 |
NRL Monterey, Marine Meteorology Division
|
|---|
|
NRL Monterey, Marine Meteorology Division
|
|---|
| Nighttime Low Cloud | Next Morning Low Cloud (Visible) |
|---|---|
) |
) |
| 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 emissive 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 bi-spectral nighttime image, on the other hand, highlights low clouds well at night in white gray shades. On loops, 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 all 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 visibilities 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 cannot 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 additional 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. At night,
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 clouds and which are 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. At night, 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. |
| Predawn Low Cloud | Corresponding Infrared |
|---|---|
|
) |
| The image on the left represents low cloud coverage just before dawn. Low clouds cover the Florida panhandle and parts of Mississippi, Alabama, and Georgia. High clouds in black appear west of Cuba. The low cloud movies (left) show that stratus is dominant at night, but during the daytime (visible images) it dissipates quickly under the influence of surface heating. The infrared image on the right shows little trace of low clouds over the inland Gulf Coast region. The infrared movies (right) are not helpful in detecting low clouds. They can be used to track high clouds, which appear as white. | |
| Daytime Low Cloud (Early Morning Visible) | Corresponding Infrared |
|---|---|
|
) |
| During the daytime, the low cloud product is simply the visible image. The image on the left shows this product just after dawn. A patch of stratus over the Alabama/Mississippi border is starting to dissipate. The low cloud movies (left) show the transition of low clouds from day to night. The infrared image and movies on the right are useful for tracking high cloud features in white, not low clouds. | |
Author: Tom Lee Last Updated: Tue Dec 3 07:20:06 2002 Produced by: The Composer (Ver: 1.1.1 ) |
|