Oklahoma Climatological Survey


Heat Transfer





The driving force for atmospheric motion is the sun and, in particular, the uneven distribution of solar radiation across the earth. It is the primary job of the atmosphere to redistribute energy in order to achieve a balance from pole to equator. We need to understand methods of heat exchange in the atmosphere to appreciate how the thin atmosphere keeps us alive.




  • Energy is the ability or capacity to do work on some form of matter.

    There are several forms of energy, including the following:

    1. Potential energy is the energy which a body possesses as a consequence of its position in a gravitational field (e.g., water behind a dam).
    2. Kinetic energy is the energy which a body possesses as a consequence of its motion (e.g., wind blowing across a wind generator). It is dependent upon an object's mass and velocity (e.g., moving water versus moving air).
    3. Internal energy is the total energy (potential and kinetic) stored in molecules.
    4. Heat (or thermal) energy is kinetic energy due to motion of atoms and molecules. It is energy that is in the process of being transferred from one object to another because of their temperature difference.
    5. Radiant energy is the energy that propagates through space or through material media in the form of electromagnetic radiation.


  • The First Law of Thermodynamics states that energy lost during one process must equal the energy gained during another.



Figure 1- Evaporation and Condensation

Latent Heat

  • Latent heat is the heat energy required to change a substance from one state to another.

    There are basically three states of matter: solid, liquid, and gas. The difference between them is how the molecules are arranged. Solids have tightly-packed molecules, liquids are still bound together but not strongly enough to keep them from flowing, and gas molecules are free-flowing, not bound to one another at all. Energy is required to change from one state to another because bonds must be loosened, broken, tightened, or made. Energy must be given to the molecules if bonds are to be loosened or broken and taken from the molecules if they are to be tightened or made.

  • Energy is required to change from solid to liquid, liquid to gas (evaporation), or solid to gas (sublimation). Energy will be released to change from liquid to solid (fusion), gas to liquid (condensation), or gas to solid.


  • Latent heat of evaporation is the energy used to change liquid to vapor.

    IMPORTANT: The temperature does not change during this process, so heat added goes directly into changing the state of the substance. About 600 calories of energy are needed for every gram of water at room temperature. This is why you cool when you step out of the shower. Heat is taken from your skin to evaporate the water on your body.

  • Evaporation is a cooling process.


  • Latent heat of condensation is energy released when water vapor condenses to form liquid droplets.

    An identical amount of calories (about 600 cal/g) is released in this process as was needed in the evaporation process. This is one mechanism of how thunderstorms maintain their intensity. As moist air is lifted and cooled, water vapor eventually condenses, which then allows for huge amounts of latent heat energy to be released, feeding the storm.

  • Condensation is a warming process.


  • Latent heat of fusion describes both changing from solid to liquid and from liquid to solid.

    From solid to liquid, about 80 calories per gram are needed. From liquid to solid, about 80 cal/g are released.

  • Latent heat of sublimation describes both changing from solid to gas and gas to solid.

    Sublimation is rare as compared to the other changes of state. From solid to gas 600 + 80 = 680 calories per gram are needed. From gas to solid, 680 cal/g are released.



Figure 2- Conduction, Convection and Radiation

Methods of Heat Transfer

  • Conduction- energy is transferred by the direct contact of molecules, not by the movement of the material

    Example: putting your hand on a stove burner. The amount of energy transferred depends on how conductive the material is. Metals are good conductors, so they are used to transfer energy from the stove to the food in pots and pans. Air is the best insulator, so good insulating products try to trap air and not allow it to move.

  • Convection- energy is transferred by the mass motion of groups of molecules resulting in transport and mixing of properties

    Example: holding your hand over a stove burner. In meteorology, we speak of convection predominantly as that caused by rising currents of warm air. We refer to all other mass motions of air as advection.

  • Radiation- energy is transferred by electromagnetic radiation

    Example: heat felt when standing away from a large fire on a calm night. Everything that has a temperature above absolute zero radiates energy. Radiation is not "felt" until it is absorbed by a substance. It does not require a medium to transfer energy through as do conduction and convection.



Specific Heat

  • Specific heat is the amount of heat needed to raise the temperature of one gram of a substance one degree Celsius.


  • The specific heat of water is very high compared to other substances, so water can store energy longer than most other substances.

    For example, the Gulf of Mexico remains warm during the night, when air and soil temperatures decrease rapidly.

    Why is the Southern Hemisphere summer generally not warmer than the Northern Hemisphere summer although Earth is closer to the sun during the Southern Hemisphere summer? Because most of the Southern Hemisphere is water, which regulates the seasonal temperatures.






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