Today's feature compares cloud frequency images derived from GOES-8
Visible imagery over Costa Rica and surrounding countries for 1515 UTC
(0900 local time) and 2015 UTC (1415 local time) from February 1997 and 1998.
A "cloud free" image was compiled from all available imagery for the specified
time by finding the lowest brightness count for each pixel over the series of
images. The "cloud free" image was then used to discriminate cloud versus no-cloud
regions in each of the images for the specified times. This method does not
distinguish between levels or types of clouds. The resulting cloud frequency
images for the morning (1515 UTC, Figures 1a,b and 2a,b below )
and afternoon (2015 UTC, Figures 3a,b and 4a,b )
for the month of February during 1997 and 1998 are shown in Figures 1 a,b
through 4 a, b. Imagery has been shown with color tables depicting both 25% frequency
of occurrence (Figure x.a) and 10% frequency of occurrence (Figure x.b) to demonstrate the differences between the
2 years.
| Figure 1a |
Figure 1b |
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GOES-8 Visible image, click to enlarge
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GOES-8 Visible image, click to enlarge
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| Figure 2a |
Figure 2b |
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GOES-8 Visible image, click to enlarge
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GOES-8 Visible image, click to enlarge
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What first becomes evident in the morning and afternoon imagery is that overall,
the cloud frequency is less in 1998 than in 1997. This is normally a dry month
for the Central American countries. This year (1998) is an El Nino year and
many of the Central American countries have been experiencing drier than normal
conditions.
What next becomes evident in the imagery is the strong influence that
topography has on the formation of clouds. Compare the topography of the
region (Figure 5) with the cloud frequency composites. Surface level and 850 mb
winds are predominantly easterly during this month and low level stratus back up
against the mountains on the eastern side of Costa Rica. Although it is difficult
to see in the 2 image times, the effect of the sea-breeze and local terrain slope flows
are noticable on the western side of Costa Rica.
Bernadette Connell
| Figure 3a |
Figure 3b |
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GOES-8 Visible image, click to enlarge
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GOES-8 Visible image, click to enlarge
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| Figure 4a |
Figure 4b |
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GOES-8 Visible image, click to enlarge
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GOES-8 Visible image, click to enlarge
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Back to text
| Figure 5 |
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Topography image
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