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S'COOL : Comparisons for Phase 1

Three days out of the week were clear days. The satellite images for one of these days show an area of bright clouds over the ocean on the right side of the picture, but the rest of the area has no clouds. Notice, however, the varying brightness of the land surface in the visible image. This variable background makes it much harder to identify clear sky from space than against the uniform blue sky background seen from the surface. This is a challenge to the satellite data algorithms, so the determination of clear sky by the S'COOL participants is of particular value.

January 13, 14, 17, 1997
Gloucester AVHRR Agree?
Cloud Type None None Yes
Cloud Fraction Clear 0 % Yes
Cloud Height No Cloud No Cloud Yes
Cloud Opacity No Cloud Optical Depth = 0 Yes

One day had cirrus clouds. The satellite images show them as bright (cold) in the infrared and moderate (thin) in the visible wavelength channels.

January 15, 1997
Gloucester AVHRR Agree?
Cloud Type Cirrus High-Altitude Ice Cloud Yes
Cloud Fraction Scattered 8.75 % Yes
Cloud Height High 5.5 km Yes, but...
Cloud Opacity Thin Optical Depth = 0.9 Yes

The "Yes, but..." on cloud height is there because CERES defines high clouds arbitrarily as anything above 6 km. However, this is clearly cirrus cloud which is generally classified as high cloud.

And one day had water clouds. The satellite images show them as moderate (warmer) in the infrared and bright (thick) in the visible wavelength channels.

January 16, 1997
Gloucester AVHRR Agree?
Cloud Type Stratocumulus Low-Altitude Water Cloud Yes
Cloud Fraction Scattered 10.0 % Yes
Cloud Height Medium 0.5 km No, but...
Cloud Opacity Variable Optical Depth = 9.1 but varying in field Yes
The "No, but..." on cloud height is there because the students did properly identify the cloud type, which is generally classified as a low cloud. It was determined that the cloud chart they were using incorrectly classified this as a mid-level cloud.

See some other examples from the test in Big Timber, Montana, USA.

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