In geographical studies, a drought is defined as an extended period of deficiency of water supply that the region is subjected to. While the same is generally attributed to low precipitation - i.e. less than average rainfall/snowfall over a course of time to be precise, there do exist several other factors which are considered responsible for a drought. It is important to take the causes and effects of drought into consideration to understand the severity of this issue, as understanding one makes it easier for you to understand the other. That being said, one has to get well-versed with the effects of drought on the planet before getting into the details on what causes it.
Overview of Droughts
Drought can be assessed on the basis of amount of precipitation (from the meteorologists point of view), on the basis of assessment of water sources in the said region (from the hydrologists point of view) and on the basis of difficulty faced by agricultural crops in growing owing to lack of water (from the farmers point of view). Similarly, the intensity of a drought differs from one geographical location to another - on the basis of what has triggered the same.
While lack of precipitation can trigger a drought anywhere, the interior regions are most vulnerable to such harsh conditions as rain bearing clouds fail to make it to these regions. Even though droughts are not considered natural disasters as such, extended period without precipitation can cause havoc on the planet as water is very important for all living things on the planet. If droughts continue for extended period, it will result in severe effects on all the living things which inhabit the planet.
The myth that drought only affects regions which are dependent on agriculture has become quite popular with time. However, the fact is that it can have a serious impact on economically well-to-do nation like the United States and Europe as well. While the developed nations might not face food shortage or other such direct problems, a drought can have its implications on the economy of the nation - which will reflect in the form of price rise and related economic issues. This makes it all the more important for a person to understand what causes drought, and how it affects the country as a whole, instead of just the region which is affected.
Causes of Drought
There is no questioning the fact that a drought is triggered by lack of precipitation over a course of time, but there has to be some underlying cause for this alteration in precipitation pattern, and this underlying cause has to be taken into consideration when discussing drought causes. Precipitation is one of the three attributes of the water cycle, while the other two are evaporation and condensation. If either of these steps of water cycle get disturbed, it results in severe alterations in other steps involved and eventually triggers a drought.
In most of the drought prone regions, the process of condensation is severely hampered as a result of the prevailing high pressure conditions which are not conducive for cloud formation. While low pressure system generally replaces high pressure system as the latter moves on, some conditions - such as presence of jet streams or cold and hot water currents in the ocean, etc. can stall it and inhibit formation of clouds.
Water vapor is a basic necessity when it comes to formation of clouds, and if the wind fails to carry required amount of moisture, it can hamper the cloud formation process. In south east Asia, water vapor that is formed over the Indian Ocean is brought on to the land by the winds blowing from the southwest direction. The winds in question here need to be strong enough to take the moisture from Indian Ocean across the Indian subcontinent. If they are not strong enough, they fail to carry the required amount of moisture, which - in turn, stalls condensation and eventually hampers the process of precipitation.
Many a time we get to see that the windward side of the mountain receives abundant rain, while the leeward side of the same mountain experiences severe drought. This generally happens when these mountains block the moisture-laden winds from going to the other side. As they precipitate on the windward side, they become light as a result of which they rise and cross over. However, by the time this happens most of the moisture content in them is lost. That explains why windward side of the mountain is lush green, while the leeward side is a barren desert.
The ability of soil to capture and hold water is hampered as a result of foul agricultural practices, deforestation, erosion of top layer of soil, etc. and these changes have the tendency to trigger a drought in the long run. Recent reports also associate droughts with climate change and global warming, whilst citing that their occurrences have been increasing with rising global near-surface temperature.
While droughts are natural occurrences which are attributed to certain environmental factors, the fact that they worsen over the course of time makes it difficult for us to assess the damage they do. Droughts are considered to be one of the three most severe threats to humans on the planet - alongside famine and floods. A look in the history, and we find quite a few examples of the havoc wreaked by them on the planet, and the Dust Bowl i.e. the drought which occurred in the Great Plains of the USA between 1931 and 1938 (as a result of foul agricultural practices), is one of the best - or should we say the worst, example of the same.