Tropical Storm Katrina is shown here as observed by NASA's QuikScat
satellite on August 25, 2005, at 08:37 UTC (4:37 a.m. in Florida). At this
time, the storm had 80 kilometers per hour (50 miles per hour; 43 knots)
sustained winds. The storm does not appear to yet have reached hurricane
strength.
The greater danger may be not with her winds, but with Katrina's rains.
The storm is moving slowly, just 13 km/hr (8 mph), and is expected to
slow as it moves over land. This means that Katrina 's heavy rains will
linger longer over one area, dumping 15-25 centimeters (6-10 inches) of
rain over Florida and the Bahamas and possibly up to 38 cm (15 inches) in
some regions, the National Hurricane Center warns.
The image depicts wind speed in color and wind direction with small barbs.
White barbs point to areas of heavy rain. The highest wind speeds, shown
in purple, surround the center of the storm.
Measurements of the wind strength of Tropical Storm Katrina show sustained
winds similar to those shown by these QuikScat observations, though not
identical. This is because the power of the storm makes accurate
measurements difficult. The scatterometer sends pulses of microwave
energy through the atmosphere to the ocean surface, and measures the
energy that bounces back from the wind-roughened surface. The energy of
the microwave pulses changes depending on wind speed and direction, giving
scientists a way to monitor wind around the world.
Tropical cyclones (the generic term for hurricanes and typhoons) and to a
lesser extent, weaker storm systems like Katrina, are difficult to
measure. To relate the radar energy return to actual wind speed,
scientists compare measurements taken from buoys and other ground stations
to data the satellite acquired at the same time and place. Because the
high wind speeds generated by cyclones are rare, scientists do not have
corresponding ground information to know how to translate data from the
satellite for wind speeds above 50 knots (about 93 km/hr or 58 mph). Also,
the unusually heavy rain found in a cyclone distorts the microwave pulses
in a number of ways, making a conversion to accurate wind speed difficult.
Instead, the scatterometer provides a nice picture of the relative wind
speeds within the storm and shows wind direction.
For more information about the storm, please visit the National Hurricane Center.
QuikScat Background
NASA's Quick Scatterometer (QuikScat) spacecraft was launched from
Vandenberg Air Force Base, California on June 19, 1999. QuikScat carries
the SeaWinds scatterometer, a specialized microwave radar that measures
near-surface wind speed and direction under all weather and cloud
conditions over the Earth's oceans. More information about the QuikScat
mission and observations is available at http://winds.jpl.nasa.gov.
QuikScat is managed for NASA's Science Mission Directorate, Washington,
DC, by NASA's Jet Propulsion Laboratory, Pasadena, CA. JPL also built the
SeaWinds radar instrument and is providing ground science processing
systems. NASA's Goddard Space Flight Center, Greenbelt, MD, managed
development of the satellite, designed and built by Ball Aerospace &
Technologies Corp., Boulder, CO. The National Oceanic and Atmospheric
Administration has contributed support to ground systems processing and
related activities.