity.

• sensible heat: The heat we can feel and measure with a thermometer.

• temperature: A measure of the degree of hotness or coldness of a substance.

• Energy is the ability to do work. The two major categories of energy are (1) kinetic energy, which can be thought of as energy of motion; and (2) potential energy, energy that has the capability to do work.

• Temperature is a measure of the average kinetic energy of the atoms or molecules in a substance.

• Heat is the transfer of energy into or out of an object due to temperature differences between that object and its surroundings. Heat flows from regions of higher temperature to regions of lower temperature.

• Latent heat is the energy involved when water changes from one state of matter to another. During evaporation, energy is stored as latent, or “hidden,” heat within the escaping water vapor molecules. This same heat is eventually released when water vapor condenses to form water droplets in clouds.

• Sensible heat is the heat we can feel and measure with a thermometer, and it does not involve a phase change. It is called sensible heat because it can be “sensed” as a change in temperature.