Atmospheric Energy Transfer

Heat Energy

Heat energy available at the Earth's surface as a function of solar irradiance tends to converge in the warm season and diverge in the cold season. Variance in the direction of the net flux is due to seasonal and subseasonal temperature gradients.

During the warm season, heat convergence is typically large, with gradients operating both upward into the cooler overlying air and downward toward the cooler soil that lies beneath the surface. While there are many thermal characteristics to consider with respect to the material through which heat flows via conduction, the upward flux of heat (and in many cases water vapor) is usually very strong due to the superadiabatic (where temperature decreases with height at a rate of greater than 10 °C per kilometer) nature of the lower planetary boundary layer (PBL).

In general, heat energy is not particularly well transmitted through air, since the molecules are only briefly in contact with one another; however, the ground surface is in immediate and frequent contact with atmospheric molecules in the lower atmosphere. From here the heat is transferred upward or downward in response to an upward-or downward-directed temperature gradient. If the gradient is directed downward, then the heat is transferred back into the surface layer, and if it is upward, then the transfer is accomplished via conduction through the convective boundary layer (CBL). It is at the top of the CBL that the upper boundary of the PBL is found.

As long as the capping inversion at the top of the PBL is not strong (generally less than 2 °C), the convection process continues to drive heat upward until a layer of neutral static stability is found. If this does not occur until the air reaches the upper levels of the troposphere, then deep moist convection (DMC) is the likely result.

Heat is also transferred upward via long-wave radiative flux. In this case, one must remember that Wien's law tells us that the wavelength of emission is inversely related to heat intensity. In cases where the greenhouse gasses of water vapor and ozone are highly concentrated, the upwelling long-wave radiation is rapidly absorbed and counterradiated in all directions, with some being directed downward toward Earth's surface. For this reason, humid and/or cloudy nights generally result in greater than average overnight low temperatures. This is especially true in cities, where upwelling radiation is contributed by asphalt surfaces and concrete buildings, not to mention heat input from the buildings’ mechanical systems. Ozone levels also tend to be much greater in cities due to automobile traffic, which improves the ability of the air to absorb upwelling long-wave radiation.

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