Those who spend much time in greenhouses know that they are often very humid places because water evaporates from plants and from surfaces that get wet when the plants are watered. Meteorologists typically refer to the water vapor content of the air as relative humidity which is how close the air is to holding as much water vapor as it can hold at its current temperature.
Unfortunately many climatologists waste so much time on the nonexistent impact of radiation on air temperature that they don't provide sufficient emphasis to the significant impact of water vapor on air temperature. Those who want to blame climate changes on humans ignore the fact that the combustion of hydrogen containing fossil fuels increases the amount of water vapor in the air. Other human actitivies such as watering yards, washing cars and operating public fountains also add water to the atmosphere.
Water has some special thermal characteristics that can significantly affect atmospheric temperatures. Water heats and cools significantly slower than other components of the atmosphere. Water vapor needs to absorb over four times more heat energy than the same mass of other air molecules to raise its temperature the same amount. When it cools it releases four times more heat than other gases.
Thus as the water vapor content of the air increases the atmosphere will heat and cool slower than when the air is drier. This process tends to keep the temperature from rising as high during the day or cooling as much at night, although the increase in the overnight low may lead to an increase in the daytime temperature because the air doesn't have to heat as much to reach a higher temperature. In equatorial areas deserts have higher maximum temperatures and lower minimum temperatures than jungle areas where the humidity is higher.
Water vapor possesses what physicists have traditionally called "latent" heat. Latent heat refers to the heat energy water molecules must absorb to go from a solid to a liquid (heat of fusion 80 calories/gram) or a liquid to a gas(heat of vaporization 540 calories/gram). This energy isn't reflected in the temperature of the water vapor. However, when water vapor condenses back to a liquid, or freezes, the release of this latent heat can raise the temperature of the air. A gram of water vapor releases enough heat energy when it condenses to raise the temperature of 2 kg of air by 1 C.
At a dew point of approximately 65 F water vapor in the atmosphere holds as much heat energy as the rest of the atmosphere. This condition explains why dew points above 65 F are associated with the strongest thunderstorms.
Physicists define a "calorie" as the amount of heat required to raise the temperature of a gram of water 1 C. 27 C (82 F) is the same temperature as 300 Kelvin [the absolute temperature scale]. At 300 K water vapor has 300 calories of heat from its temperature and 620 calories of latent heat.
The dew point is the temperature at which water vapor will condense on objects or aerosals. The dew point normally is the lowest temperature the air will fall to. As the water vapor content of the air increases the dew point rises and the air doesn't get as cool at night.
The situation is more complex than I am presenting it in this post. I . The important facts to consider are that increases in humidity can raise the low, or minimum temperature, and limit the high, or maximum temperature, each day. In areas where significant snowfall occurs, the increase in low temperature can increase the melting of snow and ice by keeping the temperature above freezing for longer periods of time.
I recently came across a 10 year old study done by David R. Easterling of the National Oceanic and Atmospheric Administration's National Climatic Data Center in Asheville, N.C., indicating that humidity had increased and, as should have been expected, the minimum temperature had been increasing and the difference between the minimum and maximum daily temperatures, diurnal temperature range (DTR), had been declining.
The potential impact of changes in atmospheric water vapor are real science. Water vapor holds a substantial amount of heat energy. The only potential impact climatologists can find for carbon dioxide is the highly questionable claim about absorbing and re-radiating low energy IR. But then, if would be difficult for the politicians behind the global warming scare to make a case for getting rid of water.
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