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NRL Monterey, Marine Meteorology Division
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NRL Monterey, Marine Meteorology Division
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| Pre-dawn Product (Low Clouds White -- High Clouds Black) | After-dawn Product (All Clouds White) |
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| This product shows low clouds and fog as white at all times during a twenty-four hour period. During the daytime the product is based on the visible imagery; at night the image is produced by a combination of GOES infrared channels. This combination is based on the 3.9 and 11 micron wavelength channels. Stratus and fog have different emmissive properties in the two wavelengths. This difference is used to produce an image of nighttime low clouds.Visible images are the most ideal images for viewing low clouds. Reflecting solar radiation, low clouds and fog appear as white against darker land and sea surfaces on visible images. However, visible images are only available during the daytime. Infrared images, available at night, are a poor substitute, showing low clouds and fog poorly or not at all. This is because low clouds and fog have a very similar radiating temperature as the underlying land or ocean. Thus, low clouds and fog appear nearly invisible on infrared images. The bispectral nighttime image, on the other hand, highlights low clouds well at night in white gray shades. On loops, therefore, the user can identify low clouds and fog (white gray shades) during both day and night. | |
| The evolution of low clouds and fog can be seen at times during a twenty- four hour period. Traditionally, fog and low clouds have only been easily viewable during the daytime only with visible images. This product extends the ability to view low clouds around the clock. The product is especially useful for unobserved areas like oceans where surface reports of low clouds, fog or poor surface visibilties are rare. On this product low clouds always appear white. High clouds appear as black during the nighttime and white during the daytime. |
| The nighttime product has been known elsewhere as the "fog" product. But it is also a "stratus" product and can not distinguish between low stratus clouds and actual fog. This is because the satellite only observes the top of clouds; it receives no information about cloud bases. Thus, the forecaster may need to use additonal data and knowledge to distinguish between low clouds and fog, such as surface observations of visibility. The nighttime portion of the loop is only useful for stratus, fog, or stratocumulus. It will not perform well over cumulus clouds. During the nighttime high clouds will appear in a variety of appearances, often as black gray shades. At times it will be difficult to distinguish nighttime high clouds from the ocean/land background. This difficulty is alleviated by use of the loop with buttons below. The user can then discern from the movement of clouds which are high vs. low clouds. This product uses the 1 km spatial resolution GOES visible data, available during the daytime. Thus, daytime images have a crisp, highly-defined appearance. During the nighttime, the product uses a combination of two infrared channels, both with a resolution of 4 km. Thus, the nighttime images have less sharpness and definition. |
| Pre-dawn Low Cloud Image | Corresponding Infrared |
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| The image on the left is the low cloud image a few hours before dawn. Low
clouds, mostly in the western one-thrid of the image, are white. Cloud-
free ocean is dark gray. High clouds in the north central portion of the
image over the northern Gulf of California are black. Do not be confused
by the black shading of the high clouds. The contrast between the black
high clouds and white low clouds makes it possible to distinguish between
clouds at different heights, something that it not possible on visible
images. On visible images all clouds are white. On the right is the corresponding GOES infrared image. On this image high clouds are white. However, low clouds are nearly invisible. This is because there is little thermal contrast between the sea surface and the relatively warm cloud tops. | |
| Post-dawn Visible Image | Corresponding Infared |
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| On the left is the visble image taken a few hours after the low cloud image just above. Both high clouds and low clouds are white. High, convective clouds can be identified by their bumpy appearance and the early-morning shadows they cast. Low clouds in the western portion of the image can be identified by their smooth appearance. On the right is the corresponding infrared image. Only high clouds (white) can be identified. | |
Author: Tom Lee Last Updated: Tue Dec 10 16:49:07 2002 Produced by: The Composer (Ver: 1.1.2 ) |
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