From: Subject: Monday Weather Topic in Greater Depth Date: Thu, 12 Nov 2009 10:49:37 -0800 MIME-Version: 1.0 Content-Type: text/html; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable Content-Location: http://www.ametsoc.org/amsedu/dstreme/learn/m_sup.html X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.5579 Monday Weather Topic in Greater Depth

SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

To complement the Daily Summary for Monday, 9 = November 2009=20

HAZARDS OF ANTICYCLONES

While most high-pressure weather is fair, high-pressure cells, often = called=20 anticyclones, may sometimes produce some adverse weather conditions. The = following hazards can result:

COLD WAVES

The cold core wintertime anticyclones that are essentially synonymous = with=20 arctic air masses may be responsible for "arctic outbreaks" or "cold = waves" as=20 they move into midlatitudes. A cold wave represents a potentially = hazardous weather condition marked by a rapid fall in temperature over a = 24-hour=20 interval; the exact criterion depends upon the season and local = conditions. Cold=20 waves are especially apparent to the east of the Rockies in the Midwest = and=20 Eastern United States. A cold wave is associated with a shallow, rapidly = moving=20 anticyclone containing continental polar (cP) or arctic = (A) air,=20 that originates in the polar plains of Siberia or northwest Canada and = slides=20 southeastward across the Plains and the Midwest. Combined with wind, low = temperatures can be deadly for persons and animals not adequately = protected.=20 These cold waves are especially disastrous to the major citrus and the = year=20 round agricultural areas of southern Florida and Texas. The majority of = Florida=20 citrus freeze conditions are associated with strong anticyclones that = reach=20 Florida from the Canadian border in one to two days. Freezing = temperatures occur=20 because of a combination of cold advection and nighttime radiational = cooling.=20

HEAT WAVES AND DROUGHTS

During the summer, a persistent warm core anticyclone that becomes = stationary=20 over a portion of the country may be responsible for producing a heat = wave or a=20 drought. These two hazardous phenomena may be related, but conditions of = one=20 without the other have been observed. A heat wave is a prolonged = interval=20 of abnormally and uncomfortably hot air temperatures, when the daily = ambient air=20 temperature exceeds 90 degrees Fahrenheit; a heat wave, such as the 1995 = killer=20 heat wave that affected the Midwest, may persist for a week or more. =

A dry spell or drought, such as the 1986 drought in the = Southeast or=20 the 1988 Midwest drought, represents a condition with abnormally small = amounts=20 of precipitation and abnormal moisture deficiency over long time = intervals;=20 increased danger of wildfire may occur. In either the heat wave or the = drought=20 case, a large, deep high-pressure system may remain over an area for an = extended=20 time interval, diverting moisture and thunderstorms around the periphery = of the=20 cell. Under this high, skies remain clear, allowing for intense solar = heating. A=20 strong subsidence inversion keeps a cap on cloud development from any = residual=20 moisture that has been evaporated from the surface.

POLLUTION EPISODES

An air pollution episode is a weather event where the = concentration of=20 air pollutants increases to dangerous or hazardous levels because the = pollutants=20 are not dispersed quickly by the atmosphere. Many of the notable air = pollution=20 episodes have occurred when a warm core anticyclone has become stagnant = above an=20 extended pollution source, such as the industrial or urban areas of the = Los=20 Angeles Basin, the Northeast Corridor or the Chicago-Milwaukee = metropolitan=20 area. Because of sinking air within the center of the high-pressure = cell, these=20 highs have a subsidence temperature inversion that increases the = stability in=20 the lower troposphere. This increased stability, coupled with light = surface=20 winds, reduces the mixing of air near the surface and traps the = pollutants.=20 Clear skies within the region dominated by the high may stimulate = photochemical=20 reactions that produce such atmospheric pollutants as smog and low level = ozone.=20

Return to the Monday = Daily=20 Summary

Return to DataStreme = Atmosphere=20 website

Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu =
=A9=20 Copyright, 2009, The American Meteorological Society. =