These images show cloud vortexes which formed with unusual clarity over Selkirk Island, Chile. We ordinarily show as few clouds as possible in shots, since they block our view of the land, but here we want to show off their scientific and especially their esthetic value.
Landsat satellites are very different from weather satellites. Weather satellites typically have high temporal resolution (they take images frequently) but low spatial resolution (they take coarse images of large areas). The Landsats see smaller areas (115-mile swaths) in more detail, but only every 16 days or so-- so they are too slow for clouds, too slow and too coarse for armies, but just right for lakes and fields and glaciers.
But sometimes land satellites do show weather well, and vice-versa. The AVHRR sensor, operated by the National Oceanic and Atmospheric Administration, is really geared for studying clouds, but has proven to be surprisingly useful for looking at large land areas, by using clear-day images, or by piecing together composites-- from the cloud-free bits of images taken on successive days. Likewise, though we usually ignore cloudy Landsat images as flawed or even useless, they are often prettier than dry-sky pictures, and they sometimes show weather phenomena at a nice high resolution.
Vortexes over Selkirk Island
Selkirk Island is named for the obstinate Scottish sailor who marooned himself on a nearby island, inspiring Daniel Defoe's Robinson Crusoe. These islands were discovered by Europeans in the mid-1500s, and Alexander Selkirk lived there from 1704 to 1709. They have always been sparsely populated, serving as penal colonies, wildlife refuges, and lobster-fishing ports. The islands are tips of volcanic peaks, rising sharply from the sea; though only 33 square miles, Selkirk Island rises over a mile into the sky. 1
In September 1999, as sensor technicians here at the U.S. Geological Survey Center for Earth Resources Observation and Science (EROS) ran some normal checks on Landsat data, they noticed unusual clouds over Selkirk Island. They referred this image to EROS scientists, who identified the series of swirls as a "Karman vortex street," a fluid flow pattern which is rarely seen so clearly outside of a laboratory. The American Meteorological Society featured the Selkirk image on the cover of their monthly bulletin. 2 We at EROS have since become vortex-conscious, seeing them suddenly everywhere, but none so nice as the first.
How did the Selkirk vortexes develop? On that day the wind was carrying northward a layer of stratocumulus clouds (flat-bottomed puffballs). The mile-high island caused this cloud layer to slow about the island, while remaining fast farther out on either side. So on each "wing," left and right, the air started rotating toward the inside-- clockwise on the left, counter-clockwise on the right. The rotational momentum made each side swirl in on itself. The whorl-cores were clear because the swirling pulled dry, clear air (from above or below) into the wet layer, a bit like the air-funnel formed when you stir orange juice. These clear, spinning pockets trailed off down the "street" from the island like soap bubbles from a toy wand-- drifting downwind, weakening, filling with clouds, and breaking up.
More Selkirk cloudscapes
Since its launch in 1999, Landsat 7 has not again seen such nice vortexes over Selkirk Island, as you can see in these images. A single vortex formed on 25 March 2000, from a similar southerly wind, but then the pattern broke up. That morning the clouds were too unstable, as shown by the turbulent convection cells in the northeast.
In its first year of trying, Landsat 7 never saw the whole island. Even on a clear day like 18 November 1999 the island's heat and elevation heave up damp marine air into the cold, until it reaches its dewpoint and condenses into a kind of permanent parasol. This sometimes trails downwind a bit on windy days, as on 22 February 2000.
Pretty big MODIS-scapes
While we're at it, let's look at some MODIS images. The MODIS sensor, on the new Terra satellite, has high resolution temporally (frequent images), spectrally (many bands), and radiometrically (many bits per pixel, like a 32-bit scanner). But MODIS's moderate spatial resolution, between a Landsat and a weather satellite, zooms us out to see giant land-swaths, and cloud-swaths too. (Hence the MOD.)
A note on the images
The Landsat images in this article (except the full-resolution 15 September 1999 image) are Landsat 7 preview images-- shrunk-down samples of the full scenes, used mainly for scene-shopping. Though obviously inferior to full scenes, previews are free, easy to work with, and appear on the Internet within hours. (Try our Earth Observing System Data Gateway or Earth Explorer.)
These previews show different bands from most of the images in Earthshots. Normally we show bands that simulate a color-infrared photograph (MSS 4 2 1 or TM/ETM+ 4 3 2), with red signifying the near-infrared wavelengths just beyond our vision. But Landsat 7 previews (ETM+ bands 5 4 3) show farther, thermal-infrared energy as red, which gives the preview a new level of information.
In the case of Selkirk, Band 5 just hints at a feature not visible in bands 4 or 3. Do you see it? Band 7, a valuable afterthought to Landsat 4, shows it even better. Northeast of the island, just east of the vortexes, there is a bright spot, and a bright plume trailing from it northeast. This may be a westbound ship, the spot its smokestack and the plume its heat trail.
Such ephemeral human artifacts are commonplace. Look at this Landsat 1 image, taken right inside the crook of Florida south of Tallahassee over the Gulf of Mexico. Can you make out what happened? Landsat 1 shot this to catch a bit of land, visible in the northeast, but most of the image is ocean. The popcorn and puffs are clouds, and the subtle diagonal lines are from Landsat 1's sensor. The long, looping white line is a condensation trail from an airplane, where the jet fuel's byproduct water vapor cooled into droplets. The trail's shadow loops alongside on its northwest. We don't know why the plane took that course, but from the trail's dissolution you can tell where the plane was when the image was acquired, and in which direction it was meandering.3
Question
Have you noticed that in these images, clouds look white but ocean looks black? Both are made of the same substance, water-- why would they appear as opposite colors?
(See the answer below.)
Footnotes
1. Encyclopedia Britannica, online edition, "Selkirk, Alexander," "Juan Fernandez Islands."
2. Thomas P. DeFelice and others, "Landsat-7 Reveals More Than Just Surface Features in Remote Areas of the Globe," Bulletin of the American Meteorological Society 81, no. 5 (May 2000), p. 1047-1049.
3. Nicholas M. Short and others, Mission to Earth: Landsat Views the World (Washington: NASA, 1976).
Selkirk vortex street: Landsat 7 ETM+ image, WRS2 path 6, row 83, 15 September 1999.
Selkirk images on other days: Landsat 7 ETM+ previews, WRS2 path 6, row 83, from the Earth Observing System Data Gateway:
Landsat images elsewhere: Landsat 7 ETM+ previews from the Earth Observing System Data Gateway:
The
photograph of Selkirk Island was taken by Dr. Jose Rutllant, University of Chile, on 10 November 1999, about 20 miles northeast of Selkirk Island. Thanks to Dr. Rene Garreaud for his assistance.
Landsat satellites see solar energy that reflects off the Earth (or atmosphere) and back at the satellite. When light hits water, whether ocean or cloud-droplet, most of the light reflects at the same angle it came in, like a basketball bounce-pass. Calm water lets the light bounce away, like a mirror, so little light reflects toward the satellite; the ocean looks dark. But a cloud's millions of droplets bounce the light around and around like pinballs, so some light always scatters toward the satellite; the cloud looks bright. For this reason, a choppy ocean with whitecaps looks brighter than a calm ocean.
If you liked that explanation, consider this: this is the same reason plants are so bright in the infrared band; leaves are tight cell-clusters, just as clouds are loose droplet-clusters. The cell walls admit visible light (since it has the right energy for photosynthesis), but they repel infrared energy (since that would just heat up and dry out the plant).
This article was finished 14 August 2000 and released 12 January 2001.
Satellite images
AVHRR images:
MODIS images: from the MODIS Land Image Gallery:
Photograph