Geoscience Reference
In-Depth Information
JET
STREAM
H
(a)
(b)
(c)
(d)
Figure 7.28 Drought in the central United States.
(a) Drought conditions in the region develop when strong high-pressure systems
develop and persist, causing air to compress, temperatures to rise adiabatically, and relative humidity to fall. (b) U.S. drought moni-
tor on July 24, 2012. Severe to exceptional drought conditions existed over much of the central United States at this time (
Source
: The
U.S. Drought Monitor is produced in partnership between the National Drought Mitigation Center at the University of Nebraska-Lincoln,
the United States Department of Agriculture, and the National Oceanic and Atmospheric Administration. Map courtesy of NDMC-UNL.)
(c) Dead corn plants in the farm belt. Scenes like this were common in the drought areas due to high temperatures and reduced available
moisture. (d) Dried farm ponds like this one were common in the central United States during the drought due to lack of water and high
evaporation rates.
pressure pushes the midlatitude jet stream and associated storm
track to the north (Figure 7.28a).
Given the descending air associated with the high, the
air over the central United States becomes remarkably stable
and temperatures rise due to compression of the air molecules.
Dallas, Texas, had over 100 days with temperatures >100˚F be-
tween 2011 and 2012. Temperatures in central Kansas exceeded
105 for several days in July 2012. Overall, July 2012 was the
hottest month ever recorded in the contiguous United States,
with over 1000 daily temperature records broken the first week
of the month. With such persistent searing temperatures wide-
spread, rainfall was scarce because specific humidity was low
and maximum humidity was high. Relative humidity was often
only 20% to 30%, which decreased the dew-point temperature
to the point where the chance of measurable rain was effectively
zero. In many places accumulated rainfall was less than 50% of
normal. The problem of water availability was further magni-
fied because the high temperatures increased evaporation rates
considerably, causing soils to crack. Overall, the U.S. Depart-
ment of Agriculture recognized severe to exceptional drought
conditions throughout much of the heartland (Figure 7.28b).
The recent drought has had enormous impacts on agricul-
ture in the heartland. Corn and sorghum production was down
in 2012 by about 27% and 7%, respectively, from average. This
decrease was no surprise considering that many fields were
baked dry (Figure 7.28c). By the time of the fall harvest, only
35% of the soybean crop was rated excellent. Water levels in
farm ponds were very low (Figure 7.28d) due to evaporation.
Given reduced water supplies and degradation of grass cover on
ranchlands, ranchers were forced to sell cattle at a much greater
rate than normal to reduce the size of their herds. Given the im-
pact of the drought on crop and livestock production, the price
of many farm commodities rose dramatically for consumers in
the latter part of 2012. As of May, 2013, much of the region
remained in severe to exceptional drought (Figure 7.29). Al-
though the drought had eased considerably in the Mississippi
and lower Ohio River valleys, it had intensified in the central
and northern Great Plains. Although the drought may break in
the coming months, concerns abound that dry conditions may
be the “new normal” in much of the U.S. farm belt due to ongo-
ing climate change. This relationship will be discussed in more
detail in Chapter 9.
