The atmosphere and the Weather

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.

Energy Transfer

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 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).

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. 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.

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:

• Temperature difference between the objects (Larger temperature difference, the faster the heat transfer)
• Conductivity of the material (e.g., conductive heat transfer in water much faster than in air)

Differences in conductivity between water and air also explains why swimming in water at a temperature of 70°F feels cold, while standing outside when the air temperature is 70°F 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. In addition, water has a very high heat capacity, which means water can absorb lots of energy without much change in temperature.

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.

Two types of convection are important in the atmosphere:

• Dry convection
• Natural - warm air rising, cold air sinking.
• Forced - winds stir up the air, mixing it around. 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. Winds can blow away this warmed layer of air, increasing heat loss.
• Moist convection (Involves phase changes of water)
• 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).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.

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.