For more than a century, meteorologists have measured the near-surface atmospheric humidity using various instruments including hair hygrometers, sling psychrometers, and dewpoint hygrometers. However, relative humidity and dewpoint readings simply reflect the humidity within the lowest several meters of Earth's surface.
Observations of the atmospheric humidity, along with air temperature and air pressure, are difficult to measure in the free atmosphere, which extends upward to tens of kilometers above Earth's surface. Meteorologists need to sample the atmosphere at various levels and construct a sounding or plot of the vertical variations of observed weather elements above a station.
During the 19th century some in situ measurements of atmospheric properties were made by instruments carried on manned balloon ascents. An in situ measurement is one where the instrument is immersed in the environment that is being measured. At the end of that century, the predecessor to the National Weather Service began conducting upper air kite observations. Recording instruments were attached to box kites, but could only be read once the kite was reeled back to Earth. Kite observations, along with subsequent airplane observations made in the early 1930s were too limited in terms of altitudes attained, too time consuming, and too costly.
Since about 1936, routine upper air observations have been made by relatively small, cheap and expendable instruments carried aloft by inflated balloons to altitudes of approximately 30,000 m (100,000 ft). An on-board FM radio transmitter within this instrument package transmits the collected temperature, humidity, and air pressure readings back to Earth for processing. The entire instrument package is called a radiosonde, a term that apparently was derived from a combination of the words "radio" for the onboard radio transmitter and "sonde," which is an old English word for messenger. Wind speed and direction are determined at various atmospheric levels during the ascent of the radiosonde by using a ground-based radio direction-finding antenna that tracks the motion of the balloon. Together the radiosonde data and wind information form what is termed a rawinsonde observation.
Currently, about 70 radiosonde stations are distributed across the continental United States. Radiosondes are launched from these stations twice daily, just prior to 0000 UTC and 1200 UTC. The upper-air data collected from each of these stations are transmitted worldwide over standard communications networks to a centralized location for use in a variety of upper air charts and numerical weather prediction models.
One such upper air chart that appears on the WES Homepage is the 500 mb chart. Such a chart is a nationwide display of the upper atmospheric conditions at a level where the radiosondes report a pressure of 500 millibars (mb). These charts portray conditions at an average altitude of roughly 5500 m (18,000 ft) above sea level--slightly higher where the air column is relatively warm and slightly lower in a relatively cold air column. While some of the specifics of the upper air station model differ from those of the surface station model, temperature is plotted in degrees Celsius at roughly the 10 o'clock position and the dewpoint is plotted in degrees Celsius at roughly the 8 o'clock position. A specimen upper air station model appears in the highlighted Map Symbol Explanation entry on the WES Homepage.
These 500 mb charts can be used to check the atmospheric humidity at an altitude of approximately 5500 m. Typically, when the 500-mb dewpoints are within 5 Celsius degrees of the reported 500 mb air temperatures we can assume relatively humid conditions at that level, with a possible cloud deck.
Precipitable water is a measure of the total water vapor content in an atmospheric column extending upward from Earth's surface. Specifically, the precipitable water is the depth of liquid water that would be collected if all the water vapor in a column of unit area, say 1 square centimeter, were condensed. Computer programs calculate the precipitable water from the dewpoint data collected from each radiosonde ascent. A current Precipitable Water chart is available on the WES Homepage. The precipitable water data obtained by individual radiosondes are plotted in hundredths of a millimeter at the location of the reporting station. For example, a value of 220 plotted on the map indicates 2.20 mm of precipitable water in the atmospheric column above that station.
Unlike the in-situ humidity observations made at surface weather observation sites and onboard radiosondes, environmental satellites are currently using remote sensing techniques to measure atmospheric humidity. Two techniques are used, that is, a scanning mode and a sounder mode.
We are familiar with the satellite images that appear on television or on the WES Homepage. These images are produced when geosynchronous satellites are in what is known as a scanning mode. The instruments onboard the satellite that are sensitive to the infrared and water vapor channels collect the long-wave radiation emanating from various components of the Earth's system, including clouds, the atmosphere, or Earth's surface. (The instruments sensing the water vapor channel are tuned to a slightly different portion of the electromagnetic spectrum than those that produce the traditional infrared imagery.) These sensors sweep across the field of view, in a scanning technique. The data are transmitted to Earth receiving stations where satellite images are produced. One of the satellite imagery products that we can use to estimate the atmospheric humidity above Earth's surface is the current Water Vapor (WV) Satellite image on the WES Homepage. As described previously, water vapor sensors detect atmospheric water vapor and cloud particles (both droplets and ice crystals) at altitudes on the order of 4000 to 6000 m, which is near the level depicted by the 500 mb chart. Areas appearing white on the water vapor image are the most humid and are often indicative of rising motion. On the other hand, dark regions indicate relatively dry conditions and sinking motions.
The other technique used to determine atmospheric water vapor concentration remotely using a satellite-based instrument operates in the sounder mode. In this technique, a special sensor samples the infrared radiation reaching the satellite from a small region in the sensor's field of view. The sensor looks at emitted radiation over small wavelength intervals. The intensity of the long-wave radiation emitted by water vapor in certain regions of the spectrum depends to an extent upon the pressure, which ultimately identifies the level in the atmosphere. By using this technique, the sensor operates as a sounder, retrieving temperature or humidity information from various levels in the atmosphere. By sampling at many locations, the sounder onboard the satellite can produce a display of the precipitable water in the atmosphere as derived from the satellite.
For more information on the operation of satellite soundings, go to: http://cimss.ssec.wisc.edu/sounder/desc.html. Also, the latest precipitable water image from GOES is at: http://orbit-net.nesdis.noaa.gov/goes/soundings/html/tpwus.html.
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URL: WES/supl.html
Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 2001, The American Meteorological Society.
