Portal:Atmospheric Sciences:Portal Atmospheric Sciences 10/2006

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Lower Antelope Canyon was carved out of sandstone over millions of years by flash floods

A flash flood is a rapid flooding of low-lying areas, rivers and streams, that is caused by the intense rainfall associated with a thunderstorm, or multiple training thunderstorms. Flash floods can also occur when ice jams block the normal course of a river, or when a man-made structure, such as a dam, collapses, i.e.; the Johnstown Flood of 1889. Flash flooding occurs when the ground becomes saturated with water that fell so quickly that it could not be absorbed. The runoff collects in low-lying areas and rapidly flows downhill with sudden rising water. Flash floods most often occur in normally dry areas that have recently received precipitation, but may also be seen anywhere downstream from the source of the precipitation (even dozens of miles from the source). The U.S. National Weather Service gives the advice "Turn Around, Don't Drown" in reference to flash floods; that is, it recommends that people get out of the area of a flash flood, rather than trying to cross it. Most people tend to underestimate the dangers of flash floods.

Did you know ...

Cloud and fog organization - organization of clouds or fog into distinct pattern such as cellular convection closed cells, cellular convection open cells, longitudinal rolls, cloud streets, and other types. Besides atmospheric scientists studying such phenomena it is, somewhat suprisingly, of great interest to olympic class sailors ^[1] who associate various organized cloud convection with steady, unsteady (related to the cellular mechanism), pulsing (associated with the transverse rolls), oscillating (longitudinal rolls), and ribboning winds (boosted roll pattern). Seminal book by Bethwaite (1993) is a definite sailing perspective source of discussed phenomena.

The Saharan Air Layer (SAL) is an intensely dry, warm and sometimes dust-laden layer of the atmosphere which often overlays the cooler, more-humid surface air of the Atlantic Ocean. In the Sahara Desert region of North Africa, where it originates, it is the prevalent atmosphere, extending from the surface upwards several kilometers. As it drives, or is driven, out over the ocean, it is lifted above the denser marine air. This arrangement is an inversion where the temperature increases with height. The boundary between the SAL and the marine layer suppresses or "caps" any convection originating in the marine layer. Since it is dry air, the lapse rate within the SAL is steep, that is, the temperature falls rapidly with height.

Bénard cells are convection cells that appear spontaneously in a liquid layer when heat is applied from below. They can be obtained using a simple experiment first conducted by Henry Bénard, a French physicist, in 1900.

Meteorology and history

The phrase perfect storm refers to the simultaneous occurrence of events which, taken individually, would be far less powerful than the result of their chance combination. Such occurrences are rare by their very nature, so that even a slight change in any one event contributing to the perfect storm would lessen the its overall impact. The 1991 Halloween Nor’easter is also known as the Perfect Storm. The nor'easter ravaged the Atlantic Ocean over the course of several days, resulting in the deaths of several Massachusetts-based fishermen and billions of dollars in damage. In October 1991, the merging of two low-pressure areas, a large flow of warm air from the south, cold air from the north, and moisture feeding into the storm from the Gulf stream all conflated with cold air from strong northwesterly winds and warm air from strong northeasterly winds to create an exceptionally powerful storm across a very large area. Had the storm been more concentrated, it might have resembled a hurricane. Because the storm occurred without the typical hurricane warnings, fishermen and smaller vessels at sea were caught off-guard in hurricane-like conditions.

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Atmosphere - Atmospheric sciences - Climate - Climate change - Climate forcing - Climatologists - Climatology - Clouds - Earth sciences - Environmental science - Greenhouses gases - Hydrology - Lightnings - Meteorological instrumentation and equipment - Meteorologists - Meteorology - Noise pollution - Oceanography - Paleoclimatology - Precipitation - Space science - Storms - Tornadoes - Tropical cyclones - Weather - Weather events - Weather hazards - Weather satellites - Winds

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• Atmospheric Sciences Portal 09/2006

Picture of the month

Fog rolls observed around Half Moon Bay, California (CC-BY-SA Jacek Walicki).