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. Conduction (discussed below)
  2. Convection (discussed below)
  3. Radiation (discussed later in the semester)

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).

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)

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
• 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.

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. We have only covered the processes of conduction and convection. The next page describes how the human body deals with heat and cold stress and how weather conditions impact heat loss from the body.