感恩节的前一天通常是美国年度内交通最为繁忙的一天。但是 2012 年 11 月 21 日，天气状况对乘飞机进出密苏里州圣路易斯的人们构成了巨大了挑战。当然也给地面交通在早上的通勤造成了巨大的麻烦。据媒体报道， 20~21 之间的夜晚，浓雾突然降临； 21 号早上能见度低至 0.1km 。 直到 MODIS （携带于 Terra 上的传感器）在 11 月 21 日（圣路易斯）当地时间上午 10 ： 45 分采集上面所示的真彩色遥感影像的时候，雾依然徘徊在该地的上空。从图像中可以看出，大雾覆盖了密西西比河、密苏里河以及他们的支流所在的大部分河谷地带，从西北部的杰斐逊市一直蔓延到东那不得开普吉拉多市；位于密苏里河和密西西比河交汇之处的圣路易斯被低云笼盖着。 雾可以视作一种特殊的云，只不过位于近地面的区域。与其他的云相似，雾的形成需要局地空气达到露点——该温度条件下，水蒸气凝结成水珠。秋天时，空气的湿度（通常）相对较高，但夜间的温度也较低。夜间，地面相对于空气冷却更快，空气受冷而下沉，冷暖空气的接触面朝河谷谷底方向运动。再加上秋天的夜晚较长，所以雾就更加容易形成。 原文如下，引自NASA网站： http://earthobservatory.nasa.gov/IOTD/view.php?id=79736 。 翻译不妥，各位指正！ The day before Thanksgiving is one of the busiest travel days of the year in the United States. But on November 21, 2012, weather posed a challenge for people flying in and out of St. Louis, Missouri. It also posed a challenge for drivers making their morning commutes. Thick fog descended on the city overnight on November 20–21, and on the morning of November 21, visibility was as low as one-sixteenth of a mile (0.1 kilometers), according to news reports. Fog lingered over the region when the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite captured this natural-color image at 10:45 a.m. local time (16:45 UTC) on November 21. Fog filled the Mississippi River Valley as well as the valleys of the Missouri River and many of its tributaries. Fog extended from Jefferson City in the northwest to Cape Girardeau in the southeast. Situated at the confluence of the Missouri and Mississippi Rivers, St. Louis was engulfed by low clouds. Fog is simply clouds that have formed at ground level. Like all clouds, fog forms when the local air reaches its dew point —the temperature at which water vapor condenses into tiny droplets. In the autumn, the moisture content of the atmosphere remains relatively high, but nights are cool. Overnight, the ground tends to cool faster than the atmosphere, and air in contact with a cooling, sloped surface will slide down and settle in the valley floor. Because autumn brings longer nights, fog has more time to form and is a fairly common occurrence. 1. References 2. AccuWeather. (2012, October 16) Mountain valley fog: What causes it? Accessed November 26, 2012. 3. Blume, B. (2012, November 21) Dense fog settles over St. Louis, cutting visibility to near zero. CBS St. Louis. Accessed November 26, 2012. 4. Department of Atmospheric Sciences. Observed dew point temperature. University of Illinois at Urbana-Champaign. 5. Hudson, K. (2012, November 21) Fog affecting Lambert flights on busy Thanksgiving travel day. 6. KMOV. (2012, November 21) Dense fog blankets the News 4 viewing area. Accessed November 26, 2012. NASA image courtesy Jeff Schmaltz, LANCE MODIS Rapid Response Team at NASA GSFC. Caption by Michon Scott. Instrument: Terra - MODIS
标签: 密苏里
