Water vapor in the atmosphere varies considerably in time and from place to place. These variations are related to both weather and climate. Clouds are formed from water vapor. Water vapor is the primary greenhouse gas that helps control temperatures in the lower atmosphere. The interactions of water vapor with other constituents of the atmosphere are complex and global in scope.
Using the Relative Humidity Protocols, you measure the amount of water near Earth's surface, but how much water vapor is in the whole column of air above you? Using this protocol enables you to answer this question. It also will help scientists answer these questions:
Despite its importance, the global distribution and temporal variability of water vapor is not well known. As with other global measurements, scientists use satellite-based observing systems to study atmospheric water vapor. A primary motivation for conducting this protocol is to provide measurements to help support the GIFTS (Geosynchronous Imaging Fourier Transform Spectrometer) instrument, part of NASA's New Millenium Program IOMI (Indian Ocean METOC Imager) spacecraft. GIFTS will observe weather patterns, atmospheric temperature, water vapor content and distribution, and the concentration of certain other atmospheric gases. From its geostationary orbit high above the earth, GIFTS will provide unprecedented detail about the spatial and temporal variability of these quantities. [For an introduction to GIFTS, see http://oea.larc.nasa.gov/PAIS/GIFTS.html.]
As helpful as satellite-based measurements are to an improved understanding of the global distribution of water vapor, ground-based measurements are still needed. For example, when GIFTS views the Earth/atmosphere system from space, its spatial resolution (one pixel) is about 4 km x 4 km. At this level of resolution scientists can track storm systems, since large systems have dimensions on the order of hundreds or thousands of kilometers. However, smaller scale phenomena, such as individual cumulus clouds, cannot be resolved. Ground-based measurements provide a way to study such small scale phenomena, complementing the satellite observations. Ground-based observations also help scientists by making possible comparisons of atmospheric properties calculated independently from satellite and ground-based data.
By reporting water vapor measurements regularly, you provide scientists with some of the data they need to better understand the global distribution of water vapor, and you learn about the atmospheric water vapor that is over your own observing site. While all water vapor data are beneficial, data that can be compared directly with satellite-based measurements are especially valuable. In some cases, ground-based measurements should be timed to coincide with the passage of Earth-observing satellites over your site. This is true for spacecraft in NASA's Earth Observing System (EOS) program, for example, as they are in near-polar sunsynchronous orbits and pass over or near virtually all sites on Earth's surface every day at specific and predictable times.
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Instruments such as GIFTS are in geostationary orbits around the equator. The altitude of these circular orbits (nearly 36,000 km above Earth's surface) is chosen so that their orbital periods are equal to one day. If a satellite orbits in the equatorial plane, it maintains a fixed position over the same place on Earth's equator (hence the "geostationary" designation). Figure AT-WV-01 shows a geostationary orbit. The diameter of the orbit is roughly to scale with the diameter of Earth.
A vantage point above Earth's equator allows spacecraft instruments to
take virtually continuous measurements of a specific portion of Earth's surface and atmosphere.
Some measurements require the observed region to be in sunlight, but other measurements can be made at any time.
If there is a geostationary satellite observing your region, it will almost always be useful to take
ground-based measurements at any time during the day.
Because of the seasonal variability of water vapor, it is important to build a water vapor data record that
extends across several seasons. Long-term records are more valuable for scientists, and they will give you a
better understanding of your own local environment
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Imagine a column of atmosphere above an observing site (see Figure AT-WV-02). This column will contain all the atmospheric constituents, including water vapor. Now imagine collecting all the water vapor in the column, transforming it into liquid form, and bringing it down to the ground. The thickness of the layer of water is typically a few centimeters, and is known as precipitable water (PW). The unit for expressing PW is cm (of water), just the same as rain. Actually, precipitation is reported to GLOBE in units of mm, not cm.
One way of measuring water vapor is to examine how it affects the transmission of sunlight through the atmosphere. Water vapor (molecules of H2O in their gas phase) absorbs sunlight in specific wavelength bands, including two bands in the near-infrared part of the solar spectrum. This absorption reduces the amount of sunlight reaching Earth's surface at those wavelengths.
Figure AT-WV-03 shows three sets of data. One is the distribution of solar energy as a function of wavelength just outside Earth's atmosphere. The second is the distribution of solar energy on Earth's surface assuming an atmosphere with no water vapor. The third is the distribution of solar energy with a "standard atmosphere" containing an average amount of PW. As the amount of PW increases, the amount of solar energy reaching Earth's surface at these wavelengths decreases.
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Now suppose that two detectors respond to sunlight at different wavelengths — one at a wavelength within a water vapor absorption band (at about 940 nm) and one just outside this band (at about 870 nm). [NOTE: The wavelength values in the printed protocol are wrong!] Assuming the position of the sun relative to the observer doesn't change, the amount of light seen by the detector for the wavelength outside the band will not change if the amount of atmospheric water vapor changes. However, the detector for the wavelength inside the band will respond to changes in the amount of water vapor. Hence, the ratio of the response of two such detectors will change with the amount of water vapor, and can be used as a measure of the water vapor amount.
PW is related to other properties of the atmosphere, including those described in other GLOBE Atmosphere Protocols. It varies hourly, daily, seasonally, and geographically. Hence, it is helpful to consider water vapor as part of a broader discussion of the atmosphere and its properties. Ideally, water vapor measurements should be taken over an extended period of time to observe seasonal effects. The measurements will make more sense if they are combined with other GLOBE atmosphere observations, including the basic meteorological protocols and aerosols. In fact, some of these other protocols can be used to provide the metadata that must be reported along with the water vapor instrument data.
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Measurements taken with the GLOBE/GIFTS water vapor instrument are in units of volts. These values must be converted into PW using calibration data that have been determined for each instrument. The calibrations require access to specialized equipment and data that cannot be duplicated by students in the lab or in the field. The PW calculations are done by the GLOBE Data Server when data are reported and the calculated values are returned for students to use.
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1
m = ———
sin(θ)
To compensate for the fact that your instrument is measuring the water vapor through a longer portion of the atmosphere along the slant path, the water vapor detected by your instrument (the slant path water vapor) is divided by the relative air mass to estimate the amount of water vapor in the vertical column of atmosphere directly over your head:
slant path PW
PW = ——————
m
The basic meteorological conditions for using the GLOBE/GIFTS water vapor instrument are the same as for the GLOBE sun photometer: You must have an unobstructed view of the sun that is not blocked by clouds. Also, you should have an overall view of the sky that allows you to make reasonable cloud type and cover, sky color, and haziness observations. If your view of the sky is severely restricted (as it might be at urban sites, for example), you will need to note the restrictions in your study site definition.
The decision about when you should take water vapor measurements depends on whether you wish to associate your measurements with a particular satellite instrument and, if so, the orbit of that satellite. For most orbits, including the near-polar sunsynchronous orbits of many Earth-observing satellites, measurements need to be timed to coincide with overflights of your site. NASA's current major Earth-observing sunsynchronous satellites fly over in mid morning or early afternoon. The precise times at which they fly over your observing site are readily available online. For instruments in geostationary orbits (such as GIFTS), or if you are not associating your measurements with specific satellite measurements, you can take measurements any time during the day. For developing a long-term record of water vapor over your observing site, it is helpful to take measurements at about the same time every day.
With a little care, this instrument will work reliably for many years. If it appears not to be working correctly, consult with GLOBE before doing anything else.
Every three months or so, or right away if you accidentally leave your instrument turned on for an extended period of time, check the charge on the battery and replace it if necessary. See the Checking and Changing Your GLOBE Sun Photometer Battery Lab Guide (in the Aerosols Protocol) for instructions. Replacing the battery will not change the calibration of your instrument and measurements made with the old battery will be OK as long as you replace it before its voltage falls below 7.5 V.
If you have access in the classroom to an electronic device that is controlled by a remote IR controller, it may be helpful to experiment with this device. How do we know IR light (radiation) is really there? Does it appear to behave like "light" even though we can't see it? What will block the IR signal from the controller? What will allow its passage?
You should spend some time in your classroom familiarizing your students with the water vapor instrument, including reading the digital voltmeter on the top of the case. In the classroom, the voltages displayed on the voltmeter will be small — only a few millivolts. If you can point the instrument at the sun, even through a closed window, you will get much higher values.
A Classroom Preparation Guide is provided to help you prepare for implementing this protocol. It describes in detail the steps involved in recording a complete set of measurements, along with some discussion for each step. It parallels the Field Guide that simply lists the steps in order with no further explanation. As part of their preparation for this protocol, students and teachers should study the Classroom Preparation Guide to make sure they understand each step.
What kinds of weather conditions and climates are associated with high (low) PW?
To what extent is water vapor related to other atmosphere variables such as aerosol optical thickness, temperature, cloud type and cover, precipitation, relative humidity, dewpoint temperature, barometric pressure, or ozone concentration?
Can observations of PW improve your weather forecasts?
The site description needs to be done only once unless, of course, you change the location of the site or add an additional site. Interpretation of your measurements requires knowledge of the longitude, latitude, and elevation of your observing site.
The basic condition for taking water vapor measurements is that you must have an unobstructed view of the sun and a view of the sky that allows you to make reasonable cloud cover and type estimates. These measurements can be done in an urban setting.
The note in the printed version of this protocol describing how and where to report barometric pressure values is inconsistent with the Water Vapor Data Sheet. On that sheet, you must indicate whether barometric pressure is station or sea level pressure, and you must specify the source of the value.
In many parts of the world, accurate barometric pressure values are readily available online (option #1), and are therefore preferable.
Many U.S. newspapers publish a daily weather almanac that gives weather information for the previous day, including barometric pressure. Use the value closest to the time of your data collection. For example, if the barometric pressure is given at noon, and this would be the value to use for most water vapor measurements. Depending on whether pressure is rising, steady, or falling, it is reasonable to interpolate between noontime and early morning or late afternoon values (6:00 am and 6:00 pm local time are often given in addition to 12:00 noon).
The following description of how to convert sea level pressure to station pressure is no longer
required. There is no longer any need to convert sea level pressure to station pressure. GLOBE will do this conversion
based on which boxes you check on the Water Vapor Data Sheet.
Regardless of the source of your barometric pressure, it will almost certainly need to be corrected to the elevation of your observing site. This is because analysis of your data depends on knowing the actual pressure, called station pressure. The internationally accepted meteorological standard for reporting pressure is to give the equivalent sea level value. (Otherwise it would be impossible to draw weather maps.) Occasionally, you may find station pressure reported by some government or other research center web sites. Barometric pressure at every observing site with an elevation of more than a few meters must be corrected for elevation. The correction adopted by GLOBE is:
station pressure(mbar) = sea level pressure(mbar) - elevation (meters)/9.2 (meter/mbar)
That is, the pressure decreases by 1 millibar for each 9.2 meters of elevation above sea level.
In the U.S., the pressure may need to be converted from inches of mercury to millibars (hectopascals), which is the international and GLOBE standard:
pressure (mbar or hectopascals) = pressure (inches of Hg) • 33.864 (mbar/inch of Hg)
It is sufficient to report station pressure to the nearest millibar.
For the most accurate measurements, it is important to maintain the air inside the case at approximately room temperature -- in the low 20's. There are some simple steps you can take to minimize temperature sensitivity problems. Keep your water vapor instrument inside and bring it outside only when you are ready to take measurements. In the winter, transport it to the observing site under your coat or in an insulated bag. In the summer, transport it in a small picnic cooler. You can construct an insulating shell for your instrument from rigid foam plastic sheets (Styrofoam) held together with aluminum tape. Especially in the summer, keep your instrument shielded from direct sunlight whenever you are not actually taking a measurement.
Time should be reported to an accuracy of no less than the nearest 30 seconds. A digital watch or clock that displays seconds is easier to use than an analog one, but in either case you must set your timepiece against a reliable standard. Even an analog wristwatch can be read to the nearest 15 seconds if it has one-minute marks on its dial. The time accuracy requirements for this and the related Aerosols Protocol are stricter than for most other GLOBE protocols.
It is not difficult to set your clock or watch accurately enough to meet the standards required for this protocol. You can get time online or from a handheld GPS receiver. In many parts of the world, you can buy a clock that sets itself automatically by detecting a radio signal from an institution that maintains a reference clock.
It may be tempting to use the clock maintained by your computer as a standard. However, this is not a good idea, as computer clocks are often inaccurate, and they should be reset periodically according to a reliable standard. Note that modern computer operating systems will automatically switch your computer clock back and forth between standard and daylight savings time. You should be aware of when this change occurs if you need to manually convert time from your local clock time to UT.
Water vapor measurements can be taken any time during the day. Indeed, it is an interesting project to study the variation of water vapor during the day. However, the water vapor instrument will give the most reliable readings when you take measurements between mid-morning and mid-afternoon. In temperate and higher latitudes, with low maximum solar elevation angles, you should take measurements near solar noon if possible, especially in the winter.
If you are taking measurements that correspond to satellite overflights, then the times of those overflights determine when measurements should be taken. How closely must your measurements match the time of the overflight to be useful? This is a question that should be discussed with scientists working with the space-based instruments. In general, the times should match within just a few minutes. However, it is always better to collect data than not, even if you cannot time the measurements precisely with satellite overflights.
Another difficult situation occurs in typical summer weather, especially near large urban areas. In this environment, polluted skies and humid conditions may make it difficult to distinguish cloud boundaries. It is important to describe such conditions whenever you report measurements. Observing the sky (away from the sun!) through orange or red sunglasses or a plastic filter will make cloud boundaries easier to see.
Whenever you try to determine cloud conditions in the vicinity of the sun, you must block the sun itself with a book, a sheet of paper, a building or tree, or some other object. A sensible rule is that if you can see even faint shadows on the ground, you should not try to look directly at the sun. If in doubt, or if you believe you cannot determine sky conditions near the sun, then do not take a measurement.
Safety Reminder: Never look directly at the sun, even through colored sunglasses or plastic filters. This can seriously damage your eyes!
Cloud condition reports should follow the Cloud Protocols. The categories given on the Water Vapor Data Sheet are described in these protocols.
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You can determine sky clarity by using a distant object — a tall building or mountain range, for example — as a reference. When this object appears sharply defined in its natural colors, then the sky is clear. As the object becomes less distinct, then there are probably more water vapor and aerosols in the atmosphere. However, please note that this method of determining haziness is more directly related to horizontal visibility, which may not always be an accurate indicator of the condition of the atmosphere above your site.
When there are obvious reasons for unusual sky conditions, the users of your data need to know about them. Urban pollution, dust, and smoke are examples of conditions that need to be reported in the Comments section of the Water Vapor Data Sheet.
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You should be familiar with the parts of the GLOBE/GIFTS Water Vapor Instrument, as shown in Figure AT-WV-07. Make sure you have all required materials and, if you are working as a team, that each team member understands her or his role. This is especially important if several different students participate in these measurements on a rotating basis. The comment in the printed version about using a computer interface does not apply to this version of the protocol.
Practice runs can be made inside by pointing your instrument at the sun through a window — even a closed window. (Actual measurements should not be made through a closed window!) The water vapor instrument should be at room temperature — 20-25°C — before collecting data. Place the instrument in an insulated container before you take it outside.
1. The GLOBE/GIFTS water vapor instrument uses light-emitting diodes (LEDs) as sunlight detectors. What is an LED?
A light-emitting diode is a semiconductor device that emits light when an electrical current flows through it. The actual device is a tiny chip only a fraction of a millimeter in diameter. The chip will be housed in either a small metal case with a flat glass cover about 5 mm in diameter, or an epoxy cylinder about 5 mm in diameter.
The physical process that causes LEDs to emit light also works the other way
around. When light shines on an LED, it produces a very small current.
The electronics in your water vapor instrument amplify this current and convert it to a voltage.
LEDs are found in a wide range of electronic instruments and consumer products. The most familiar LEDs emit visible light -- red, yellow, green, or blue. The LEDs in your water vapor instrument emit (and respond to) infrared light. This radiation is invisible to the human eye. LED transmitters and detectors are commonly used in the familiar handheld remote control devices often included with consumer electronics devices such as TVs and audio equipment.
2. What does the GLOBE/GIFTS water vapor instrument measure?
As noted in Question 1, sunlight striking the detectors in your instrument causes a very small current to flow. Each detector responds to sunlight over a different narrow band of infrared wavelengths. When the current is amplified it produces a voltage that is proportional to the amount of light striking the detector within that wavelength band. Water vapor absorbs sunlight traveling through the atmosphere in one of the wavelength bands, but not the other. Your instrument is calibrated so that the amount of water vapor in the atmosphere can be related to the ratio of voltages from the two channels.
3. What is the field of view of the GLOBE/GIFTS water vapor instrument and why is it important?
The water vapor instrument is a sun photometer. The equation that describes
theoretically how to interpret sun photometer measurements requires that the instrument should see only direct light
from the sun — that is, light that follows a straight-line path from the sun to the light detector. This
requirement can be met only approximately because all sun photometers see some scattered light from the sky around the sun.
The cone of light that a sun photometer's detector sees is called its
field of view, and it is desirable to have this cone as narrow as possible. The GLOBE/GIFTS water vapor instrument's
field of view is about 2.5 degrees, which is a reasonable compromise between desires for accuracy and practical
considerations that arise in building a handheld instrument. The basic tradeoff is that the smaller the field of view,
the harder the instrument is to point accurately at the sun. Very expensive sun photometers, with motors and electronics
to align the detector with the sun, can have fields of view of 1 degree or less. However, studies have shown that the
error introduced by somewhat larger fields of view is negligible for the conditions under which the GLOBE/GIFTS water
vapor instrument should be used.
4. How important is it to keep the water vapor instrument from getting hot or cold while I'm taking
measurements?
The LED detectors in your instrument are temperature-sensitive,
so their output is slightly influenced by their temperature. Therefore, it is important to protect your
instrument from getting too hot or too cold. Keep it inside, at room temperature, when you are not actually
collecting data. Never leave your instrument outside or in direct sunlight for extended periods of time.
When you are collecting data, the important temperature is not the outside air temperature, but the air
temperature inside the case. You can monitor the case temperature by selecting the "T" channel on your instrument.
(Multiply the voltage reading by 100 to get the temperature in degrees C.) This temperature should be in the
low 20's. If the temperature is in this range when you start taking measurements, and if you work as quickly as
possible, the temperature inside the case should not change by more than a degree or two and you can minimize
undesirable temperature effects.
5. I dropped my water vapor instrument. What should I do now?
Fortunately, the components inside your water vapor instrument are very rugged, so they should survive being dropped. If you have made an insulated housing for your instrument, then it will be very well protected. However, you should still check the case for cracks. Even if the case is cracked, it may still be OK. Just tape over the cracks using something opaque, such as duct tape or aluminum tape. Open the case and make sure that everything looks OK. In particular, make sure that the battery is still firmly attached to its connector. If the alignment brackets have moved or are loose as a result of the fall, your instrument should be returned to the Science Team for recalibration.
6. How do I know if my water vapor instrument is working properly?
When you turn your water vapor instrument on without pointing it
at the sun, you should measure a small DC voltage no larger than a few millivolts. When you point your instrument
directly at the sun, the voltage should increase to a value in the range of about 0.5 to 2 V. If you do not observe
such voltage changes when you point your instrument at the sun, then it is not working.
The most likely reason for a water vapor instrument to stop working is that
the battery is too weak to power the electronics. As indicated in the procedure for changing the battery (see the
Aerosols Protocol), you should replace the battery if its voltage (with your instrument turned on) is less than 7.5 V. You should check the battery three or four times per year unless you know your instrument has inadvertently been left on for an extended period of time.
Changing the battery will not affect the calibration of your instrument. If you replace the battery and your instrument still appears not to work, contact GLOBE for help.
7. Can I make my own water vapor instrument?
Yes. You can purchase a basic GLOBE/GIFTS water vapor instrument kit. Constructing this device involves soldering some electronic components, which is a skill students need to learn from someone who has done it before. You can start taking measurements as soon as you have assembled your instrument. However, at some point, you must send your water vapor instrument to the GLOBE Science Team for calibration before your data can be accepted into the GLOBE Data Archive.
8. How accurate are measurements taken with the GLOBE water vapor instrument?
This is a difficult question whose answer is the subject of ongoing research.
Unlike some other GLOBE measurements, there is no accepted reference standard against which these measurements can
be compared. All measurements of total atmospheric water vapor content are subject to errors and uncertainties.
Calibration of the GLOBE/GIFTS water vapor instrument depends on measurements made with other techniques.
Therefore, its accuracy depends on the accuracy of these other techniques. Other sun-photometer based measurements
of water vapor do not claim accuracies better than 10%. Although this seems like a large error,
it is sufficient to be useful for improved understanding of the distribution and transport of water vapor.
9. How is total precipitable water vapor related to atmospheric properties measurable at the ground?
Practically by definition, it is not possible to infer precipitable
water (PW) directly and accurately from other measurements made on the ground. If that were possible, we wouldn't
need a water vapor instrument! However, atmospheric scientists have long understood that there is an approximate
relationship between PW and the surface dewpoint temperature -- the air temperature at which relative humidity would be 100%. About 40 years ago, C. H. Reitan [Surface Dew Point
and Water Vapor Aloft, J. Applied Meteorology 2, 776-779, 1963] derived an empirical relationship:
ln(PW) = 0.1102 + 0.0614Td
where ln(PW) is the natural logarithm of the precipitable water in centimeters and Td is the dewpoint temperature in degrees Celsius. This equation represents a statistical "best fit" relationship between dewpoint temperature and PW. It does not imply that dewpoint temperature can be used to predict PW to an accuracy of four significant figures! Because the physical relationship between PW and dewpoint temperature is only approximate, it cannot substitute for an actual measurment of PW. Testing this relationship is a good research project for advanced secondary school students.
10. Can my GLOBE/GIFTS water vapor instrument be used to measure aerosol optical thickness at infrared wavelengths?
This question might occur to you if you are also doing the GLOBE Aerosols Protocol.
The GLOBE/GIFTS water vapor instrument is nothing more than a sun photometer that has been calibrated in a
particular way to determine atmospheric water vapor. However, it can also be calibrated as a sun photometer
that can be used to determine aerosol optical thickness at two near-IR wavelengths. You can continue to use the
same instrument to meausure water vapor, too. Typically, you will not be able to do this calibration yourself.
If you are interested in this project, which is well worth doing, please contact the Science Team.
Typically, precipitable water (PW) or column water vapor varies between a few tenths of a centimeter and several centimeters. At high elevation sites in arid climates, PW can approach 0. PW is only rarely above 6 cm. Much larger values may indicate that cirrus clouds were in front of the sun during the measurement. If a particular instrument regularly produces PW values outside the normal range, it indicates that something is wrong with the instrument (e.g., the battery needs to be changed or the instrument needs recalibration). Negative values of PW are physically impossible and indicate serious problems with the instrument or with the observer's understanding of how to collect data.
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It is clear from Figure AT-WV-08 that PW values are higher in the summer than in the winter. PW measurements made by students in temperate climates should exhibit this seasonal cycle. Note that major volcano eruptions, such as Mt. Pinatubo, and El Nino events can influence the seasonal PW cycle. Measurements made in other climates, such as tropical regions that have wet and dry seasons, should have PW cycles that are related to these seasons. PW values at high-elevation observing sites will be smaller than those for sites nearer to sea level. (Unlike barometric pressure, for example, and like aerosol optical thickness, PW values are not "normalized" to sea level; they represent the actual amount of water vapor in the atmosphere above the observing site.)
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Although the relationship between PW and dewpoint is interesting (see also item 9 in FAQs), it is clear from Figure AT-WV-09 that you cannot use dewpoint temperature as a replacement for actual measurements of atmospheric water vapor. (Otherwise, there would be no reason for this protocol!) The relationship between dewpoint and water vapor breaks down when the weather is changing rapidly — when a cold front is passing, for example. A primary calibration will associate the instrument response with an independent measurement of PW. Instantaneous direct measurements of the total water vapor content of the atmosphere at a particular place and time are basically not feasible and even indirect measurements require many assumptions. Hence there is significant uncertainty even in "professional" measurements of PW.
Click here to get the Water Vapor Data Sheet. (It's in a separate file so you can print just the data sheet without printing the entire protocol.)