The Visible Infrared Imaging Radiometer Suite (VIIRS) aboard the NOAA-NASA Suomi NPP satellite captured these two images of Florida before (l) and after (r) Hurricane Irma made landfall in the Sunshine State. Notice anything different between them?
Apart from all the clouds, which are always different between two images, you can see a change in the color of the water surrounding Florida and, to a lesser extent, the Bahamas. What causes this change? The heavy rains, high winds, and storm surge brought by strong storms like Irma.
The winds and waves churn up sediment at the bottom of the ocean, at least in shallow areas like the Florida Keys and the Bahamas. The storm surge causes beach erosion and flooding along the coasts, while the heavy rains cause inland flooding (both “flash” and "river" flooding). If you look closely, you may even see this sediment and pollution getting swept along by the currents in the Gulf of Mexico as well as on the Atlantic side of Florida. And, remember that the Atlantic side of Florida is home to the Gulf Stream.
Although true-color images like these may appear to be photographs of Earth, they aren't. They are created by combining data from the three color channels on the VIIRS instrument sensitive to the red, green and blue (or RGB) wavelengths of light into one composite image. In addition, data from several other channels are often also included to cancel out or correct atmospheric interference that may blur parts of the image.
Before, After, and Beyond
Beyond comparing before-and-after images, we also can monitor the movement of sediment and pollution after storms for as long as it’s there, such as in this video showing an image from before the storm (31 August 2017) and then the following 6 days (12–17 September 2017).
If you view this animation a few times, you should see two things:
- Sediment around the Florida Keys does get pulled into the Gulf Stream, with visible eddies where the polluted water meets the clean water
- The polluted water generally gets darker with time, because more of the dirt and sand and garbage settle out with time, allowing the ocean to slowly return to its pre-Irma appearance.
You might also notice the ocean around the Bahamas is always lighter in color. This is true even in the “before” image, because the water is very shallow in the Bahama Banks, allowing satellites to "see" all the way to the bottom. However, offshore, on the west side of the largest island (Andros), the water becomes nearly white after Irma’s passage.
To see other satellite images of Irma, visit our gallery on the NESDIS website.
This article is based on a blog post by Curtis Seaman that first appeared on the Suomi NPP (National Polar-orbiting Partnership) blog published by the Cooperative Institute for Research in the Atmosphere. Click here to read that post in its entirety.