Water in Earth's atmosphere
Is mostly nitrogen
This chart shows the composition of *dry* air. But worldwide, the average concentration of water in the atmosphere is about 0.4% with typical concentrations close to the surface of 1-4%.
We know that water vapor is a greenhouse gas. There is much more water vapor in Earth's atmosphere than $CO_2$.
Furthermore, burning fossil fuels *also* releases water. Consider the chemical reaction for combustion of natural gas (methane):
American Chemical Society
Why doesn't that Kalmus chart of radiative forcing include the effects of anthropogenic water vapor emissions from fossil fuels??
Water vapor observations
Below is a NASA image of the average amount of water in earth's atmosphere (in a "column" of air...), world wide, in January of 2020.
Can you guess from this image whether hot air or cold air typically "holds" more water vapor?
How would you guess that this map would look different if we looked at August, instead of January?
*Relative* humidity
When the
weather report says "100% humidity" does that mean that all the
molecules in the air are water?
[Can you think of any undesirable consequences for humans of having 100% of the molecules in air be water molecules? 😊]
Actually there is a maximum density of water molecules in the gas state that depends on temperature. 100% relative humidity means that the water density has reached this maximum concentration.
- At 68 F and "100% humidity", there are about 18 grams of water in each cubic meter of air in a gaseous state. About 2% of all molecules in the air are water molecules.
- At 77 F and "100% humidity" there are about 22 grams in a cubic meter of air.
Imagine that you have a cubic meter box of air at 77 F and 100% humidity. Now you cool it to 68 F. What happens to those extra 4 grams of water?
The water cycle
All discussions of the water cycle should be accompanied audibly by the Banana Slugs:
Atomic basis for condensation
All gas molecules are attracted to each other.
Phase changes, including condensation can be understood as a competition between
- the mutual attraction of gas molecules, which causes them to clump together
- and thermal motion of molecules, which tends to break up the clumps.
Some molecules (such as water) have a very strong mutual attraction (due to their "polar nature"...)
Other molecules (such as Neon and Helium) have a relatively weak mutual attraction.
Which kind of molecule (strong or weak attraction) do you think has a higher boiling point (and also a higher condensation point)?
Once again, we should visit the PHET simulation.
- Choose Phase Changes
Then selecting Adjustable Attraction and a temperature of about -190 C
- Try playing with the interaction strength
It turns out that water boils / condenses at +212 F (+100 C) and $CO_2$ boils and condenses at -109 F (-78 C).
Room temperature is roughly +70 F (21 C).
Look up: How long does water stay in the atmosphere?
Consequences
- Whatever water we add (as emissions) to the atmosphere rapidly drops out again.
- But there is no $CO_2$ "rain" near room temperature.
- Higher temperature air can "hold" more water.
So, as Earth's average temperature increases, we would expect higher average amounts of water vapor in the atmosphere. What are some consequences of that??
- Hurricanes don't necessarily happen more often with climate change, but when they do occur:
Models project an increase on the order of 10-15% for rainfall rates averaged within about 100 km of the storm center by the end of the 21st century. There is already medium confidence for a detectable human contribution to past observed increases in heavy precipitation in general over global land regions and the United States
- NOAA report on Global Warming and Hurricanes [2019, revised Feb 2020] - 60 inches of rain fell from Hurricane Harvey in Texas shattering U.S. storm record [Washington Post, 2017]
- The Indiana Climate Change Impacts Assessment[Purdue / IU consortium, 2018] notes already-observed increases in annual precipitation of 5" (Near Goshen) over the period 1895-2016. And
During winter and spring months, nearly all of the climate models suggest increasing precipitation in all three future periods, with greater increases over time for both emission scenarios.
- Spring of 2019 was so wet...
