Drought is one weather extreme that’s defined not by what happens but by what doesn’t happen — namely, rain. It’s as if nature forgot that it was supposed to provide a certain amount of moisture each year. The meteorological spigots are shut down, and nobody can say just when they’ll be turned on again, although El Nino may provide a hint.

Drought is usually pictured through its effects: withered crops, empty reservoirs, dust storms. The most poignant impacts are on people, of course. Few weather-related images are as burned into our consciousness as the worry-lined, dust-scratched faces of America’s drought-stricken 1930s and the hollow-eyed Ethiopians who faced famine in 1985. If a flood can destroy people in minutes, a drought is far less merciful in accomplishing the same task, and far slower in bringing relief.

What is a drought?

As plain-as-day as a drought may seem, academics and scientists have wrangled long and hard over a precise definition. Recently they’ve coined a set of criteria for drought types and agreed that a given drought may only meet some of these.

Meteorological drought is the obvious definition: the rains stop, or diminish to a certain degree, for some length of time. In many places it doesn’t have to be bone-dry to register as a drought, especially where it normally rains year round, such as London or Fiji. In these locales, a drought might mean it’s simply raining less hard when it does rain. To assess a drought here, you might note how long you’ve gone without getting what you consider to be a decent day’s rain. In more arid climates, as in Denver or Melbourne, you could evaluate how many days it’s been since it’s rained at all. This is especially true in desert climates, where a single day can produce the amount of rain you’d normally expect in a year.

However you gauge it, what matters when assessing a drought is how much rain a place has received compared to the average. A year with 500mm/20in of rain would be manna for parts of Mongolia but a disaster for Miami Flora and fauna adapt to the long-term average rainfall in a given locale, so it’s the departure from that average that takes a toll. Cities that function well in a nonnal rain cycle may be in big trouble when a drought strikes, especially if they’ve expanded during times when water was plentiful.

Since drought affects us by drying up our water supply and damaging our crops, it can also be viewed through the lens of hydrology and agriculture. How low is the water level in the reservoir that your city relies on for water? Are the lands upstream getting enough rain to help recharge the system? Crops can be affected in complex ways by drought. If a dry spell is just starting, the topsoil may be parched but the deeper subsoil still moist enough to keep an established crop going.

Conversely, a long-standing drought may break just in time to help germinating plants get started, even if the sub-soil remains dry.

When drought becomes destiny

There’s a certain self-perpetuating aspect to drought. A persistent weather pattern that inhibits rainfall for a few weeks is enough to get a drought started Once it’s going, a more subtle interaction between land and air takes place. The dry ground heats up quickly and sends its heat to the atmosphere more readily. As green vegetation dies off, the brighter ground reflects even more heat than before. This may help sustain the giant, cloud-free domes of air so prevalent across a drought-stricken region. However, the relative importance of this land-air interaction in keeping a drought going isn’t totally clear.

Another factor, one that’s become a boon to drought prediction, is the discovery of the El Nirto/La Nifta influence on world climate. These cycles, tied to the tropical Pacific, tend to create a set of preferred weather patterns that can persist for one or two years around the world. El Nino is strongly associated with drought across northeast Brazil and South Africa, and it commonly diminishes the strength of the Asian monsoon, thus triggering drought from India to Indonesia and northern Australia. La Nina tends to cause drought in places where El Nino encourages heavy rain, such as eastern equatorial Africa and the central Pacific; it’s also been tied to drought across the western and central United States. These relationships aren’t iron-clad, but they do serve as powerful tools in helping farmers and policymakers manage their water resources better.

Does it follow the plough?

Places like the central Sahara, where it virtually never rains, can’t really experience a true drought. However, the margins around these barren lands can go from long-term drought into a more ominous, semi-permanent state of aridity, perhaps abetted by the actions of society. Desertification, as this is called, jumped onto the political landscape in the 1960s and 1970s, when the Sahel of western Africa slid into a drought that lasted off and on for more than two decades. In the moist years beforehand, government policy had encouraged farmers to cultivate northward toward the desert’s edge. In turn, that pushed nomadic farmers and their herds into even more marginal territory further north. When the drought hit, both grazing and farming were too intensive for their land’s now fragile state. Wind and erosion took hold, and the zone of infertile land grew southward.

It wasn’t the first time land use had been fingered as a partner to drought The catastrophic Dust Bowl across the central US in the 1930s — made famous in John Steinbeck’s The Grapes of Wrath — followed several decades of giddiness as Europeans settled what was once called the Great American Desert. Despite the warning signs of several intense droughts during the late 1800s, the land was green and fruitful by the 1920s. But the above-average rains abruptly stopped in the early 1930s, and the overworked soil soon blew away in choking dust storms that paralysed towns and cities from Texas to the Dakotas. The drought’s timing — just as the Great Depression was setting in — seemed especially cruel, but it was the activist government of the New Deal that ended up educating farmers on land use and creating such innovations as shelter belts of trees to break the fierce prairie wind Another southern-plains drought was nearly as intense in the 1950s, but it caused far less damage, thanks in large part to the New Deal. Some of the lessons were transplanted to Africa; shelter belts of neem trees reduced the impact of the 1970s Sahel drought in parts of Niger.

Boosters of expansionist farming once thought that “rain follows the plough” — if a marginal land were cultivated, then water would evaporate above the greened land and help keep it moist As US political scientist Michael Glantz has noted, the events of the twentieth century pointed toward a more bleak aphorism: drought follows the plough. We still can’t fathom how long a long-term drought influenced by human intervention might last, even in the case of the Sahel, where more regular rains had returned by the 1990s. Agricultural practices may expose the vulnerability of a given land, but the atmospheric forces that induce drought and take it away could be the main player after all. Humans may yet help trigger more extensive and intense droughts in another way: by warming the global atmosphere.