Drought is caused by persistent high-pressure air systems that suppress cloud formation and block rainfall from reaching a region. While that’s the immediate trigger, the full picture involves ocean temperature cycles, human land use, climate feedback loops, and rising global temperatures, all of which can start a drought, deepen it, or make it last longer than it otherwise would.
High-Pressure Systems: The Direct Cause
Every drought begins with the same basic atmospheric event: a mass of sinking air, called subsidence, settles over a region and compresses as it descends. That compression warms the air, lowers its relative humidity, and prevents clouds from forming. The result is a dome of high pressure that acts like a lid, deflecting storms and moisture away.
When this pattern lasts a few days, you get a dry spell. When it persists for months or seasons, you get a drought. The extreme drought that hit the United States and Canada in 1988, for example, resulted from exactly this kind of large-scale circulation anomaly that locked in place and refused to move. Regions that sit under semipermanent high-pressure zones year-round, like the Sahara, Kalahari, and Gobi deserts, are essentially in a state of permanent drought for this reason.
Ocean Temperatures and El Niño/La Niña
The temperature of the tropical Pacific Ocean has an outsized influence on where rain falls across much of the world. During a La Niña phase, when surface waters in the central and eastern Pacific cool below normal, rainfall patterns shift in ways that starve certain regions of moisture. In the U.S. Southwest, La Niña events were strongly associated with drought from 1948 through 1977. During El Niño phases, when those waters warm, the same region tends to receive above-average rain.
These cycles don’t just determine whether a drought happens. They shape how frequent and intense droughts become over decades. Modeling studies comparing 480-year simulations with and without these ocean oscillations show that the El Niño/La Niña cycle meaningfully alters both the frequency and severity of drought in affected regions. That said, the relationship isn’t perfectly reliable. The connection between Southwest U.S. precipitation and Pacific Ocean temperatures weakened noticeably after 1999, and some recent droughts have developed even under neutral conditions when neither El Niño nor La Niña was active.
How Rising Temperatures Make Droughts Worse
Higher air temperatures don’t directly cause a drought, but they dramatically intensify one that’s already forming. Warmer air pulls more moisture out of soil and plants, a process that accelerates the transition from a simple rainfall shortage into a full agricultural or water-supply crisis. Research on the contiguous United States shows that the Midwest and Southeast, two of the country’s most important farming regions, are experiencing the largest increases in drought intensification driven by rising temperatures and declining precipitation.
The practical effect is that droughts are becoming more severe even when rainfall deficits aren’t much worse than historical norms. Under high-warming scenarios projected through 2050 to 2080, the probability of mild agricultural drought actually decreases, but the probability of extreme drought categories climbs across all U.S. regions. In other words, moderate droughts are being replaced by more intense ones. Some western ecoregions have already seen drought exposure increase by up to 200% compared to the mid-20th century, and projections suggest average annual drought exposure could rise by 377% by the second half of this century.
Deforestation and the Water Cycle
Forests are not passive bystanders in the water cycle. Trees pull water from the soil and release it into the atmosphere through their leaves, a process called transpiration. That moisture rises, forms clouds, and falls as rain, often over the same forest or regions downwind. When large areas of forest are cleared, this moisture recycling breaks down. Less water vapor enters the atmosphere, which means less precipitation locally and for areas that depend on moisture carried by wind from forested regions.
Five global climate models consistently show that deforestation decreases evapotranspiration, reduces leaf area, and shifts the balance so that more water runs off the surface immediately rather than cycling back into the atmosphere. Most deforested areas in these simulations experienced reduced rainfall as a direct consequence. This creates a feedback loop: fewer trees means less rain, which stresses remaining vegetation, which further reduces moisture recycling.
Human Water Use as a Drought Multiplier
Natural rainfall deficits tell only part of the drought story. How people manage and consume water can make a drought significantly worse, or in some cases, partially cushion its impact. California’s severe 2014 drought illustrates both sides. In Southern California, reservoir operations during low-flow periods offset roughly 50% of the drought deficit. But in the Central Valley, heavy irrigation demands increased drought duration by 50% and the total water deficit by 50 to 100%. Human water consumption in that agricultural region more than doubled the probability of an extreme drought event compared to what natural variability alone would have produced.
Groundwater over-extraction compounds the problem. When surface water runs short, farmers and cities pump more from underground aquifers, which can take years or decades to recharge. This creates a hydrological drought, where rivers, lakes, and underground water supplies drop to critically low levels, even after rainfall eventually returns to normal.
Dry Soil Feeds on Itself
Once a drought takes hold, the land itself can prevent recovery. Wet soil promotes evaporation that adds moisture to the lower atmosphere, encouraging moderate cloud growth and eventually more rain. Dry soil does the opposite. With little moisture to evaporate, the ground heats up, the boundary layer of air above it grows rapidly but stays dry, and the conditions needed for rain-producing clouds don’t develop. This soil-moisture feedback loop means that a drought can sustain and deepen itself even without any ongoing atmospheric anomaly pushing it along.
Plants hit their own tipping point during this process. Globally, the critical soil moisture threshold where plants begin experiencing water stress averages about 0.19 cubic meters of water per cubic meter of soil. Below that level, plants can no longer extract water efficiently through their roots, transpiration drops, and daytime land surface temperatures spike. In arid ecosystems, that threshold is even lower (around 0.12), meaning plants there are adapted to drier conditions but still face a hard limit. In humid ecosystems, the threshold sits higher (around 0.26), so plants accustomed to abundant water feel the stress sooner.
Flash Droughts: Weeks, Not Months
Not all droughts build slowly. Flash droughts develop over a matter of weeks when extreme atmospheric conditions converge: little or no rainfall, above-normal temperatures, strong winds, and clear skies. If those conditions persist for several weeks, soil moisture and vegetation health can collapse rapidly. Researchers define a flash drought as requiring a minimum development period of about 30 days, with soil moisture dropping below the 20th percentile of normal values for that time of year.
Flash droughts are particularly dangerous for agriculture because they outpace the typical monitoring and response timeline. A farmer watching seasonal forecasts for a gradual dry spell can be caught off guard when soil moisture crashes in a month. These events are becoming a growing concern as rising temperatures increase the atmosphere’s ability to pull moisture from soil and plants at faster rates.
How Droughts Are Measured
Climatologists track drought using indices that combine temperature and precipitation data to estimate how dry conditions are relative to what’s normal for a given location. The most widely known is the Palmer Drought Severity Index, which simulates a water balance for the soil: how much rain fell, how much water evaporated, how much soaked into the ground, and how much ran off. The gap between actual precipitation and what would be “climatically appropriate” for existing conditions produces the drought score.
The original version of this index estimated evaporation using only temperature and day length, which works reasonably well in many settings but misses important factors like wind speed, solar radiation, and humidity. Modern updates use more complete energy-balance calculations that account for all of these variables, giving a more accurate picture of how much moisture the atmosphere is actually pulling from the landscape. This matters because in some regions, changes in wind patterns or cloud cover drive evaporation more than temperature alone.