Amount of Water in the Atmosphere

We know, all too well from personal experience, that the amount of water in air varies quite a bit over a day, over a month and over a year. We can sense humidity levels and have experienced how we sweat more and the sweat does not evaporate when air humidity is high. 


3413 Trillion US Gallons

     The US Geological Survey estimates the total amount of water in the atmosphere as 3,100 cubic miles or (at 1 cubic mile = 1.10111715 × 10¹² US gallons) 3413.463165 × 10¹² US gallons.

     This is only about 0.001% of the total water on Earth .

Temperature & Humidity control amount
     The specific amount of water in the air varies with temperature and humidity.
     At 30 degrees Celsius, the amount of water varies from 0 grams per cubic meter in dry air to 30 grams per cubic meter (0.03 ounce per cubic foot) when the air is saturated.

        At a temperature of 50 degrees Celsius, there are 83 grams per cubic meter (0.08 ounces per cubic foot) when the humidity is 100% 

Relative humidity	20 %	40 %	60 %	80 %	100 %
+50 °C	16.6	33.2	49.8	66.4	83.0
+40 °C	10.2	20.5	30.7	40.9	51.1
+30 °C	6.1	12.1	18.2	24.3	30.4
+20 °C	3.5	6.9	10.4	13.8	17.3
+10 °C	1.9	3.8	5.6	7.5	9.4
0	1.0	1.9	2.9	3.9	4.8
-10 °C	0.5	0.9	1.4	1.9	2.3
-20 °C	0.2	0.4	0.5	0.7	0.9

3 ways water ends up in the air 
     Evaporation - the process by which water is converted from liquid to vapor state and added into the air. Nearly 90% of the water in the atmosphere is due to evaporation from oceans, lakes and other large bodies of water
     Transpiration - The process  by which plants release water vapor directly into the air. This process accounts for nearly 10% of the water vapor in the atmosphere
     Sublimation -  The process by which ice changes directly (without entering the liquid state) into water vapor that ends up in the air. This is why ice, that has been in your freezer or the ice chest for a long time, looks 'different', tastes hollow, feels 'less cold' and crushes easily.

Mean Annual Precipitation = 1 Meter

    The mean annual precipitation every year is about 1 meter (about 3 feet and 3 inches). As this is water vapor condensed from regions close to the surface, it gives us a measure of the amount of water that circulates in and out of the atmosphere on an annual basis.

     Attempts to extract this volume of water through human endeavour would keep the hydrological cycle in harmony with its natural performance

99% of water is close to ground level

     75% of the air in the atmosphere and 99% of all the water in the atmosphere lie in the first 9 miles (15 kilometers) above the ground. This part of the atmosphere is called the Troposphere.

     The composition of the troposphere is uniform throughout its height (i.e. does not vary with elevation above the Earth) except in one way, namely, the proportion of water vapor is greatest closest to the ground and decreases as elevation increases.

References:

http://ga.water.usgs.gov/edu/watercycleatmosphere.html

http://en.wikipedia.org/wiki/Water_vapor
 http://en.wikipedia.org/wiki/Troposphere
http://books.google.com/books?id=oh8lqM5obuYC&pg=PA156&lpg=PA156&dq=water+vapor-