In simplest terms, drought is a prolonged dry period due to a chronic shortage of rainfall in a region. This shortfall results in hydrologic imbalance, which means a region's precipitation can't keep up with the local rate of evaporation (the vaporization of water) and transpiration (the movement of water through soil and plants).
Drought strikes a region for complex, myriad reasons, but we can put our finger on at least one direct and immediate cause: Sinking air leads to high-pressure zones, which result in lower humidity, fewer clouds and less precipitation [source: Folger]. Air pollution doesn't do drought any favors either.
Over the past decades, meteorologists have linked drought patterns in the United States to abnormally cool sea surface temperatures in the Pacific tropics -- also known as La Niña conditions. Scientists continue to study this correlation, but we're learning that La Niña can trigger droughts in North America, Europe and Asia [source: Folger].
In a larger geographic and seasonal sense, you can look at drought in four key categories. First up, there's permanent drought, which you find in desert climates where severe water shortage is the norm. Next there's seasonal drought, which refers to a region's seasonal dry spells -- such as the dry seasons experienced in the tropics.
These first two drought categories don't offer any surprises: The dry season is dry, and the desert is a desert. But the next category, unpredictable drought, does. The term refers to the typically brief and irregular dry spells often experienced in humid or subhumid climates.
Finally, there's invisible drought, so named because rain showers tend to mask its severity. It occurs when even frequent precipitation can't keep up with the sweltering summer heat and its correspondingly high levels of evaporation. It looks like you're getting enough rain, but it's actually evaporating too fast to make a difference.
The U.S. National Weather Service identifies four different operational definitions that also illustrate the escalating severity of drought.
First up, there's meteorological drought, which compares a bout of dryness to typical regional and seasonal weather patterns. A period like this might only interest local weather bugs as it hasn't yet affected the economy or even become all that apparent to the naked eye.
Next up, there's agricultural drought, which focuses on specific crop needs. If a prolonged dry period hurts local crops, then under this definition, drought has reared its ugly head. And indeed, agriculture is generally the first economic sector affected by drought. Life outside the farm may just carry on like normal, but at this point, the weather bugs aren't the only ones shaking their head at the empty rain gauge.
When hydrological drought sets in, everyone else begins to take notice. Both surface and subsurface water supplies drop below normal. This level of drought generally makes itself known in reduced stream flow rates and plummeting lake, reservoir and groundwater levels.
Finally, there's socioeconomic drought, in which extreme dryness begins to affect everyone. It might mean water restrictions, or it might escalate into food shortages, reduced tourism, hampered shipping or even displaced populations.
In underdeveloped portions of the world, drought can easily escalate into starvation, outbreak of disease, panic, political unrest and armed conflict.