FOR IMMEDIATE RELEASE:                  NIST 94-40
Nov. 14, 1994

Contact:  Linda Joy (NIST)              COMPARISON OF UV
          (301) 975-4403                MEASUREMENTS WILL
                                        IMPROVE FORECASTS
          Dane Konop (NOAA)
          (301) 713-2483

     A recently completed Commerce Department field test will
help environmental scientists worldwide more accurately measure
ground-level changes in damaging ultraviolet radiation from the
sun.

     Such measurements are crucial in assessing effects of ozone
depletion in the upper atmosphere on human health, agriculture,
fisheries and materials such as concrete, plastics and paints.
As ozone levels in the upper atmosphere fall, scientists project
that increased levels of harmful ultraviolet radiation, or UV,
will reach ground level.

     In the Sept. 22 to Sept. 28, 1994, field test on Table
Mountain Mesa, 10 miles north of Boulder, Colo., U.S. and
Canadian scientists compared readings from seven
spectroradiometers, instruments that measure UV radiation.

     The intercomparison was led by Ambler Thompson of the
Radiometric Physics Division of the National Institute of
Standards and Technology.  The National UV Calibration Facility
of the National Oceanic and Atmospheric Administration's Air
Resources Laboratory hosted the intercomparison.

     In the week-long test, scientists from NIST, NOAA, the
National Science Foundation, the U.S. Department of Agriculture,
Colorado State University, the University of Georgia, the
Smithsonian Institution, the State University of New York at
Albany, the Environmental Protection Agency and the Atmospheric
Environmental Service of Canada compared simultaneous UV
measurements.  Data from the intercomparison will provide a
reference base for UV monitoring networks established by the EPA,
the USDA and NOAA.

     While most UV radiation from the sun is absorbed by the
protective ozone layer in the upper atmosphere, some UV does
reach the Earth's surface.  Scientists and the public are
concerned that more UV radiation will reach the Earth's surface
due to human-induced depletion of the Earth's stratospheric ozone
shield, particularly affecting the middle latitudes where most
people live.

     It is well known that there is a direct relationship between
decreased stratospheric ozone and increased UV at the surface,
but presently there are no reliable long-term measurements to
ascertain trends in UV at the surface.  UV monitoring networks
are being established in the United States and worldwide;
however, it will take several years or more to develop a record
from which meaningful trends can be extracted.  This is due to
variability in stratospheric ozone as well as clouds and air
pollution, which also affect the transmission of UV. 
Furthermore, UV trends are not likely to be large, so it is
extremely important that the measurements be done with high
accuracy.

     Thompson's goal is to help the various UV monitoring
agencies achieve high levels of accuracy in UV measurements so
that readings from different networks in different locales will
be reliable in calculating global trends.  He would like the
networks to achieve accuracies of 90 to 95 percent and says the
recent intercomparison of instruments in Boulder illustrated that
some agencies are making very accurate measurements and others
need to improve. 

     Continuous calibration will be needed to ensure the long-
term data quality.  To do this, routine intercomparison of
spectral instruments, such as the event that just happened in
Boulder, will be an integral part of each network's operations.

     The spectroradiometers that were tested separate the sun's
rays into distinct wavebands, much as a prism does.  Most of the
sun's radiation is visible light that may be separated into the
colors of the rainbow.  Only a small part of the solar spectrum
is in the ultraviolet, which comprises higher energy wavelengths
that are harmful to living things.  These harmful high-energy
wavelengths cannot be seen by the human eye but can be detected
by sensors in these special instruments.

     Clouds generally decrease the amount of ultraviolet
radiation reaching the Earth's surface.  NOAA's experimental
ultraviolet forecasts account for likely cloud cover and observed
and predicted changes in stratospheric ozone for over 50 U.S.
cities. 

     NOAA, an agency of the U.S. Commerce Department, describes
and predicts changes in the Earth's environment and promotes
global environmental stewardship.

     As a non-regulatory agency of the Commerce Department's
Technology Administration, NIST promotes U.S. economic growth
by working with industry to develop and apply technology,
measurements and standards.

                                   -30-

Note:  This announcement is being released jointly by NOAA and
NIST.