Drought
#1 weather-related cause of deaths worldwide
Drought affects larger areas over larger timescales than all other types of hazardous weather.
Meteorological or Climatological Drought: an unusually long period of below normal precipitation for an area.
Hydrological Drought: an unusual deficiency of ground water and/or streamflow (water levels are below normal) for an area.
Agricultural Drought: a period of deficient moisture in soil layers from which crops and plants normally draw their water.
Socioeconomic Drought: when moisture shortage is sufficiently large that is affects the supply of, or demand of, some economic goods; lags behind other types of drought.
Cause of Drought? No single cause. Many unknowns; tough to predict onset of drought and ending of drought. Sea Surface Temp (SST) anomalies have shown some correlation in shifting general circulation patterns.
Drought in Central U.S.: High pressure, both at surface and aloft, builds over central U.S.; blocks influx of moisture from Gulf of Mexico and Atlantic Ocean; incoming solar radiation heats dry ground and air, none used for evaporation; heating strengthens high pressure ridge; "Feedback Mechanism": the drought feeds itself. [1930s "Dust Bowl, 1950s, 1988.]
Drought in Eastern U.S.: upper level and surface high sets up over Midwest/OH Valley, or over the Carolinas; effectively blocks moisture from Gulf of Mexico; storms pass by to the north in Canada, dragging dry cold fronts across region; warm and humid air, with a downsloping component east of the Appalachians. [Recent droughts: 1962-65, 1998-99 ? some to 2002; T.S. "Dennis" and Hurricane "Floyd" ended drought in Fall of 1999, sending many places from drought to flooding.]
Drought in Western U.S.: most precip falls from OCT to APR (heaviest in DEC to MAR); high pressure ridge develops over western U.S. deflecting storms well to the north, while setting up a dry, downsloping easterly wind flow (note: usually marked by cold air outbreak in central and eastern U.S.). [Recent droughts: 1976-77, 1986-87 through 1990-91: this 5 winter drought dropped So-Cal Reservoir System down to 16% of capacity; 1987-90 was the driest 4-year stretch on record. Cause? Not known. El Nino 1976-77, 1986-87; La Nina in heart of 1986-91 stretch.
Flood Forecasting & Safety: NEXRAD Doppler Radar Network; the NWS issues Flash Flood Watches & Warnings, River Flood Watches & Warnings, Small Stream Flood Advisories, Coastal Flood Watches & Warnings; 6" of water can sweep a person away; a foot of water can wash a car away; abandon any car that stalls in flood waters; "Stupid Motorist Law" in AZ.
Historical Perspective on Drought: 1930s-type drought occurs 1-2 times each century. Droughts lasting a decade or more occurs once over 500 years; Multidecadal drought - "Megadrought" - can last for many decades, as happened in the late 1200s and late 1500s.
Hotter Days & Cooler Nights during a drought.
The Drought of 1988.
The Drought of the 1950s.
The Drought of the 1930s: The "Dust Bowl".