TOPEX/Poseidon surveys sea-level heights by measuring the
time required for pulses generated by the onboard radar
altimeters to bounce back to the satellite from the sea
surface. Water vapor in the atmosphere can delay the return
of the radar pulses to the satellite, this interfering with
the accuracy of sea-level measurements. To correct for this
delay, a microwave radiometer aboard TOPEX/Poseidon measures
the atmospheric water vapor content, expressed as the total
grams of water vapor in an imaginary column between the
satellite and a square centimeter of the sea, at any
particular point of the globe.
This image shows the global distribution of water vapor
measured by the satellite's radiometer from October 3 to
October 12, 1992. The highest values (indicated by red)
occur in the western tropical Pacific Ocean and the eastern
tropical Indian Ocean where the ocean surface waters are the
warmest. Without this correction, a water vapor content of 5
grams per square centimeter in these regions would cause an
altimeter measurement error of 32 centimeters; such an error
could interfere significantly with the study of ocean
circulation.
The atmospheric water vapor is important in itself
because the heat released by its condensation into vapor
fuels atmospheric circulation. Water vapor is also related
to sea- surface humidity, which controls the transfer of
latent heat (heat required by evaporating water) from the
oceans to the atmosphere. In addition, it is a greenhouse
gas that affects Earth's radiative heat balance. Monitoring
the global water vapor content is thus an important task for
understanding the role of the oceans in weather and climate
change.
NASA/JPL/Caltech
JPL Identification #: P-41499
