Page 129 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 4 atmosphere and Surface Energy Balances 93
shape and occupies space.) Humans have learned to manipulate energy so that it does work for our benefit, such as the chemical energy used to run motor vehicles or the gravitational energy used within dams for hy- dropower production.
Kinetic energy is the energy of motion, produced when you run, walk, or ride a bicycle, and produced by the vibrational energy of molecules that we measure as temperature. Potential energy is stored energy (stored either due to composition or position) that has the ca- pacity to do work under the right conditions. Petro- leum has potential energy that is released when gasoline is burned in a car’s engine. The water in the reservoir above a hydropower dam has potential energy that is released when the pull of gravity impels it through the turbines and into the river downstream. (Potential energy is converted into kinetic energy in both these examples.) Both kinetic energy and potential energy produce work, in which matter is moved into a new po- sition or location.
Types of Heat In Chapter 3, we mentioned that heat is the flow of kinetic energy between molecules and from one body or substance to another resulting from a temperature difference between them. Heat always flows from an area of higher temperature into an area of lower temperature; an example is the transfer of heat when you wrap your warm hand around a snowball, or a piece of ice, and it melts. Heat flow stops when the temperatures—that is, when the amounts of kinetic energy—become equal.
Two types of heat energy are important for under- standing Earth–atmosphere energy budgets. Sensible heat can be “sensed” by humans as temperature, be- cause it comes from the kinetic energy of molecular mo- tion. Latent heat (“hidden” heat) is the energy gained or lost when a substance changes from one state to an- other, such as from water vapour to liquid water (gas to liquid) or from water to ice (liquid to solid). Latent heat transfer differs from sensible heat transfer in that as long as a physical change in state is taking place, the substance itself does not change temperature (although in Chapter 7 we see that the surroundings do gain or lose heat).
Methods of Heat Transfer Heat energy can be trans- ferred in a number of ways throughout Earth’s atmo- sphere, land, and water bodies. Radiation is the transfer of heat in electromagnetic waves, such as that from the Sun to Earth, discussed in Chapter 2, or as from a fire or a burner on the stove (Figure 4.2). The temperature of the object or substance determines the wavelength of radia- tion it emits (we saw this when comparing Sun and Earth in Chapter 2); the hotter an object, the shorter the wave- lengths that are emitted (Wien’s Law). Waves of radiation do not need to travel through a medium, such as air or water, in order to transfer heat.
▲Figure 4.2 Heat-transfer processes. infrared energy radi-
ates from the burner to the saucepan and the air. Energy conducts through the molecules of the pan and the handle. The water physi- cally mixes, carrying heat energy by convection. latent heat is the energy absorbed when liquid water changes to steam (water vapour).
Conduction is the molecule-to-molecule transfer of heat energy as it diffuses through a substance. As mol- ecules warm, their vibration increases, causing colli- sions that produce motion in neighbouring molecules, thus transferring heat from warmer to cooler material. An example is energy conducted through the handle of a pan on a kitchen stove. Different materials (gases, liquids, and solids) conduct sensible heat directionally from areas of higher temperature to those of lower tem- perature. This heat flow transfers energy through matter at varying rates, depending on the conductivity of the material—Earth’s land surface is a better conductor than air; moist air is a slightly better conductor than dry air.
Gases and liquids also transfer energy by convec- tion, the transfer of heat by mixing or circulation. An example is a convection oven, in which a fan circulates heated air to uniformly cook food, or the movement of boiling water on a stove. In the atmosphere or in bodies of water, warmer (less dense) masses tend to rise and cooler (denser) masses tend to sink, establishing patterns of con- vection. This physical mixing usually involves a strong vertical motion. When horizontal motion dominates, the term advection applies.
These physical transfer mechanisms are important for many concepts and processes in physical geography: Radiation and conduction pertain to surface energy bud- gets, temperature differences between land and water bodies and between darker and lighter surfaces, and temperature variation in Earth materials such as soils; convection is important in atmospheric and oceanic circulation, air mass movements and weather systems, internal motions deep within Earth, and movements in Earth’s crust; advection relates to the horizontal move- ment of winds from land to sea and sea to land, the for- mation and movement of fog, and air mass movements from source regions.
Latent heat
in steam Conduction (vapour)
Convection
Radiation