Editing Water vapor (section)

=== Impact on air density ===
Water vapor is lighter or less [[Density of air|dense than dry air]].<ref>{{cite news| url=https://www.washingtonpost.com/blogs/capital-weather-gang/wp/2013/08/05/why-dry-air-is-heavier-than-humid-air/| title=Why dry air is heavier than humid air| newspaper=The Washington Post| date=August 5, 2013| access-date=28 December 2014| author=Williams, Jack}}</ref><ref>{{cite web| url=http://www.wwrf.org/humidity101.htm| archive-url=https://archive.today/20130416080406/http://www.wwrf.org/humidity101.htm| url-status=dead| archive-date=16 April 2013| title=Humidity 101| publisher=World Water rescue Foundation| access-date=28 December 2014}}</ref> At equivalent temperatures it is buoyant with respect to dry air, whereby the density of dry air at [[standard temperature and pressure]] (273.15 K, 101.325 kPa) is 1.27&nbsp;g/L and water vapor at standard temperature has a [[vapor pressure]] of 0.6 kPa and the much lower density of 0.0048&nbsp;g/L.

==== Calculations ====
[[File:dewpoint.jpg|right|frameless|upright=1.15]]

Water vapor and dry air density calculations at 0&nbsp;°C:
* The [[molar mass]] of water is {{nowrap|18.02 g/mol}}, as calculated from the sum of the [[atomic mass]]es of its constituent [[atoms]].
* The average molar mass of air (approx. 78% nitrogen, N<sub>2</sub>; 21% oxygen, O<sub>2</sub>; 1% other gases) is {{nowrap|28.57 g/mol}} at standard temperature and pressure ([[Standard temperature and pressure|STP]]).
* Obeying [[Avogadro's Law]] and the [[ideal gas law]], [[Humidity|moist air]] will have a lower density than dry air. At max. saturation (i. e. rel. humidity = 100% at 0&nbsp;°C) the density will go down to 28.51 g/mol.
* STP conditions imply a temperature of 0&nbsp;°C, at which the ability of water to become vapor is very restricted. Its [[concentration]] in air is very low at 0&nbsp;°C. The red line on the chart to the right is the maximum concentration of water vapor expected for a given temperature. The water vapor concentration increases significantly as the temperature rises, approaching 100% ([[steam]], pure water vapor) at 100&nbsp;°C. However the difference in densities between air and water vapor would still exist (0.598 vs. 1.27 g/L).

==== At equal temperatures ====
At the same temperature, a column of dry air will be denser or heavier than a column of air containing any water vapor, the molar mass of diatomic [[nitrogen]] and diatomic [[oxygen]] both being greater than the molar mass of water. Thus, any volume of dry air will sink if placed in a larger volume of moist air. Also, a volume of moist air will rise or be [[Buoyancy|buoyant]] if placed in a larger region of dry air. As the temperature rises the proportion of water vapor in the air increases, and its buoyancy will increase. The increase in buoyancy can have a significant atmospheric impact, giving rise to powerful, moisture rich, upward air currents when the air temperature and sea temperature reaches 25&nbsp;°C or above. This phenomenon provides a significant driving force for [[Cyclone|cyclonic]] and [[Anticyclone|anticyclonic]] weather systems (typhoons and hurricanes).