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Regional and Mesoscale Meteorology Branch

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Percent Cloud Cover Composites/Climatologies over Eastern U.S.

Summertime Climatologies

Monthly cloud cover percentage products can be generated using the GOES-8 visible channel for each hour. A time series loop of composites (combined monthly percentage products), taken over the Eastern and Midwestern United States during the summertime months of June, July and August of 1998 and 1999, from 1245 UTC through 2245 UTC.

Cloud Cover Percentage at 1245 UTC for Summers of 1998-99

During the summer, weather patterns over the United States are predominantly convective. One of the most obvious examples seen in the loop referenced above is over the Florida peninsula. Beginning with the 1245 UTC(8:45 EDT) image, the percentage of cloud cover starts at under 20% for most of the state, steadily increasing throughout the day to greater than 70% over some areas by late afternoon, early evening (1845-2145 UTC), then gradually decreasing during the late evening. Another sign of convection is the speckled appearance of the composite over the state. This is the result of convective clouds reappearing each day in areas more conducive to development (for instance, over the Everglades) while it is often suppressed in others (over the cool Lake Okeechobee).

This same convective pattern can be seen in other parts of this sector. Cloud percentage greatly increases during the day over the Appalachian Mountains, while the Midwest and southern states show the same kind of speckling as Florida.

Cloud Cover Percentage at 1845 UTC for Summers of 1998-99

There are several other noticable features. One is the higher percentage of cloud over the warm gulf stream water as oppose to the surrounding ocean. Another is that the cloud percentages over the cool Great Lakes tend to stay in the 30-45% range throughout the entire day, while the percentages greatly increase over the heating land. One exception is along Lake Michigan’s eastern shoreline, where the land area also stays within the 30-45% range. This is probably due to the prevailing west winds and breezes coming off the lake, keeping the shoreline cool and convection to a minimum.

Currently, there are only two years in this composite. More years are required to make a true climatology. As additional years are added, various trends will be observed and verified. By using these and other composites, areas conducive to convection can be identified and studied.

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