Defining a drought is difficult. A drought occurs when there is significantly less rainfall than would normally be expected. For different countries, and for different regions within a country, normal rainfall varies greatly.
What might be considered a drought by one country would be considered an inconvenience by another. Generally, the word drought applies to an extended period without rainfall which has a detrimental affect on population and resources.
There are some drought indices which are used internationally to give some idea of drought. This might be for a local area or on a wider scale, for the entire country perhaps. These include:
Percent of normal - Calculated by dividing real rainfall by what is normally expected and multiplying by 100%. This tends to be used for seasonal or monthly forecasts for a small area.
Standardized precipitation Index (SPI) - Developed in 1993, this calculates rainfall, or lack of, over a variety of timescales, a year, a month, 6 months, etc. In use by the National Drought Mitigation Center in the U.S.
Palmer Drought Severity Index (PDSI) - This was the first comprehensive drought index developed in the U.S. This index is based on a soil moisture algorithm.
Crop Moisture Index (CMI) - An offshoot of the Palmer system and used for short-term predictions rather than major droughts.
These and other similar systems are described in greater depth at this link.
There are also certain types of drought. These include:
Meteorological - These are droughts specific to a region and based on dryness, i.e. how dry it is in comparison to the norm.
Agricultural - Linked to meteorological drought and concerning the impact of decreased rainfall on the agricultural resources of a region.
Hydrological - Usually related to meteorological droughts, concerned with natural water systems such as streams and rivers. Humanity can have an impact on this kind of drought, especially if there is some form of damming or other activity that affects the water courses.
Socioeconomic drought - This is directly related to all three of the aforementioned types of drought. It is defined as happening when the demand for economic goods outstrips supply due to drought conditions.
More about all of these variations can be found at this link.
Drought has multiple effects. Images of starving children and cracked, parched landscapes come readily to mind. Hunger and famine are probably the most visible effects when we see drought reported on our televisions and in newspapers, but there are other effects. These include:
Thirst - People can and do die of thirst during droughts.
Wildfires - Perhaps less reported but wildfires will burn through fields and forests alike when everywhere is tinder dry. Crops are lost, people are forced out of their homes and off their lands, and animals are killed and displaced.
Disease - When there isn't enough water people become sick. Sanitation fails, what little water is available is often contaminated and disease spreads as quickly as the wildfires.
Displacement - People will move in search of water. They will abandon lands, failed crops and burned out homes, taking to the road in search of water for their families and their animals. This, in turn, places more strain on the areas that these people migrate too, exacerbating the problem.
Unrest and war - Lack of water leads to fights over what water resources are available. People become scared and angry which can easily lead to civil unrest, especially if it appears that there is unfairness in the distribution of water resources. This can lead to war as unrest escalates.
Drought is a terrifying prospect and yet is it actually, for the most part, a naturally occurring phenomenon and happens regularly. It is only because we have been deluged with images of disastrous droughts in Africa that we have come to think of droughts as major disasters that happen infrequently. They happen all the time, but usually on a much smaller scale.