Cloud streets in the Gulf of Mexico
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Cloud streets in the Gulf of Mexico

December 9, 2010
On the first day of meteorological winter, December 1, 2010, a massive cold front moved across the eastern coast of North America, dumping up to 20 cm of snow to parts of Ontario, Canada and chilly, windy conditions as far south at the Gulf of Mexico. On that date, the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Aqua satellite passed over the Gulf of Mexico, capturing the visible effects of this cold front - spectacular clouds - as it passed overhead.

In the lower right corner of the image, a band of clouds form a broad arc from the southwest to the northeast. This is the cloud formation along the cold front, where the following cold air heaves the warmer air upwards. The clouds formed by this unstable line of air forms a visible boundary as seen in this image. As the cold front passed, the temperature dropped precipitously. The city of Tampa, which can be seen as a gray smudge on Tampa Bay well north of the cold front, enjoyed a balmy high of 86° F on November 28, but on the day of this image, the high was an unseasonably chilly 66° F.

Behind the cold front, cold, dry air aloft encountered the warm, moist air rising from the Gulf waters, causing water vapor to condense into clouds. Because of the strong winds, the clouds formed parallel formations called “cloud streets”. At the point where the clouds first form, the streets appear very narrow and well-defined. As they age, they begin to spread out and rejoin each other, as well as to blend into the clouds of the cold front.

Meteorological winter is a convention adopted by meteorologists based on “sensible weather patterns” (or shortest days and lowest annual temperature) for the purpose of record keeping. Astronomical winter, in contrast, is based on the position of the Earth in its orbit around the sun. The winter solstice, when the Earth's axial tilt is farthest from the sun, marks the first day of astronomical winter, and is usually on or near December 21. In contrast, for most of the northern hemisphere, meteorological winter begins on December 1 and lasts from December until the last day of February.

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