Nowadays, the determination of the humidity of air in the atmosphere is done on a global scale by satellite measurements.
But of course the technique of humidity measurements did not start in space.
The first measurements were performed on the ground.
These ordinary techniques are still used today as ground-based measurements.
The question of how much water is present in the atmosphere is as important for short-term weather forecasts as for long-term projections on the future of our climate.
Where the water is to be found in the
atmosphere can also be just as
important. The water vapor concentration at the equator can be 500 times bigger than the water concentrations at the North or the South
Poles. At the same time, the concentration varies strongly at one location, depending on season and daytime.
In addition to the spatial dimensions
of altitude, latitude, and longitude, the water
vapor concentration in the Earth´s atmosphere changes in a very irregular way with time.
Rise of a balloon sonde |
Meteorologists have long measured water
vapor in the air. Every day, standard measurements of temperature, pressure and water
vapor concentration are performed.
Often balloon sondes are used for these measurements.
In this procedure, measurement equipment is attached to a
ballon, which is released into the air and rises up
to 10 km, all the while measuring a so-called vertical profile of water
vapor and other meteorological data above a certain point on the Earth
surface and relaying the
data via radio back to the
surface. The equipment
used nowadays on sondes and
at the surface is a psychrometer, or a
dewpoint meter.
Long ago, the meteorologists used a
"hair-hygrometer".
Degreased hair has the characteristic that its length changes with
humidity, and this was used to determine the water content of
air!
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Most of these instruments have to be calibrated.
This means that for the case of a hair-hygrometer (hygro is Greek for moisture), the length of a hair has to be measured at an exactly known water
vapor concentration in a lab. From this model value, the actual value of the concentration in the atmosphere can be calculated if you measure the length of the same hair somewhere
else in the atmosphere. Nowadays, much more advanced and precise equipment is used.
Even so, scientists are regularly
discovering sources of systematic error in these modern measurement techniques
(one was recently found of up to 10%), due to different conditions during the calibration in the lab and in the real
atmosphere, and thus instruments are
constantly improving. For this reason, it is important to use as many techniques as possible, in our case to measure the humidity of air.
Water vapor measurement of a balloon sonde at the Royal Dutch Meteorological Institute (KNMI) on 25th October 1995.
| Satellite
Measurements (Remote Sensing) |
Measurements of water vapor concentrations by means of equipment on a satellite is one of the most recent alternatives to ground-based measurements.
Since it is like taking measurements from a great distance,
many people call it "remote sensing". Satellites fly over every point of the Earth within the time
frame of a couple of days, and can therefore give
full coverage of the global water vapor
concentration over the earth's
surface. The disadvantage is that a "point" for a satellite is usually an area of several hundreds
of square kilometers. So one can only measure the average humidity of a very large area.
The measurement of a "vertical profile", like in balloon sonde measurements, is very difficult as well, because you look through the entire atmosphere and cannot distinguish between certain altitudes.
The latest satellite techniques try to tackle this problem by not looking straight down on the Earth surface, but in a certain diagonal angle. By changing this angle, the direction of where the satellite is "looking" is changed continually.
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Newest Developments
The satellites that were used until today could either look perpendicular on the Earth, or take measurements diagonally. A new satellite,
though, is in production:
The newest satellites can look at the Earth surface
both sideways through the atmosphere and straight at it.
Where the two looking directions cut each other, the solar intensity in the volume can be determined.
Picture-insert comes from IFE/Bremen
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| Spectrometer measurements from satellites |
The radiation from the sun consists of photons with different energies. Every energy is represented by a certain wavelength of the light.
Depending on the characteristics of a compound in the atmosphere,
such as a molecule of water or carbon dioxide, it is able to absorb light only in certain energies or on certain wavelengths.
In a spectrometer, this characteristic of the compound is used to identify the compound by the means of the absorption spectrum.
If a spectrometer is used from a satellite, it can measure the spectrum of the light that is scattered back from the Earth and it can identify all kinds of compounds from this spectrum.
This method is also used to determine the amount of water
vapor in the atmosphere, because, as is seen in chapter 2, water
vapor only absorbs light in certain areas.
When the spectrometer counts the photons that come from the Earth, and the photons that water can absorb are missing, you can tell that there was water
vapor in the atmosphere at the time.
A simplified picture:
The sun radiates from space (black) into the atmosphere (blue). The "light particles" (the photons, the yellow beam) reach the Earth surface (brown). We now look at the light particles that bounce back from the surface, that are reflected. Particle
no. 1 reflects and reaches the satellite (blue-green), which receives a signal (becomes red). The same happens with photon
no. 2. No. 3, though, encounters a water molecule on the way to the satellite.
This absorbs the energy of the photon, and the satellite
no longer detects the photon. The more
water molecules there are in the air, the
fewer photons will reach the satellite.
If the number of photons with a wavelength
absorbed by water reaching the satellite from the Earth is compared to the number of photons which is emitted by the sun, it is possible to derive the concentration of water molecules in the atmosphere.
This is only the overall concentration in the area that the satellite covers on the moment the spectrum is taken.
Because more than 99% of the water vapor in the Earth atmosphere is in the 5000 meter above the surface,
and the atmosphere itself is over a hundred kilometers thick and satellites are usually at an altitude of about a thousand kilometers, you can imagine that the photon-counter in the spectrometer must be very precise. To achieve this, the track of the satellite must be calculated in advance by a computer.
Click to enlarge!
Global water vapor measurements in July with the satellites SSM/I, TOVS und TRIOS
Source: David L. Randel et.al., BAMS - June, 1996 Vol 77, No 6
| Meaning of the water
vapor measurement |
In the chapters "Water vapor - A greenhouse gas" and "Absorption", we already saw that the water molecule absorbs photons in the far ends of the energy spectrum, especially in the infrared,
which is the wavelength emitted back from the Earth.
The greater the number of the warmth-absorbing water molecules, the more heating of the atmosphere
occurs. This is called the greenhouse effect.
Carbon dioxide, produced when fossil fuels like oil or
coal are burned, and methane, primarily emitted by rotting plant
material and animals, are both molecules that absorb in the areas in the infrared spectrum where water does not.
The increasing concentrations of carbon dioxide and methane cause an additional warming, which causes even more water
vapor to get in the atmosphere. The water concentration is
a thousand times higher than the methane concentration, which makes water by far the most important greenhouse gas.
When it is possible to precisely determine the water
vapor concentration on every spot in the atmosphere, it will be possible to quantify the human contribution in the form of CO2 and methane to the greenhouse effect.
One would expect a constant global water vapor
concentration if no human influence was to be detected.
Only if the water vapor concentration could be
determined in a very precise way on every point in the atmosphere, will
it be possible to quantify the human contribution to the greenhouse effect.
The precision of the determination of water
vapor concentrations by means of satellite measurements at the time is about 30 to 40%, sometimes even worse. This is mainly due to the complex spectrum that water has in relation to other gases in the atmosphere. In addition, most of the water is near the Earth surface, which is very far from the satellite and therefore hard to detect. The determination of water
vapor concentrations in a way which is more reliable, say within 5% precision, is still very far away.
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Summary:
It is very hard to quantify water
vapor in the atmosphere.
Its concentration changes continually with time, location and altitude.
To measure it at the same location every day, you would need a hygrometer, which in earlier days made use of the moisture-sensitivity of a hair, and by now of for instance condensators.
A vertical profile is obtained with a weather balloon.
To get a global overview, only satellite measurements are suitable.
From a satellite, the absorption of the reflecting sunlight due to water
vapor molecules is measured.
The results are pictures of global water
vapor distributions and their changes.
The measurement error, however, is still about 30 to 40%.
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text: Ruediger Lang
FOM-Institute for Atomic and Molecular Physics
Atmospheric Photophysics Group
Amsterdam
Lang@amolf.nl
translation: Heleen de Coninck - MPI Mainz 16/10/01
Edited by Stephen Gawtry - University of
Virginia
