Cold Waves

• "Cold Wave": an influx of unusually cold air into middle or lower latitudes, as determined by departure from normal regionally and seasonally; Figure 13.1

• Cold Waves affect larger areas than other winter hazards (e.g., blizzards, ice storms, etc.)

• An average of 35 deaths per year are directly attributed to extreme cold; disproportionately in the south and nearly 75% are male

• Citrus crop losses from a Cold Wave can result in losses in billions of dollars (e.g., Florida 1983: $3.6 billion; 1985: $2.9 billion)

• Economic losses: broken water pipes, commercial slowdowns, substantially increased heating costs

• Clear skies and the long (or continuous) winter nights enhance the radiational cooling process that creates the Polar airmasses (cP) over the high latitudes; coldest polar airmasses are those that form over Siberia, northern Canada & Alaska; typically -50 F to -70 F; Figure 13.4-5

• Arctic airmasses (cA) are those that form farther north over the Arctic Ocean; the cracks (called "leads") that form in the Arctic Ocean ice allow significant amounts of heat into the lower atmosphere, thereby keeping this type of air mass a bit "warmer"; typically -40 F to -50 F

• Polar airmasses are very shallow (1-2 km above the surface) are characterized by strong and deep temperature inversions

• Cold Outbreaks over North America: cold air mass is found under the eastern flank of a strong upper level high pressure ridge; warm air (mT) off Pacific Ocean builds ridge over Alaska and western Canada, thereby deepening the trough to the east, over central and eastern U.S.; Figure 13.2

• A strong cyclone, originating east of the Rockies, can transport the cold air south on its western flank; Figure 13.3

• Deep snowcover can lead to record-breaking lows in Cold Waves, if conditions set up for ideal radiational cooling (i.e., clear & calm, ridge aloft)

• Extreme Cold in Europe & Asia: Eurasia is the largest landmass in the world and has the coldest temps in the Northern Hemisphere; Siberian cold is enhanced by: 1) the large distance to the nearest unfrozen ocean, thereby isolated the region from warmth & moisture; 2) the presence of mountain barriers to the east & south, serving to isolated and trap the cold air; Figure 13.6

• Sometimes, the Siberian air spills into Europe; descent of air from about 725 mb to around 1000 mb actually "warms" the air by 35 F; Figures 13.7-9

Cold Waves (continued)

Major Cold Outbreak Summary for the Central & Eastern U.S.

• 1) the buildup of an upper level ridge in the jetstream over western North America, often as a result of warm air transported northward in the lower troposphere ahead of a strong cyclone in the Pacific;

• 2) intensification of a surface high pressure center over northern Canada or Alaska through rapid cooling of the air near the surface and convergence aloft downstream of the ridge;

• 3) movement of the cold airmass south-eastward in response to steering by the upper level winds and confinement of the cold-air pool on its western side the by Rocky Mountains;

• 4) extensive snow cover over central North America to keep the cold airmass "refrigerated";

• 5) a mechanism to enhance the winds that transport the cold air southeastward, thereby reducing the transit time of the cold air; the "trigger mechanism" is often a strong winter cyclone crossing central or eastern North America.

• "Wind Chill Chart:" wind removes heat from skin and also increase evaporation of moisture, causing even further cooling; only applies to living entities, not inanimate objects (such as cars); the NWS developed a new "Wind Chill Chart" in November 2001 (Table 13.2), based on a model of the human face at a height of 5 feet above the ground (old method was for 33 feet above ground); skin freezes at about -37 F

• "Wind Chill Advisory:" issued by the NWS when wind chill temps of -25 F are expected for a period of at least 3 hours

• "Wind Chill Warning:" issued by the NWS when wind chill temps of -40 F are expected for a period of at least 3 hours

• "Great Cold Wave of 1899"