Global Warming VS. the Greenhouse Effect

Global warming posits the questions “will an increase in concentration of greenhouse gases lead to an increased global average temperature?” Whereas, the greenhouse effect is a fact that states that the average global temperature is warmer than it would be in the absence of an atmosphere due to greenhouse gases that are being absorbed and re-radiated.

Incoming shortwave (visible light and infrared) radiation gets absorbed by the Earth thus causing the ground to heat and emit longwave (infrared) radiation. The longwave radiation, then, gets absorbed by greenhouse gases (CO2, H2O, CH4) which, in turn, heats up and re-emits radiation out in every direction (some towards the Earth). Notably the cycle continues when some of that radiation, again, gets absorbed by the ground, heats the ground even more, emits even more radiation, and so on.

Wien's Displacement Law

Wien’s displacement law says that the wavelength of the maximum emitted radiation is inversely proportional to the absolute temperature (°K). In other words, hotter objects radiate more energy at shorter wavelengths than do cooler bodies at all wavelengths. This allows us to determine the temperature of other stars depending on its color. Something that glows blue hot is much warmer than one that glow red hot!

Stefan-Boltzmann Law

            Blackbodies are purely hypothetical bodies—they do not exist in nature—that emit the maximum possible radiation at every wavelength. The single factor that determines how much energy a blackbody radiates is its temperature. Although, the amount of radiation emitted by an object is not linearly proportional to its temperature which is where the Stefan-Boltzmann law comes into play. The blackbody version of the Stefan-Boltzmann law expresses that the intensity of energy radiated by a blackbody increases according to the fourth power of its absolute temperature.

I = σT⁴

where I denotes the intensity of radiation in watts per square meter, σ (Greek lowercase sigma) is the Stefan-Boltzmann constant (5.67 × 10−8 watts per square meter per K4), and T is the temperature of the body in kelvins.

            At any rate due to the fact that blackbodies do not exist in nature most liquids and solids can be treated as graybodies, meaning they emit some percent of the maximum amount of radiation possible at a given temperature. Which brings us to the graybody version of the Stefan-Boltzmann law that includes the emissivity factor, meaning that the electromagnetic energy emitted by any graybody will be some fraction of what would be emitted by a blackbody.

I = ƐσT⁴

That percent of energy radiated by a substance relative to that of a blackbody is considered emissivity (ε), ranging from just above zero to just below 100 percent. However, the atmosphere is an exception to this because emission depends on a number of factors (i.e. the amount of water vapor and other gases in the air). Still, we can say that the atmosphere is not a perfect emitter of radiation because it emits less radiation at any particular temperature than would a blackbody.

Rayleigh Scattering--Why is the Sky Blue?

            Scattering agents smaller than about 1/10^th the wavelength of incoming radiation disperse radiation (both forward and backward) in a manner known as Rayleigh scattering. Rayleigh scattering is performed by individual gas molecules in the atmosphere and primarily affects shorter wavelengths. It is particularly effective for visible light, especially for those colors with the shortest wavelengths (i.e. blue).

Combined with greater effectiveness in scattering shorter wavelengths than longer wavelengths, this characteristic leads to three interesting phenomena:

1.    The blue sky on a clear day

2.    The blue tint of our atmosphere

3.    The redness of sunsets and sunrises

Why is the sky blue?

            Gases and particles in the atmosphere scatter some incoming solar radiation in all directions. Air molecules scatter shorter wavelengths most effectively and blue light is among the shortest (and therefore most readily scattered) of visible wavelengths thus the scatter contains a higher proportion of blue light. And, since Rayleigh scattering occurs at every point in a clear atmosphere and diverts energy towards a viewer from all directions, no matter where you look on a cloudless day, the sky is blue.

Some more interesting facts…

Laws of Thermodynamics

0^th Law of Thermodynamics—if body A is in thermal equilibrium with body T, and so it body B, then A and B are in thermal equilibrium.

1^st Law of Thermodynamics—a measure of heat transferred into a system will result in an increase in temperature and in the system’s ability to do work. In other words, energy is conserved property that is neither created nor destroyed but, may change form and travel from place to place. Or, in terms of an internal combustion engine in an automobile, the first law describes the underlying principle of what occurs in the cylinder.

Other forms of the First Law…

2^nd Law of Thermodynamics—only in transferring heat from a warmer body to a cooler body can heat be converted into work, in a cyclical process. A cyclic process is a series of operations by which the state of the substance (working substance) changes but the substance is finally returned to its original state in all respects. In other words, heat is always transferred from regions of high temperature to regions of low temperature. Heat can be transferred by three processes…

1.    Conduction—the movement of heat through a substance without appreciable movement of the molecules.

2.    Convection—the transfer of heat by mixing of a fluid.

3.    Radiation—the transfer of energy by radiation that can occur through empty space.

Forms of the Second Law, considering the Carnot Cycle…

3^rd Law of Thermodynamics—there is no finite series of steps that can get you to absolute zero. In other words, since absolute zero cannot be reached an engine cannot be perfectly efficient.

Conclusion—heat can be converted into work, in a cyclic process, but can only be perfectly efficient at absolute zero, which is unattainable. 

Focus on the Environment and Social Impacts: Radiation Inversions and Human Activities

Understanding Weather and Climate (7th Edition) (MasteringMeteorology Series) by Edward Aguado, James E. Burt

Forecasting: Dew Point and Nighttime Minimum Temperatures

Understanding Weather and Climate (7th Edition) (MasteringMeteorology Series) by Edward Aguado, James E. Burt

Physical Principles: The Sun

Understanding Weather and Climate (7th Edition) (MasteringMeteorology Series) by Edward Aguado, James E. Burt

Physical Principles: The Nature of Radiation, Absorption, and Emission

Understanding Weather and Climate (7th Edition) (MasteringMeteorology Series) 7th Edition by Edward Aguado (Author), James E. Burt (Author)