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NRL Monterey, Marine Meteorology Division
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NRL Monterey, Marine Meteorology Division
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| Late Afternoon Low Cloud | After Dark Low Cloud |
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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. |
| Late Afternoon Low Cloud | Corresponding Infrared |
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| The image on the left shows low cloud off the California Coast near sunset. This is the last visible image of the day. The image on right shows the low cloud product later that night. Status clouds have increased dramatically. Cirrus appear as black streaks on the image on the right. Care should be made not to misinterpret cirrus as cloud-free sea surface. The low cloud movies (left) can help the user from being confused about this. | |
| Nighttime Low Cloud | Corresponding Infrared |
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| The left image shows low clouds during the nighttime off the west coast of the United States. The image suggests that the low clouds are mostly offshore. Black streaks in the southwestern portion of the image are high cirrus clouds. On the right side is the infrared image on which low clouds are nearly invisible. It is difficult to detect the low clouds on this image because they have a similar temperature as the ocean background. | |
Author: Tom Lee Last Updated: Tue Dec 10 16:36:34 2002 Produced by: The Composer (Ver: 1.1.2 ) |
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