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Weather Thermodynamics

Weather we experience is primarily the result of thermal and mechanical processes. Heat is the transfer of energy that occurs when masses having different temperatures are in contact. Heat can result in a combination of temperature change, phase change, or transformation into mechanical energy (motion).

The first law of thermodynamics is energy cannot be created or destroyed. Energy can be transformed, but must come from somewhere. For example, chemical energy in the form of burning coal is transforms liquid water to high pressure steam (mechanical energy) which can be harnessed to drive a turbine. The turbine’s motion can be used to power a generator that changes the mechanical energy into electrical energy. A light bulb can be used to transform the electrical energy into light energy. The primary source of energy that powers the Earth's weather is the Sun. Sunlight absorbed by the Earth is used to melt snow and ice, evaporate water for cloud formation, and create air movement, and effect temperature changes.

The second law of thermodynamics is that energy spontaneously tends to disperse from regions of high concentration (high temperature) to regions where energy is diffuse (low temperature). This law is a driving force for weather. For example, energy from a warm ocean diffuses into the cooler air above, warming the air and increasing the velocity of the air currents.

The zeroth law of thermodynamics observes that the rate of transfer depends on the difference between the temperature of the regions. The greater the temperature difference the faster energy is transferred. This law is balanced by Carnot efficiency, which is the upper theoretical limit to the efficiency of energy transfer. Carnot efficiency, the portion of the transferred energy available for mechanical work, is higher when the temperatures of the regions exchanging energy are more similar. When the temperature of the air mass above an ocean is only slightly cooler than the surface of the water, most of the energy transferred increases wind velocity: however, there is little energy transferred and a long time is required for the transfer to take place.   Conversely, when the temperature of an air mass is much colder than the ocean below it, more energy is transferred and energy transfer is rapid, but not as much of the energy transferred is available to create air movement. Nonetheless, enough energy transfers to fuel weather phenomena such as storms and hurricanes.

Temperature of a mass is also affected by changing pressure. An adiabatic process is one in which the temperature of a mass changes only because of changing pressure. For example, an aerosol container feels cooler as its contents are sprayed. The internal pressure decreases as the container is emptied, as does the temperature of the contents. On the other hand, a bicycle pump gets hotter as it compresses air. Adiabatic processes also occur in the atmosphere. Since air pressure is greater at sea level, rising air cools. On the other hand, air coming over a mountain warms as it sinks into the valley. Chinooks in Alberta are an example of this weather phenomenon.

Temperature changes are also associated with phase changes. The energy gained or lost as a substance transforms among solid, liquid or gas is called latent heat. For example, water on the surface of an ocean absorbs energy to evaporate. This latent heat is stored and transported upward. When the water vapor condenses the energy is released. This heat results in a combination of warming the air and creating wind which influences the weather we experience.

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