# What is precipitable water vapor in millimeter-wave radioastronomy and how is it measured?

Looking through ALMA on-line documentation and articles, the quantity PWV (precipitable water vapor) is a central theme. I have two linked questions.

1. What (actually) is precipitable water vapor, and is there a "non-precipitable" component also? Why not just call it "water vapor"?
2. How is it actually measured? It seems to be monitored by radiometers, but I don't understand how that can be used.

I usually see numbers of the order of 5 millimeter to 10 millimeters for very high altitude locations like Atacama. I assume this is the column-integrated amount, expressed in units of liquid, is that right?

• This sounds like something the colleagues over at Earth science might have a lot of expertise with... Jan 29, 2018 at 11:47
• @AtmosphericPrisonEscape I'm really interested in just how the term is used by radio astronomers, and how the radiometers actually make the measurement. Water can be in the form of ice, liquid droplets, or vapor, and those would have different properties for example.
– uhoh
Jan 29, 2018 at 15:33

As defined quite clearly, with equations, here

The total atmospheric water vapor contained in a vertical column of unit cross-sectional area extending between any two specified levels, commonly expressed in terms of the height to which that water substance would stand if completely condensed and collected in a vessel of the same unit cross section.

## edit

There's, for example, an ALMA paper which discusses measurements and comparison with meteorological stations.

• Comments are not for extended discussion; this conversation has been moved to chat. Oct 21, 2016 at 15:38

The reason it is called "precipitable" water vapor (PWV) is that the water vapor is in the form of gaseous vapor which may further condense into cloud form and then into actual precipitation i.e. rain (which often overwhelm the sensors). It is often also called total column water vapor which more accurately reflects what is being measured i.e. a height of the equivalent column of liquid water, hence the measurement in mm. The American Meteorological Society glossary entry on precipitable water (or precipitable water vapor) similarly defines it as:

The total atmospheric water vapor contained in a vertical column of unit cross-sectional area extending between any two specified levels, commonly expressed in terms of the height to which that water substance would stand if completely condensed and collected in a vessel of the same unit cross section. The total precipitable water is that contained in a column of unit cross section extending all of the way from the earth's surface to the "top" of the atmosphere.

For very high and dry sites e.g. the Atacama, Antarctica will have less than 5mm PWV, sea level sites will be 50+mm of PWV. ESO's Cerro Paranel observing site (home of the VLT), has median PWV of 2.5mm (see e.g. the histograms in this ASM 2016-2018 report). The Atacama Pathfinder EXperiment (APEX) sub-mm telescope on Chajnantor (also home to the ALMA array) has had a weather station for several years and for example the APEX 2017 weather statistics show that the APEX site (~5100m altitude) had a PWV <1.5mm approx. 67% of the time.

There are several ways to measure the PWV. The Earth orbiting satellites such as the AIRS and MODIS instruments on NASA's Aqua and Terra satellites measure the radiance in several IR wavebands (typically from approx. 0.5 to 15 um). Some of these bands are essentially dominated by water absorption bands so performing differences between bands containing water and without water will give a measure of the water column.

Microwave radiometers at 210 or 225 GHz are traditionally what have been used to measure PWV, with more modern radiometers shifting to the 350um (856 GHz) window/band. An example is the one at the Caltech Submillimeter Observatory on Mauna Kea described in this link at Gemini Observatory. They work similarly by measuring the depth of the absorption by the water vapor in the molecular lines/bands. More water vapor produces deeper lines with more optical depth. More details are in this (freely available) paper by Radford which shows the effect of increasing PWV on the absorption in the sub-mm, the amount of PWV at various sites and more info on the measurement.

Finally this can also be done by dual frequency GPS receivers at the L1 (1575 MHz) and L2 (1224 MHz) and measuring the path length excess through the atmosphere above what is caused by the receiver-satellite distance alone. By using two frequencies, you can remove the effect of the variable ionosphere on the path length. The remaining zenith path distance breaks down into two parts, a hydrostatic or "dry" component which can be easily estimated using e.g. the Saastamoinen model, leaving the "wet" component caused by water vapour. Once this is corrected to the zenith, this gives a measure of the PWV.

• This is a really helpful answer, thank you! Just to double check; "precipitable" water vapor and total column water vapor are measures of liquid droplets and not vapor?
– uhoh
Apr 25, 2019 at 20:45
• You asked why it's called "precitable water vapor" rather than just "water vapor". My understanding is that most of water in the atmosphere is in vapor/small droplet form which if it got it's act together and collected up into bigger droplets, it would actually precipitate out i.e rain or snow. So "precipitable" because it could precipitate in the future, given droplet growth/merging together, but most of the time it doesn't... Apr 25, 2019 at 21:27
• When we talk about the vapor pressure of water, or even just the word vapor itself, it seems to refer strictly to the gas phase of a substance. But certainly there are colloquial uses of the word that apply to mists or other dispersions of droplets. I have indeed asked What (actually) is precipitable water vapor and if you can cite a supporting reference explaining if it includes or excludes droplets or gas phase water that would be great. Right now it's hard to tell if you are sure or just hypothesizing
– uhoh
Apr 25, 2019 at 22:34
• – uhoh
Apr 25, 2019 at 22:37
• Amended answer and added additional clarifications for the definition of PWV and links to additional sites' PWV statistics, including Chajnantor for the sub-mm/mm ALMA/APEX site Apr 26, 2019 at 15:45

You can measure the error or water vapor by looking at an edge of something over a distance. This could be possible by looking at the edge of a planet or the edge of something like a satellite.

The International Space Station travels in orbit around Earth at a speed of roughly 17,150 miles per hour (that's about 5 miles per second!). This means that the Space Station orbits Earth (and sees a sunrise) once every 92 minutes.

Its possible to find the ISS using the FREE SkyView on your phone and find the space station and perhaps study the image.

Its interesting to think about humidity measurements that we see from a weather website and compare these to what you actually see. It maybe that the ideal place for a telescope is just above the tree line? (for people who don't like numbers) However, there are some really nice urban telescopes and hopefully in the future we will have more telescopes in nice cities even if not all the nights are clear.

Its kinda fun to read an Indian paper on the topic... https://journals.ametsoc.org/doi/pdf/10.1175/1520-0450%281990%29029%3C0665%3AAPOTWV%3E2.0.CO%3B2

(the Indian and Japanese papers both seem to show that the water vapor disappears at about 10-12 KM)? However, some of the biggest telescopes in the world are being placed only about ~3000 meters up. Its sometimes nice to place telescopes in urban areas where we can get to it. Especially when the vapor doesn't really disappear until 10 KM which is way too high up anyway?!?!

For example: https://www.eso.org/public/teles-instr/elt/

(also remember that temperature is not a linear function with altitude... it goes down then up then down then up then down... i think... just look at the graph...)

Why water vapor is more "linear" is still one of those cosmic mysteries... I liked the x^2 equation the other person posted.

• I don't think this really addresses, much less answers the question as asked.
– uhoh
Jan 29, 2018 at 9:34
• Sorry, it maybe possible to measure water vapor using a acoustic profile too? Jan 31, 2018 at 10:29
• Many things are possible. I'm asking here about how radio astronomers interpret readings from microwave radiometers, and how the different phases of water (.e.g. ice, droplets, vapor) impact both the radio astronomy, and the radiometer measurements.
– uhoh
Jan 31, 2018 at 10:31
• In terms of altitude and water vapor if we are looking for real-time visibility it maybe possible to use the combination of air pressure and a regular satellite weather map? Radio Astronomy sights like the VLA (Very Large Array) in New Mexico are only about 2100 meters above sea, even ALMA in the Chile Mountains is only about 5000 and next to ALMA is the Extremely Large Telescope (the ELT) at 3000 meters up. If the graphs posted are correct it seems most of the water vapor only dispersals at about 10,000 meters and that almost 6 miles up into the air! Being careful is important. Everest. Feb 4, 2018 at 6:19