Prof. A. Balasubramanian
Centre for Advanced Studies in Earth Science,
University of Mysore, Mysore
Research in the early 1980s uncovered more
than 150 published definitions of drought. The
definitions reflect differences in regions, needs,
and disciplinary approaches.
A drought is a period of below-average
precipitation in a given region, resulting in
prolonged shortages in its water supply.
Drought refers to a prolonged period of
abnormally low rainfall, leading to a shortage of
A drought can last for several months or years.
Sometimes, droughts are declared for a full
district for a few years.
It can have a substantial impact on
the ecosystem and agriculture of the affected
region and harm to the local life and economy.
Factors Responsible :
There are several factors responsible for
The effects of drought will be disastrous.
This lesson highlights the characteristics, causes
and effects of droughts, methods of monitoring
and the methods of mitigation.
Drought is a temporary aberration from normal
It can vary significantly from one region to
another region. Drought is different than aridity.
Aridity is a permanent feature of climate in
regions where low precipitation is the norm, as
seen in any desert.
During dry and hot weather periods, it is
common to find dry and cracked earth surface
without even a single shed of water or wet
Lakes, rivers, and streams may as well run dry.
Well, these are the typical earth conditions that
define drought in layman terms.
Annual dry seasons in the tropics significantly
increase the chances of a drought developing
and subsequent bush fires.
Periods of heat can significantly worsen drought
conditions by hastening evaporation of water
Human factors, such as water demand and water
management, can exacerbate the impact that
drought has on a region.
2.0 Characteristics of Droughts:
Drought is a normal, recurrent feature of
climate that occurs in virtually all climate
zones, from very wet to very dry.
Many plant species, like Cactaceae (or cacti),
have adaptations like reduced leaf area and
waxy cuticles to enhance their ability to tolerate
Some others survive dry periods as buried
Semi-permanent drought produces arid biomes
such as deserts and grasslands.
Prolonged droughts have caused mass
migrations and humanitarian crises.
Most arid ecosystems have inherently low
Like other hazards, droughts can be
characterized in terms of their severity, location,
duration and timing.
Droughts can arise from a range of
hydrometeorological processes that supress
precipitation and/or limit surface water or
groundwater availability, creating conditions
that are significantly drier than normal or
otherwise limiting moisture availability to a
potentially damaging extent.
3.0 Drought is a disaster:
A drought is a natural event, caused by
other weather events like El Nio and high-
Drought can also be triggered
by deforestation (people cutting down forests),
by global warming, and by diverting rivers or
Drought is a disaster which usually takes place
It is often difficult to decide when a drought
started and sometimes when it ends too. Its
effects often build up slowly over a long period
of time and may last from months to years after
4.0 Causes of drought
4.1 Precipitation deficiency:
Drought comes due to very low rainfall and due
to recurring failure of monsoons.
Drought can be triggered by a high level of
reflected sunlight and above average prevalence
of high pressure systems, winds carrying
continental, rather than oceanic air masses.
Prolonged period of evapotranspiration can
worsen the situation and increase the drought
4.2 Atmospheric Water Vapor:
Droughts commonly are referred to as "dry" in
the sense that not only does less precipitation
fall, but also the air is drier than usual.
4.3 Atmospheric Circulation Patterns:
A drought is associated with persistent or
persistently recurring atmospheric circulation
4.4 Sources of Moisture:
Heated air pulls moisture from the soil,
allowing it to form clouds and return to the
Earth in the form of precipitation. If weather
patterns shift enough to create an area with little
rainfall over a period of time, there will not be
enough moisture in the soil to draw up into the
air to create clouds.
Moisture continues to be removed from the soil
on a daily basis, and no rain clouds will be able
to form to replace the moisture.
4.5 Dry season :
The dry season greatly increases drought
occurrence, and is characterized by its low
humidity, with watering holes and rivers drying
Many grazing animals are forced to migrate due
to the lack of water and feed to more fertile
4.6 El Nio:
Drier and hotter weather occurs in parts of
the Amazon River Basin, Colombia,
and Central America during El Nio events.
Winters during the El Nio are warmer and
drier than average conditions in the Northwest.
4.7 Climate change:
Activities resulting in global climate change are
expected to trigger droughts with a substantial
impact on agriculture, throughout the world,
and especially in developing nations.
Climate change affects a variety of factors that
are associated with the droughts.
When considering the relationship of drought to
climate change, it is important to make the
distinction between weather and climate.
Weather is a description of atmospheric
conditions over a short period of time, while
climate is how the atmosphere behaves over
relatively long periods of time.
The individual drought periods can be
understood as discrete weather events.
Climate changes occur over longer periods and
can be observed as changes in the patterns of
4.8 Erosion and Human Activities:
Human activity can directly trigger
exacerbating factors such as over farming,
excessive irrigation, deforestation,
and erosion adversely impact the ability of the
land to capture and hold water.
In arid climates, the main source of erosion is
wind. Erosion can be the result of material
movement by the wind.
5.0 Types of Droughts:
5.1 Meteorological drought:
Meteorological drought is defined usually on
the basis of the degree of dryness (in
comparison to some normal or average
amount) and the duration of the dry period.
Meteorological drought is brought about when
there is a prolonged time with less than average
Meteorological drought usually precedes the
other kinds of drought.
Some definition about meteorological drought
identify periods of drought on the basis of the
number of days with precipitation less than
some specified threshold.
This measure is only appropriate for regions
characterized by a year-round precipitation
regime such as a tropical rainforest, humid
subtropical climate, or humid mid-latitude
A drought in terms of meteorology takes into
account deficiencies in measured precipitation.
Each year's measurements are then compared to
what is determined as a "normal" amount of
precipitation and drought is determined from