Energy Transfer

The next topic concerns how the human body exchanges energy (or heat) with its surroundings. This will include how the body responds to both hot and cold temperatures AND how humidity and winds factor into the heat exchange. This will lead us to the concepts of wind chill factor and heat index.

We start with a few basics. Keep in mind the material presented here is somewhat simplistic. In reality energy expenditures and transfers can do more than just change the temperature of an object.

• For an object to warm, energy must be added
• For an object to cool, energy must be removed
• Energy balance for an object (that does not produce energy internally):
  • If energy in = energy out, temperature of object remains constant
  • If energy in > energy out, temperature of object increases
  • If energy in < energy out, temperature of object decreases
• Energy is transferred between objects that are at different temperatures. The direction of energy transfer is ALWAYS from hot --> cold.
• There are three mechanisms of energy transfer:
  1. Radiation
  2. Conduction
  3. Convection

Radiation

Radiation is the transmission of energy through space or through a material medium in the form of electromagnetic waves. Don't be concerned about understanding the wording of the last sentence. Only a couple of points will be made now. We will return to radiation later in the semester.

All objects in the universe emit (or give off) radiation energy. The type and amount of radiation energy emitted depends on the object's temperature. Basically, the hotter the object, the greater the amount of radiation energy it emits. For example, the Sun emits much more radiation energy than the Earth because the Sun is much hotter.

If you place a rock out in space, the rock loses energy, and hence cools down, by continuously emitting radiation. Meanwhile, the rock gains energy, and hence heats up, by absorbing radiation energy that was originally emitted by other objects, like stars. If the radiation energy absorbed by the rock is greater than the radiation energy emitted by the rock, the temperature of the rock will go up. If radiation energy absorbed is less than radiation energy emitted, the temperature of the rock will go down.

This explains much of the daily temperature changes at a given place on the Earth. At night, the ground surface cools because it is emitting radiation energy away, while there is no radiation energy coming in from the Sun. During the day, the ground surface heats up because the radiation energy being absorbed from the Sun is greater than the radiation energy being emitted from the ground.

For the most part, exchanges of radiation energy have a smaller influence on human comfort than energy exchanges due to conduction and convection ... however, when you expose yourself to direct sunlight, absorption of the Sun's radiation can make you feel warm (or hot). We will not discuss heating or cooling of the human body by radiational energy exchanges any further.

Conduction

Conduction is the transfer of energy by direct collisions of molecules (touching). Energy can be conducted from one object to another or within a single object that contains temperature variations (See Figure F). The rate at which energy is transferred within a material is referred to as its heat conductivity. For example, take a rod of steel. Heat the rod at one end and measure how quickly heat is conducted toward the other end. In general, solids and liquids are better heat conductors than gases because the molecules that make up solids and liquids are more tightly packed than in gases. Thus, water and metals are good heat conductors, while air is a poor heat conductor (or a good heat insulator). A table of heat conductivities for several substances is provided below. You don't need to worry about the scientific units of heat conductivity. Use the table to compare how well heat is transferred by conduction through various materials. The higher the heat conductivity, the faster heat flows through the material by conduction. Note that some of best heat insulators, items at the bottom of the table like wood, rabbit fur, and wool, get much of their insulating properties from trapped air within the material. For example, wool jackets keep us warm mainly because of the pockets of still air trapped within the fibers.

When two different objects touch heat is always transferred from the warmer object to the colder object. If you touch something hot, energy is transferred from the hot object to you. If you touch something cold, energy is transferred from you to the cold object.

The rate of conductive heat transfer depends on:

• The temperature difference between two objects that are touching or the temperature difference from one end to the other within a single object. The larger temperature difference, the faster the heat transfer.
• Conductivity of the material. For example, the conductive heat transfer in water is much faster than in air.

Differences in conductivity between water and air also partially explains why swimming in water at a temperature of 70°F (21°C) feels cold, while standing outside when the air temperature is 70°F (21°C) does not. Because water is a good heat conductor, it moves heat away from your body faster than air does, which results in a cold sensation. Another reason different from conduction is that water has a large heat capacity, which means water must absorb a lot of heat (energy) to raise its temperature. So if you are surrounded by a large pool of cold water, the heat from your body is easily conducted away from you and does not cause the water to warm up much, which allows the conductive heat transfer to remain rapid.

Convection

Convection is the transfer of heat by actual movement of mass within a fluid. Convection is a very important means of energy transport in the atmosphere, especially moist convection. Convection only occurs in fluids (liquids and gases), not in solids. In the atmosphere, we can think of convection happening when parcels of air (blobs of air about the size of large balloons) move around.

Two types of convection are important in the atmosphere:

• Dry convection
• Natural dry convection - This is simply warm air rising and cold air sinking(See Figure G). Parcels of air will continue to rise as long as the parcel air temperature remains warmer than the temperature of the air surrounding the parcel. Dry convection is often initiated by the uneven heating of the ground surface on sunny days. For example, a blacktop surface will become hotter than the surrounding grass. Air above the blacktop will become warmer than the surrounding air over the grass. The warmer air rises (in parcels), which moves heat or energy upward. These currents of rising heated air are called "thermals." It is common to see birds of prey, like hawks, use these thermals to carry themselves upward without expending much energy.
• Forced dry convection - When winds stir up the air, it forces the air to mix, which transfers heat or energy from warmer to colder regions. This partially explains why a fan can help keep you cool. In still air, a thin layer of warmed air forms an insulating barrier just above your skin. As the air in contact with the body warms, the temperature difference between your body and the air surrounding your body gets smaller. The rate of heat transport from you to the air slows down as the temperature difference is lessened. Winds can blow away this warmed layer of air (forced convection), forcing it to be replaced by cooler air, and thus increasing heat loss, since the temperature difference between your body and the air immediately surrounding the body remains larger.
• Moist convection
• Accounts for energy removed due to evaporation of water (usually from near the ground surface), then delivered when the water condenses (usually high in the atmosphere where clouds form). (See Figure G) I find students often have difficulty understanding this process. Recall that water vapor contains more internal energy than liquid water. When water evaporates, you can say that the energy, which was used to evaporate the liquid, is stored in the water vapor. This stored energy is released when the water vapor condenses back to liquid water. Overall, energy is removed from the region where the water evaporated and released where the water condenses, thus transferring energy from one location to another.
• The rate of heat loss via evaporation depends on the net rate of evaporation, which as we have seen depends on the relative humidity. The rate of net evaporation also depends on the wind. In still air, a thin layer of humid air forms a barrier above where liquid water is evaporating. Winds can blow away this humid layer of air, increasing evaporative heat loss. This is actually more important in understanding why a fan can help you keep cool on a hot day. In general, the faster the windspeed, the faster the rate of net evaporation. The net rate of evaporation (rate of evaporation minus rate of condensation) is determined by what is known as the vapor pressure deficit. The vapor pressure deficit is the saturation vapor pressure (based on the liquid water temperature) minus the vapor pressure in the air. In still air water vapor will accumulate just above the surface from which it is evaporating, which increases the vapor pressure, lowers the vapor pressure deficit, and results in slower net evaporation. Winds can blow away the air with high water vapor content and force it to be replaced by air with lower vapor content, and thus increases the net evaporation rate.

All three mechanisms of energy transfer, conduction, convection, and radiation, play a role in how the human body exchanges energy (heat) with the external world. The next page describes how the human body deals with heat and cold stress and how weather conditions impact heat loss from the body.