The end of a drought is difficult to predict, because droughts tend to perpetuate themselves. But in time a stronger weather system will break the circuit and good times will return.
Drought is a most unspectacular form of severe weather. It startsunnoticed, and is characterized by the absence, or at leastrarity, of notable weather with the exception of dust storms and
. Even its most common associate, the
, sneaks in without fanfare or announcement, as
each day becomes a little hotter than the last.
So what is a drought?
The first thing to remember about droughts is that they are
natural happenings, and an expected part of the climate and
weather cycle. The global engine that drives the weather is
complex, and minor variations in atmospheric and oceanic
circulation can result in significant local changes in
temperature and rainfall. If it's abnormally hot and dry,
expect a drought. If it's abnormally wet, look for higher
Droughts are also widespread and persistent, and at any
time somewhere in the USA will be experiencing a drought.
The same is usually true for floods. The classic
example of this is that during the record Mississippi
floods of 1993, parts of the south east were severely
affected by drought.
One other feature of droughts is that rain still falls.
It's just that there is not enough of it. It's also
common, particularly in shorter lived droughts, that some
parts of an area may be less affected than others. A
couple of local thunder storms can make a big difference if
your farm is under their path.
But let's get on with a definition.
As you may expect, because we are dealing with weather and
climate, there is no single definition. But drought should be
considered as a prolonged lack of expected precipitation.
Hmmmm. To translate a little, expected precipitation is rain
or snow that would normally affect an area at a particular
time of year - the average or norm. If precipitation has
been significantly less than expected for a long period -
at least three months - an area is said to be in drought,
or drought affected.
There are many ways of defining the "significantly less" part.
The most common method used in the US is the Palmer Drought
Severity Index (PDSI), which compares actual rainfall with
the average, with allowance made for temperature.
Types of Drought
There are three generally accepted types of drought, depending
on who's talking about them.
The first is the Meteorological Drought - more or less
as defined above. In a meteorological drought an area becomes
drought affected if precipitation is significantly below
average for several months, and the drought is considered to
exist until precipitation returns to normal. The key here is
the average rainfall - if an area normally gets negligible
rain in summer, the absence of rain this summer will not
mean the area is in drought.
Hydrological Drought goes a step further. A hydrological
drought occurs when an areas water supply is reduced. So an
area of low average rainfall where crops are grown using
irrigation could be experiencing a hydrological drought if
the source of irrigation water dried up, even though local
rainfall was normal.
Hydrological drought usually follows the development of a
meteorological drought. Normally rain or melted snow will
soak into the soil profile, and large volumes of water
can be stored there, particularly if the soil is sandy
or gravelly. This stored groundwater is what keeps
streams flowing after the initial runoff from rain has
dried up. It also provides the water used in shallow
wells, particularly for irrigation on river flats.
But if there is insufficient rainfall to refill this reserve,
its level, identified by saturated soil, slowly drops until
it falls below the stream bed. At this time the stream
stops flowing, and wells begin to dry. At the same time
surface water storage will be drying up, due to both
evaporation and usage.
Agricultural Drought reflects the effects of insufficient
precipitation on crops and grazing land, and more or less
combines the other two definitions. Without adequate water,
whether from rainfall or irrigation, crops are put under
If the soil around the root zone of grasses, grains and
vegetables dries out, they will die. Trees and vines are
put under stress and their growth and production
is hindered. In grazing areas, the grass dies and stock
go begin to starve.
Definitions of agricultural droughts take into account
the form of agriculture in an area, and also the stage
of growth crops are at when rainfall becomes scarce.
Newly planted wheat will be much more severely affected
than an established orchard during the early stages of
Some people identify a fourth form of drought - the
Sociological Drought. Because it's not just
streams, plants and animals that are affected by
drought - it's also the people who own and run the
farms, and the communities that service the area.
Prolonged drought can mean no income from the farms,
and if the farmer is under financial stress, so are
the businesses in the local towns, whose livelihood
depends on serving the needs of the rural community.
The effects of drought on communities and people can
continue long after the drought has broken - some
may never be able to recover. These problems were
strongly evident during the Dust Bowl drought of the
1930s, and formed the basis for John Steinbeck's
classic novel "The Grapes of Wrath".
Many droughts have no obvious cause. A small change in
normal atmospheric circulation may be enough to reduce
rainfall for a few months. this can be critical in an
area where even normal rainfall is barely adequate for
Other droughts, particularly those of regional extent,
may be related to major short term shifts in atmospheric
or oceanic circulation, such as El Nino. Now while El
Nino gets blamed for all sorts of things, some of which
are due to other causes, its part in the start of some
major droughts can be identified.
El Nino is part of a repeated if not regular shift in
oceanic currents, tied very closely to variations in
air mass position. If, as part of an El Nino event,
a warm water mass is replaced by a cooler one, then
air over that water body will pick up less water vapour
And if in the normal course events warm moist air formed
over that part of the sea is responsible for the rainfall in
an adjacent land area, any change to its character will
affect the precipitation on land. Less moisture in the
air mass may cause seasonal rains to fail, with drought the
In either case, local or regional, the affected area is
likely to be influenced by an area of high pressure. This
usually leads to higher daytime temperatures, less
instability in the atmosphere less chance of storm
development, and clear skies.
Regardless of the cause, a drought tends to perpetuate
itself. Once started, the cycle of clear skies, more
hours of sunlight, higher temperatures and higher
evaporation tends to sustain itself, and often the
situation stays the same until it receives a jolt from
a major rain bringing event.
Dust Storm in Kansas
Dust Bowl Drought, 1930s
Agriculturalists now know a lot more about drought
management than in the days of the Dust Bowl droughts
in the 1930s, and the effects of the shorter droughts
can to some extent be countered by a combination of
sound land and financial management, always assuming
that there are a few good seasons between droughts.
Any area will be drought affected at some time in its
history, and that some areas are more drought prone
than others.Particularly in the latter areas,
selection of crops and stock that are more drought
resistant is effective in the long term. Other
strategies include using practices which conserve soil
moisture, and paying attention to stocking rates to
prevent early removal of grass cover.
Hardest of all is to preserve some capital to provide
reserve funds for when normal income drops as the drought
So far no proven and reliable methods of rain making
have been developed, although cloud seeding trials
continue. So for the immediate future we will have to
assume that droughts will always be with us, that they
will cost us money, and that we should prepare for them
as best we can.
Huge 1930s Dust Storm in Texas
Note the size of houses in foreground.
Drought and the Home Weather Station
, droughts are not the most
exciting of weather events to record on a home
The typical feature of a drought is more of the same
- tomorrow is likely to be the same as today, except
maybe hotter. Yet there is always a sense of hope and
Although a drought affected area is likely to be situated
under a stationary high, air masses and pressure systems
are still active elsewhere. Fronts are on the move, but
the inertia of the high is usually too great for it to be
Nevertheless, each approaching front, low or convective
system may just be the one that brings some local relief,
or even acts as the circuit breaker that ends the
drought. And the longer the drought goes on, the greater
the anxiety level as each potentially relieving system
approaches. And unfortunately, the greater the
disappointment when there is no significant change.
But one day .....
Like To Know More?
Firstly, the photos used in this page come from publications
from the 1930s, which are included in NOAA's excellent
As usual in matters related to weather, the place to go
for resources and further information on drought is the
NOAA Drought Page
. Related types of hazardous
weather are included in the
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