Atmospheric Gas Water Vapour

Atmospheric Gas Water Vapour

Atmospheric Gas Water Vapour

There are many gases of varying quantities, in the Earth's atmosphere, although nowadays it seems only one is discussed - Carbon Dioxide (CO2).
According to NASA https://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html "by volume, in a dry atmosphere, 78.08% Nitrogen, 20.95% Oxygen, 0.934% Argon, 0.042% Carbon Dioxide, which adds up to 100.006%" then add, "other gases 0.268%" which makes 100.273%, clever, but they then say "numbers do not add up to 100% due to roundoff and uncertainty. Water is highly variable, typically makes up about 1%."

Wikipedia say "By volume, dry air contains 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and small amounts of other gases.[8] Air also contains a variable amount of water vapor, on average around 1% at sea level, and 0.4% over the entire atmosphere." https://en.wikipedia.org/wiki/Atmosphere_of_Earth It is worth reading reference [8], which states:

"Two recent reliable sources cited here have total atmospheric compositions, including trace molecules, that exceed 100%. They are Allen's Astrophysical Quantities[5] (2000, 100.001241343%) and CRC Handbook of Chemistry and Physics[4] (2016-2017, 100.004667%), which cites Allen's Astrophysical Quantities. Both are used as references in this article. Both exceed 100% because their CO2 values were increased to 345 ppmv, without changing their other constituents to compensate. This is made worse by the April 2019 CO2 value, which is 413.32 ppmv.[2] Although minor, the January 2019 value for CH
4 is 1866.1 ppbv (parts per billion).[3] Two older reliable sources have dry atmospheric compositions, including trace molecules, that total less than 100%: U.S. Standard Atmosphere, 1976[6] (99.9997147%); and Astrophysical Quantities[7] (1976, 99.9999357%)"

Interestingly neither the UK Met Office nor the World Meteorological Office, don't mention anything about the main atmospheric gases, only "greenhouse" gases, as such they do not include, nitrogen, oxygen, argon (99%), or water vapour.

My argument from the examples from NASA and Wikipedia above is, Earth most certainly does not have a dry atmosphere, so why are the percentages of gases, particularly water vapour, accounted for as a percentage of a dry atmosphere. Surely this is an oxymoron, logic tells me water vapour would be zero in a dry atmosphere, by definitions of "dry" and "water".

I would like to know what happens to the percentages of all the other atmospheric gases in a real atmosphere when water vapour is introduced. Do these other gases decrease in proportion, or does the atmosphere expand by the percentage of water vapour and the percentages of all gases stay the same, some of both, or another explanation?

To find out how much water vapour is in our atmosphere I conducted a series of investigations. Using satellite images, radiosonde soundings, and nullschool information.

Satellite Image

Fig 2 Water Vapour Channel 6 Meteosat 0900 UTC 16Aug2019

Figure 2 shows there is quite a lot of water vapour in the atmosphere (grey), the question is how much? Clouds are represented by lighter grey and white)

In figure 2, we can see a distinct pattern. Southern and Northern polar regions have high levels of water vapour and cloud. Just north and south of the equator the air appears to be much drier, these two areas are known as the Hadley Cells, large areas of high pressure, where water is squeezed out of the atmosphere. And the area around the Equator shows the Inter-Tropical Convergence Zone (ITCZ) a line of moist warm unstable air which circles the Earth and moves north and south of the equator depending on season. The ITCZ produces heavy rain and thunderstorms.

Radiosonde Soundings

As a meteorologist I have used Radiosonde data on many occasions (upper air data from weather balloons) to find out how moist the prevalent air mass is, its movement, and the stability of that air mass, plus other information.
Radiosonde data: Collected by telemetry instruments connected to weather balloons and launched twice a day 0000Z and 1200Z (full information) and 0600Z and 1800Z (limited information) from about 1,300 locations around the world. The instruments measure, altitude, pressure, temperature, dew point (to compute Relative Humidity), wind speed and direction and geographical position (to calculate wind speed and direction).

Here are two very different examples of sonde information from launches at Camborne UK, and Alice Springs Australia on 18Jun2020 at 0000Z.

Fig 3 Skew-T plot of upper air conditions Camborne, UK 18Jun2020 00Z

Fig 4 Skew-T plot of upper air conditions Alice Springs, Australia 18jun2020 00Z

The furthest left vertical numerical column is pressure in millibars/hPa (blue), alongside that is height in meters (black). Along the bottom the numbers are temperatures in °C (blue).

The helium or hydrogen filled balloons start at the surface and rise. To the left of the chart is wind speed and direction at height.

The two thick black lines rising in the centre of the charts are, temperature on the right, and dew point on the left. The closer the lines are together the moister the air, and the further apart, the drier the air. It all depends on the current air mass.
In the Camborne plot, the air is very moist up to 300 hPa (9150m/~30,000ft), where the temperature and dew point stop decreasing, this indicates the tropopause.

The closeness of the lines indicate there would be thick cloud up to the tropopause. Not going into detail about the stability of the air, but if you look at the far right hand column of data you will see CAPE 48.92, this is an indicator of the stability of the air mass, in this case quite unstable, so the cloud is likely to be cumulonimbus, with a high risk of heavy rain and thunderstorms.

The Alice Springs plot shows the temperature and dew point are widely separated showing a dry air mass, as to be expected in the middle of the Australian desert. Where the lines are closer together at 1000m and 2200m would indicate some scattered stratocumulus (3,300ft) and altocumulus (7,200ft). These two soundings show how moist and how dry the atmosphere can be, from surface to 100hPa.

%Relative Humidity - Surface to 10hPa observations

The next stage was to observe Relative Humidity via nullschool data for each location depicted in figure 1. The locations are as close as possible to the desired location due to the difficulty of precise positioning.

Once the location was fixed, I recorded Relative Humidity readings for Surface, 10000hPa, 850hPa, 700hPa, 500hPa, 250hPa, 70hPa and 10HPa. In the following images each location is marked by a green circle. The images on the right are a sample of one location 45N 00E. All the images for each location can be found in a .pdf found after the summary and conclusions section.

N.B Absolute Humidity is the mass of water vapour divided by the mass of dry air in a volume of air at a given point. Relative Humidity is the ratio of the current absolute humidity to the highest possible absolute humidity, which depends on the current air temperature.

Summary

Most of Earth's weather is between the surface and the tropopause, up to 70hPa. The global percentage of Relative Humidity from surface to 70hPa is 22%.

The air at 10hPa has very little water vapour. Apart from the South Pole which showed RH 7% and three other locations showing just RH 1%, the other locations showed RH 0%.

Generally, %RH decreases with height as would be expected, but it is interesting to note a few exceptions with 100% RH at 250hPa at 45S 90E and 0N 90W, both over oceans and the South Pole at 94%. As 250hPa is approximately 36,000ft, I would suggest these higher readings are caused by cirrus type clouds.

Fig 6 shows the results of average global % RH Surfac3 to 10hPa

Fig 7 shows the results of Global % RH Land/Water Surface to 10hPa

The air over land was noted as being drier than that over the water locations as seen in figure 7.

The lowest average RH between surface and 70hPa (48%) at 45N 90E is on the west side of the Gobi Desert in China, and to be expected. The highest average RH (90%) surface to 70hPa is the South Pole. Considering warm air is supposed to hold more water this result was unexpected.


Conclusions

I suggest the results may have been somewhat different had this exercise been conducted at a different time, given air masses, frontal activity, and hence cloud masses are highly mobile.

The high %RH up to 70hPa at the South Pole can be explained by the following: As the South Pole is on an island covered with ice and snow, winds carrying much warmer moist air from the surrounding seas overlaying a very cold airmass would naturally rise. The greater temperature differential would cause the warmer, but cooling air to rise to higher levels.

The overall results show the global average of 50% RH up to 10hPA and 57% within the tropopause, suggests water vapour, is by far the most significant "greenhouse" gas.

However, some of the higher %RH recordings are very likely caused by clouds at different levels at that point in time. A combination of the sun's input, clouds and Relative Humidity are the controlling mechanism for maintaining a suitable climate for living organisms on Earth. Plus of course CO2 which all life needs to photosynthesise.

Click here to download the PDF