Figure 1: Hurricane Katrina
Nearly the whole of the Gulf of Mexico was churning with the powerful
winds and rains of Hurricane Katrina on August 28, 2005, when NASA's
QuikScat satellite captured this image. The image depicts relative
wind speeds swirling around the calm center of the storm. The highest
wind speeds, shown in shades of purple, circle a well-defined eye, with
gradually weakening winds radiating outward. The barbs reveal wind
direction, and the white barbs show heavy rainfall. At the time this
image was taken, the National Hurricane Center reported that Katrina
had winds of 160 miles per hour (257 kilometers per hour or 140 knots)
with stronger gusts and was moving north-northwest at about 10 mph (16
km/hr). The storm weakened slightly before coming ashore, but was still a
powerfully destructive storm.
Why don't the wind speeds shown here match those reported by the National
Hurricane Center? QuikScat measures near surface wind speeds over the
ocean based on how the winds affect the ocean. The satellite sends out
high frequency radio waves, some of which bounce off the ocean and return
to the satellite. Rough, storm-tossed seas return more of the waves,
creating a strong signal, while a mirror-smooth surface returns a weaker
signal. To learn to match wind speeds with the type of signal that returns
to the satellite, scientists compare wind measurements taken by ocean
buoys to the strength of the signal received by the satellite. The more
measurements scientists have, the more accurately they can correlate wind
speed to the returning radar signal.
Storms as large as Katrina are rare. This means that scientists have
few buoy measurements to compare to the data they get from the satellite
and can;t match the satellite measurements to exact wind speeds. Instead,
the image provides a clear picture of relative wind speeds, showing how
large the strong center of the storm is and which direction winds are
blowing.
For official warnings and information about Hurricane Katrina, please
visit the National Hurricane Center. To learn more about measuring winds
from space, check out NASA's Winds web site.
