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NRL Monterey, Marine Meteorology Division
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NRL Monterey, Marine Meteorology Division
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| Pre-dawn Low Cloud | After dawn Low Cloud (Visible) |
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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 | Corresponding Infrared |
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| The image on the left shows the nighttime lowcloud product over the Hawaiian Island. White patches indicate lowcloud; black regions inthe extreme north indicate high cirrus clouds. Noticethe cloud plume to the east of the big island of Hawaii. This is caused the flow of the trades overthe island. Notice that on the corresponding ir image(right)it is difficult to detect the low clouds,including the plume over the island.The low cloud movies (left) show the transition from nightto day night, illustrating that low clouds canbe tracked across the day/night terminator. The IRmovies (right) show that IR images are not an effective wayto follow the evolution of low clouds, day or night. | |
| After dawn Low Cloud (Visible) | Corresponding Infrared |
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| The low cloud image on the left is simply the visibleimage. The low cloud movies (left) use the low cloud productsat night and switch to visible images during thedaytime. Thus, it is possible to observe the plumeoff Hawaii both in darkness and in light. Theinfrared movies (right) are far less useful in thisregard. | |
Author: Tom Lee Last Updated: Tue Dec 10 16:51:18 2002 Produced by: The Composer (Ver: 1.1.2 ) |
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