Urban Weather

In urban areas, the atmosphere is exposed to the highest concentration of water vapor from combustion.  If the amount is significant, this might increase local humidity and possibly affect weather.  Let's investigate this possibility further.

City-wide effects 

Let's determine the amount of water vapor produced from combustion in a city like Los Angeles and compare that to natural sources of humidity.

Combustion water vapor:   There are roughly 5 million automobiles in Los Angeles each burning 2 gallons of gasoline each day.   This amounts to 40 million kg of water released into the local atmosphere each day from automobiles.  Adding in all other sources of combustion, such as power plants and home heating, this is approximately 100 million kg of water added to the Los Angeles atmosphere per day. 
  

Natural water vapor:  Each day, the sun evaporates 1 trillion tons of water worldwide.  This comes out to 20 billion kg in the 100 km x 100 km Los Angeles metro area each day.

Looks like we have a winner!  Water vapor evaporated naturally amounts to 200 times that produced by combustion in urban areas.  Or, said another way, water vapor from combustion is only 0.5% of the total water vapor released into the atmosphere in urban areas.  This means that water vapor from combustion has a negligible impact on humidity and evaporation, even in an urban area like Los Angeles.  It is therefore unlikely that water vapor from combustion can significantly affect local weather in urban areas.  It should be noted that other aspects of combustion, such as carbon dioxide and aerosol formation, may have a significant effect on local weather.   Also, other aspects of urban areas, such as pavement and buildings absorbing sunlight, may have significant effects as well.   But we'll leave those topics to the other websites on the internet!

Micro-scale effects

What about possible impacts to weather immediately nearby to a power plant?  Power plants emit huge amounts of water vapor into the air; it is obvious by the thick cloud plumes they release into the air.

A study was conducted by Corti and Carnevale¹ on the water vapor plume released from a combined-cycle power plant.  Combined-cycle plants use natural gas for the gas turbines and light or heavy oil for the hot water generators to minimize pollution, but they release a lot of heat and water vapor.  

Their results showed that the plume affects relative humidity (RH) of the air to about a 200 meter radius from the center of the plume.  RH was 100% at the center of the plume, and quickly reduced to ambient (70% RH) at a distance of 200 meters from the point source.  This is not a very large area.  Additionally, their studies did not show any impact on humidity further than 200 meters from the plume, and absolutely no effect on local weather or rainfall.    Therefore, it appears that water vapor does not affect weather on a local scale either.

"Heavenly effects" (Contrails)

http://www.es.lancs.ac.uk/hazelrigg/contrails/contrail.gif

Contrails (Condensation Trails) are the lines of clouds that appear in the sky behind an aircraft.  They are produced from the water vapor released from the jet's combustion engines condensing and forming clouds in the very cold atmosphere at flying altitudes.   Contrails, as well as the cirrus clouds they become when they disperse, are optically thin and allow  sunlight to pass through them but prevent earth's reflected radiation from leaving the atmosphere.  This, in theory would act similar to the greenhouse effect, and cause the daytime temperature to be cooler, the nighttime temperature to be warmer, with an overall heating effect.

The suspension of air travel for three days in the United States after September 11, 2001 provided an opportunity to test this hypothesis. Measurements did in fact show that without contrails the local diurnal temperature range (difference of day and night temperatures) was about 1 degree Celsius higher than immediately before².

In summary, it appears that water vapor produced by combustion does not affect local weather near the ground, even in highly concentrated areas, but it can affect local weather when emitted at high altitudes where it can immediately form cirrus clouds.
 

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¹ Corti A, Carnevale E:  "Environmental impact from wet plumes in combined-cycle power plants". Applied Thermal Engineering, 18 (11): 1049-1057, November 1998.
² Travis et al., J. Climate, 17, 1123-1134, 2004.