# Why isn't $\ce{H2O}$ evenly distributed in Earth's atmosphere?

I frequently read $$\ce{CO2}$$ have a spatially constant concentration sadly rising and at 415ppm at present. That concentration do not vary a lot spatially I think. You will find ~415ppm at Argentina or at Taiwan.

However this is not the case for $$\ce{H2O}$$; some regions are dry, some others are wet.

Can you explain to someone not trained in atmospheric sciences what makes $$\ce{H2O}$$ especial?

• earthonlinemedia.com/ebooks/tpe_3e/atmospheric_moisture/… – gansub Feb 26 '20 at 9:28
• CO2 is relatively well mixed compared to SO2, say, but it's still not spatially uniform: e.g., surface and satellite obs. Whether it's important depends on what's being studied though. – Deditos Feb 26 '20 at 9:33
• Good question, I think the answer to your question is: rain. H₂O is not particularly special in being not homogeneously distributed, at least it's not unique in that sense, there are other gases that are also short-lived, but for different reasons. Since I'm not entirely certain about the answer I'm just leaving this comment as a pointer. – gerrit Feb 26 '20 at 12:03
• one is a liquid in most earth temperatures and the other a gas, that would be a pretty solid start. – John Feb 27 '20 at 22:15

I frequently read CO2 has a spatially constant concentration sadly rising and at 415ppm at present. That concentration do not vary a lot spatially I think. You will find ~415ppm at Argentina or at Taiwan.

Your reading sources are overly simplistic. Carbon dioxide concentrations are not spatially constant, and the increase is not monotonic. They instead vary by several percent over the course of a year and over the globe. This is particularly the case in the Northern Hemisphere, where CO2 concentrations increase during the winter when land plants are dormant but decrease during the summer when land plants draw down CO2 levels.

The next two images portray these seasonal effects. The first image portrays CO2 concentrations over the world on 22 March 2105 while the second portrays concentrations five months later. Note that CO2 concentrations in the Northern Hemisphere dropped during this five month interval.

The net change over the course of decades is, sadly, upward, as portrayed in the graph that follows. This graph portrays CO2 concentrations as directly measured at the observatory at the top of Mauna Loa. These measurements represent the longest continuous record of direct atmospheric CO2 concentration readings. The red curve shows the monthly averages. This curve exhibits a sinusoid-like curve atop a ramp. The black curve has the seasonal variations removed, making it much closer to a monotonic ramp.

Can you explain to someone not trained in atmospheric sciences what makes H2O special?

Water is the only common substance on Earth that exists as a solid, a liquid, and as a gas in the Earth's atmosphere. While some alcohols, some oils, and elemental bromine have somewhat similar melting points and boiling points, none of these is as stable chemically as is water, and none is anywhere as close to common as is water.

Carbon dioxide remains in its gaseous state throughout all temperature and pressure ranges experienced in the Earth's atmosphere. (Aside: if CO2 levels were 2000 times what they are now, CO2 might well precipitate as CO2 snow at the Vostok Station. But that ridiculously high concentration level is not the case; CO2 will remain in its gaseous state throughout all temperature and pressure ranges experienced in the Earth's atmosphere.)

Assuming that CO2 levels remain less than 2000 times higher than present means that CO2 in the atmosphere will always be in the gaseous state, which in turn means the CO2-bearing capacity of the atmosphere is effectively infinite. On the other hand, the water-bearing capacity of the atmosphere depends very strongly on temperature. Air at 35°C at sea level can hold about 37 grams of water per kilogram of dry air before the water vapor starts to precipitate as rain. That's over 60 times the current CO2 molar concentration level. Air at -35°C can hold less than 0.2 grams of water per kilogram of dry air before the water vapor starts to precipitate as snow, which is less than the current CO2 molar concentration level.

The following graphic portrays the huge variations ("huge" compared to the several percent variations in CO2 concentration levels) in water concentration levels. The graph in question shows the monthly average total precipitable water vapor levels for May 2009.

• All of the images are from US government websites based on instruments owned / funded by the US government. All of the posted images are in the public domain. – David Hammen Feb 28 '20 at 20:05
• IF CO₂ levels were 2000 times higher, Vostok would be hot and wet and not experience CO₂ snow... (but I get your point). – gerrit Feb 29 '20 at 15:53

The brief answer is 'the water cycle' I think.

A longer answer is that water can exist in many states on and above the surface: at least snow, ice and liquid water on and below the surface and water vapour (gas) and aerosols of liquid water or ice (clouds) in the atmosphere. On the other hand $$\mathrm{CO_2}$$ can only exist in one: gas.

What that means is that $$\mathrm{CO_2}$$ is as well-mixed as any other long-lived gaseous component of the atmosphere over time, which is 'pretty well'. Components of the atmosphere, such as $$\mathrm{SO_2}$$, which are not long-lived don't get mixed so well because they don't have time to be. For water the situation is even more complicated because there are complicated phase transitions going on within the atmosphere, and because the distribution of liquid water is extremely dependent on topography, and in particular it can move large distances from where it originates.

The phase transitions of water in the atmosphere are driven by temperature and pressure conditions in the atmosphere, and in turn alter those conditions, and the whole system is chaotic of course. The result of some of these phase transitions is water leaving the atmosphere as precipitation.

How much water gets into the atmosphere depends on conditions on the surface and in the atmosphere, and also on where water is on the surface, which is extremely non-uniform (see above).

All of these transitions are driven by energy from the Sun which varies over the planet.

So the end result of all that is that

• water vapour is not long-lived in the atmosphere as a result of all this, so it doesn't have time to become well-mixed;
• water is not evenly-distributed on and below the surface because it's mostly liquid (and because snow & ice is only stable in some parts of the surface);
• the behaviour of water entering, moving within, and leaving the atmosphere is extremely complex and chaotic (or 'there is weather'), resulting in large variations over a range of scales in distribution of the various states of water which can live in the atmosphere, and large variations in how much water leaves the atmosphere over various scales.

The influence of the sun is greatest at the equator. The sun causes air to rise in three main cells or bands around the Earth north of the equator,with a similar arrangement south of the equator. The Hadley cells which rise up closest to the equator, both north and south, are by far the most important.

As the air rises it cools and the pressure becomes lower, so it sheds its moisture as rain over an equatorial region which used to be covered in rain forest and where, despite the depredations of man, there is still a lot of rainforest left. These Hadley cells are the main reason why moisture and rainfall are unevenly distributed. Mountains and plateaus are another cause.

Having shed most of its moisture and moved north, in the northern hemisphere the Hadley cell descends over the Sahel and other desert and semi-desert areas around the globe. They are deserts, of course, because the dry air of the descending cell creates high pressure, so very little rainfall.

A similar state of affairs exists in the southern hemisphere, but as there is less land and more ocean to the south, there are fewer deserts than in the north.