As Hurricane Bill continued to track west-northwest on August 17, 2009, it encountered low wind shear and increasingly warm water, both of which are favorable for strengthening. As a result, Bill intensified. The Tropical Rainfall Measuring Mission (TRMM) satellite took this image of Bill at 10:25 pm EDT on August 17 (02:25 UTC, August 18).
Although an eye was not visible in the underlying infrared data, which shows the white clouds that make up the storm, TRMM’s microwave and radar sensors showed that Bill had a well-formed eye, an indication that the storm had become better organized. The rain bands form concentric rings in response to the increased circulation. The large symmetric shape of the storm clouds confirms that Bill was not encountering any wind shear that would distort the shape or impede its development.
At the time of this image, the National Hurricane Center estimated Bill’s maximum sustained winds to be 148 kilometers per hour (92 miles per hour or 80 knots). Shortly after the image was taken, winds increased to 157 km/hr (98 mph or 85 knots), making Bill a Category 2 hurricane.
Image produced by Hal Pierce (SSAI/NASA GSFC) and caption by Steve Lang (SSAI/NASA GSFC).
Hurricane Bill had a well-formed eye surrounded by concentric bands of rain late on August 17.