How Natural Heat Engines Operate

Internal combustion engines use fossil fuels that store solar energy captured by ancient biomass. Thunderstorms, on the other hand, use solar energy stored as latent heat in moist air created when liquid water evaporates as a result of interaction with warm air and sunshine.

Dry air cools down as it expands with altitude and is exposed to colder air, but air containing appreciable moisture does not. Water vapor in the air condenses (as cumulus clouds), releasing heat that keeps the moist air warmer than the surrounding colder air. The buoyancy of the warm, moist air lifts it through the surrounding air, providing the energy release mechanism of a thunderstorm.

For the cumulus cloud to form a thunderstorm, continued uplift must occur in an unstable atmosphere. With further vertical extension of the air parcel, the cumulus cloud grows into a cumulonimbus cloud - the classic anvil-shaped "thunderhead". The energy released by this mechanism is staggering.

For every gram of water condensed, about 600 calories of heat are released. When water freezes higher in the cloud, another 80 calories of heat are released. This energy increases the temperature of the updraft and is converted to kinetic energy by upward air movement. If the quantity of water condensed in a cloud is known, then the total energy of the thunderstorm can be calculated.

In an average thunderstorm, the energy released amounts to about 10,000,000 kilowatt-hours, which is equivalent to a 20-kiloton nuclear bomb. A large thunderstorm is10 to 100 times more powerful, and can release energy equivalent to a thermonuclear device in the megaton range.

Development of a thunderstorm in a moist, conditionally unstable atmosphere is illustrated on the adiabatic chart in the upper right. The line ABCDE represents the early morning temperature structure of the lower atmosphere. The stable layer AB is the nighttime surface inversion.

From B to D, the atmosphere is conditionally unstable since its lapse rate lies between the moist-adiabatic and dry adiabatic lapse rates. Convection from the surface cannot take place unless energy is provided either in the form of heating or lifting.

If air at A is lifted, its temperature decreases at the dry-adiabatic rate of 5.5°F per thousand feet until saturation is reached. Above that level it would decrease at the lesser moist-adiabatic rate. If the moisture content of the air is such that condensation is reached at level F, the temperature of the air would follow the dry adiabat from A to F, then the moist adiabat from F to G and up to E.

During this lifting from A to F to G, the air would be colder than the surrounding air whose temperature is represented by ABG, and would have negative buoyancy. Without energy being supplied to the parcel to lift it, the parcel would tend to return to the surface. Above the level G, the parcel, with its temperature following the moist adiabat to E, would be warmer than the surrounding air, would have positive buoyancy, and would rise freely.

The area on the graph enclosed by AFGB is approximately proportional to the energy which must be supplied before free convection can take place. It is usually referred to as a negative area. The area enclosed by GCDE is a measure of the energy available to accelerate the parcel upward after it reaches level G. It is referred to as a positive area.

In forecasting, thunderstorms are considered to be more likely if the positive area is large and the negative area is small. Whatever the size of the negative area, it represents negative buoyancy that must be overcome before the conditional instability is released.

A common method by which the negative area is reduced is through daytime heating by the sun. When the surface temperature has increased to A', mixing and heating have produced a dry-adiabatic layer from the surface to level G'. The negative area is completely eliminated, and convection of air from the surface to level G' would be possible.

The moisture content of air at this level is such that condensation takes place in rising air upon reaching G'. Above level G', which in this case would be both the convective condensation level and the level of free convection, the temperature of rising air would follow the moist adiabatic line G'E'. The air would rise freely, because it would be increasingly warmer than the surrounding air.

Average Thunderstorm Energy - Source: http://en.wikipedia.org/wiki/Thunderstorm