Monday Supplemental Summary

HAZARDS OF ANTICYCLONES

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

HEAT WAVES AND DROUGHTS

During the summer, a persistent warm core anticyclone that becomes stationary over a portion of the country may be responsible for producing a heat wave or a drought. These two hazardous phenomena may be related, but conditions of one without the other have been observed. A heat wave is a prolonged interval of abnormally and uncomfortably hot air temperatures, when the daily ambient air temperature exceeds 90 degrees Fahrenheit; a heat wave, such as the 1995 killer 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 the 1988 Midwest drought, represents a condition with abnormally small amounts of precipitation and abnormal moisture deficiency over long time intervals; increased danger of wildfire may occur. In either the heat wave or the drought case, a large, deep high pressure system may remain over an area for an extended time interval, diverting moisture and thunderstorms around the periphery of the cell. Under this high, skies remain clear, allowing for intense solar heating. A strong subsidence inversion keeps a cap on cloud development from any residual moisture that has been evaporated from the surface.

POLLUTION EPISODES

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