How Droughts Form: Weather Patterns, Soil Moisture, and Long-Term Causes

How Droughts Form

A drought begins when an area receives less rain or snow than its climate normally provides. The shortage must continue long enough to reduce available water in soil, rivers, reservoirs, snowpack, or aquifers. A few rainless days may produce dry weather, but drought develops over weeks, seasons, or years.

The local climate sets the threshold. A month with 25 millimeters of rain may be normal in a desert and unusually dry in a tropical region. Scientists therefore compare current conditions with long-term records for the same place and season.

Most droughts follow a recognizable sequence:

1. Rainfall or snowfall falls below the seasonal norm.
2. Sunny, warm, dry, or windy weather removes water from plants and soil.
3. Topsoil dries, followed by deeper root-zone soil.
4. Plant growth slows and crop water stress appears.
5. Streamflow, reservoir storage, and groundwater recharge decline if the shortage continues.
6. Water demand begins to exceed the supply available from natural and managed sources.

The speed of this sequence varies. Some droughts build slowly across several seasons. A flash drought can intensify within weeks when low rainfall combines with heat, wind, strong sunlight, and low humidity.

The Water Balance Behind Drought

Drought formation depends on the balance between incoming water and outgoing water. Precipitation supplies the land through rain, snow, sleet, and other forms of atmospheric moisture. Water then leaves through runoff, drainage, evaporation, plant transpiration, and human use.

A simplified land-water balance can be expressed as:

Change in stored water = precipitation − evapotranspiration − runoff − deep drainage

Stored water includes soil moisture, snow, surface water, and groundwater. When losses remain greater than incoming moisture, storage falls. Drought becomes more likely as this deficit accumulates.

One storm does not always correct the balance. Heavy rain may run quickly into rivers without reaching deep soil or aquifers. A gentle series of storms spread over several weeks can sometimes restore more usable water than one intense downpour.

Weather Patterns That Start Drought

Persistent High-Pressure Systems

High atmospheric pressure encourages air to sink. As it descends, the air warms and becomes less favorable for cloud formation. When a high-pressure ridge remains over a region, storms may be diverted around it for days or weeks.

Persistent ridges often produce clear skies and strong solar heating. The land loses water while receiving little precipitation. If the pattern remains stationary, an ordinary dry spell can develop into meteorological drought.

Atmospheric Blocking

A blocking pattern occurs when a large pressure system disrupts the usual west-to-east movement of weather systems. Storms may stall upstream, travel around the blocked area, or follow a different path.

The affected region can remain dry even while nearby areas receive repeated rain. Blocking patterns may also support heat waves, which accelerate moisture loss from the land.

Jet Stream and Storm-Track Shifts

The jet stream helps steer low-pressure systems and their associated rain or snow. A northward, southward, or unusually wavy jet stream can move the storm track away from its expected seasonal position.

Regions left on the dry side of the storm track may miss several precipitation events in succession. The timing matters: losing rain during a crop’s flowering stage or missing winter snow in a mountain watershed can have greater effects than the same deficit during a less water-sensitive period.

Changes in Ocean Temperatures

Ocean temperatures influence atmospheric pressure, winds, and rainfall far from the sea. The best-known example is the El Niño–Southern Oscillation, or ENSO, in the tropical Pacific.

El Niño and La Niña shift tropical rainfall and alter large-scale circulation. Their drought effects are not uniform. The same ENSO phase may favor dry conditions in one region and wet conditions in another. Its influence also changes with season, event strength, and interaction with other ocean patterns.

Other ocean-atmosphere variations include the Indian Ocean Dipole, the Pacific Decadal Oscillation, and Atlantic temperature patterns. These processes can change the probability of drought, but they do not guarantee a particular local result.

Weak or Delayed Seasonal Rains

Many regions depend on a short wet season. A delayed monsoon, an early end to seasonal rain, or long breaks between storms can sharply reduce annual water supply.

Annual rainfall totals can also hide damaging timing. A region may finish the year near its normal total after receiving heavy rain outside the growing season, while farms and ecosystems still experience drought during the months when water was needed.

Low Snowfall and Early Snowmelt

Mountain snowpack stores winter precipitation and releases it during spring and summer. This delayed flow supplies rivers, reservoirs, farms, cities, and aquatic habitats.

Snow drought can form in two ways:

• Dry snow drought: Too little winter precipitation falls.
• Warm snow drought: Precipitation falls mainly as rain rather than snow, or accumulated snow melts earlier than usual.

A rainy winter can therefore leave a watershed short of summer water if little moisture remains stored as snow.

How Soil Moisture Controls Drought Development

Soil moisture is the water held within spaces between soil particles. It links weather conditions with plant health, runoff, groundwater recharge, and local surface temperature.

Rain or melting snow may follow several paths after reaching the ground. Some water infiltrates the soil, some runs across the surface, and some evaporates. Water that enters the soil may remain near the surface, move into the root zone, drain toward groundwater, or be taken up by vegetation.

Topsoil and Root-Zone Moisture

Topsoil reacts quickly to recent weather. A few sunny and windy days can dry the upper layer after rain. Root-zone soil extends deeper and supplies established plants, so it usually changes more slowly.

Dry topsoil can delay seed germination and planting. When dryness reaches the root zone, crops, grasslands, shrubs, and trees have less water available for growth and cooling.

Field Capacity and Wilting Point

Field capacity describes the moisture remaining after excess water has drained from saturated soil. Permanent wilting point is the moisture level at which a plant can no longer extract enough water to recover from wilting.

Plant-available water lies between these two levels. Soil can still contain water below the wilting point, but that water is held too tightly for many plants to use.

Why Soil Type Matters

Soil depth, slope, vegetation, root structure, previous land use, and the intensity of rainfall also affect storage. Two fields receiving the same amount of rain may develop different levels of agricultural drought.

Evapotranspiration

Evapotranspiration combines evaporation from soil and water surfaces with transpiration from plants. Temperature, solar radiation, wind, humidity, plant cover, and available soil water control its rate.

Warm air with low humidity creates strong atmospheric demand for moisture. Wind replaces moist air near leaves and soil with drier air, allowing water loss to continue. Bright sunlight supplies the energy needed for evaporation.

Potential evapotranspiration describes how much water could be transferred to the atmosphere if moisture were freely available. Actual evapotranspiration declines once soil becomes too dry to meet that demand.

The Soil-Moisture Feedback

Moist soil uses part of the Sun’s energy to evaporate water. This process cools the land surface. Once soil dries, less energy goes into evaporation and more heats the ground and nearby air.

The warmer surface raises evaporative demand and can place plants under further stress. Reduced vegetation may then provide less shade and transpiration. This soil-moisture feedback can strengthen heat and dryness after drought has begun.

Flash Drought Formation

A flash drought develops or intensifies much faster than a conventional seasonal drought. It often starts when rainfall decreases while temperatures, wind, sunlight, and atmospheric moisture demand rise together.

Vegetation may initially release more water through transpiration as it responds to heat and sunlight. Root-zone moisture then falls rapidly. Plants reduce water loss by closing tiny leaf openings called stomata, which also limits photosynthesis and growth.

Flash drought is especially damaging during sensitive crop stages because fields can move from adequate moisture to severe plant stress before irrigation plans or water allocations are adjusted.

Rapid soil-moisture loss is the defining physical signal of many flash droughts. Rainfall records alone may not reveal how quickly conditions are changing.

Types of Drought and Their Usual Order

These categories often overlap. Meteorological drought commonly appears first, followed by declining soil moisture. Hydrological effects may arrive later because reservoirs and aquifers can buffer a short precipitation shortage.

The order is not fixed. A region beginning the season with low reservoir storage or depleted groundwater may experience water-supply trouble before severe plant stress appears.

Why Some Droughts Last for Years

Repeated Seasonal Rainfall Deficits

A wet month can interrupt a drought without ending it. Multi-year drought forms when one dry season follows another and the intervening precipitation cannot replace accumulated losses.

Reservoirs, deep soil, mountain snowpack, and aquifers carry a water-storage memory from one season into the next. Low storage at the start of a new dry period allows impacts to appear sooner.

Slow Groundwater Recharge

Aquifers are replenished when water infiltrates below the root zone and reaches saturated layers underground. Recharge may take months, years, or longer, depending on geology, soil, vegetation, and the depth of the water table.

During prolonged drought, less water reaches the aquifer while pumping may rise to replace reduced surface-water supplies. Groundwater levels can continue falling even after rainfall returns.

Large-Scale Climate Variability

Ocean and atmospheric patterns can persist or recur across several seasons. Their combined effects may repeatedly steer storms away from the same broad region.

Long droughts rarely have one isolated cause. A persistent circulation pattern, warm ocean conditions, low snowpack, depleted soil moisture, and high evaporative demand may reinforce one another over time.

Rising Temperatures

Human-caused climate change does not affect drought in the same way everywhere. Regional rainfall trends differ, and some places may receive more annual precipitation.

Warming can still make drought more severe by raising atmospheric moisture demand, increasing evaporation where water is available, reducing snow storage, and advancing spring snowmelt. Plants may use soil water earlier in the season, leaving less moisture for summer.

Climate change can also alter storm tracks and precipitation timing. Its influence must therefore be assessed by drought type, season, and region rather than through a single global rule.

Human Activities and Water Shortage

Water use does not usually create the atmospheric pattern responsible for meteorological drought. It can, however, intensify agricultural, hydrological, ecological, and socioeconomic drought.

Groundwater Pumping

When withdrawals exceed recharge, aquifer levels fall. Wells may need to reach greater depths, springs may weaken, and groundwater support for rivers can decline. A later rainfall deficit then begins against an already depleted supply.

Irrigation and Reservoir Operations

Irrigation can protect crops from short dry periods, but it transfers water from rivers, reservoirs, or aquifers. Heavy demand during a long drought may reduce the reserves available for later seasons.

Reservoirs can delay water shortages by storing wet-season flow. Their effectiveness depends on starting storage, evaporation, inflow, operating rules, and competing demands.

Land-Cover Change

Deforestation, overgrazing, soil erosion, drainage of wetlands, and urban development alter infiltration, runoff, shade, and plant water use. Compacted or sealed surfaces may send rainfall rapidly into drains and rivers instead of allowing it to enter the soil.

Land management can change how strongly a landscape responds to dry weather. It should not be confused with the atmospheric cause of a rainfall deficit.

How Drought Is Measured

No single measurement describes every drought. Monitoring systems combine weather observations with data from soil, vegetation, snow, rivers, reservoirs, and groundwater.

Indices must be read at the correct timescale. A one-month precipitation index can detect rapid drying, while a 12- or 24-month index is more useful for reservoir and groundwater conditions.

How Soil Moisture Is Observed

Ground sensors measure moisture at specific locations and depths. Models estimate soil water using precipitation, temperature, vegetation, soil properties, and other inputs. Satellites use microwave observations to estimate moisture across large areas, though many instruments are most sensitive to the upper few centimeters of soil.

Combining these methods gives a fuller picture. A ground sensor offers local detail, while satellite and model data reveal the regional pattern.

Why Heavy Rain May Not End a Drought

Drought recovery depends on where the missing water is stored. Surface soil can respond within hours, while reservoirs and aquifers may need sustained precipitation across several seasons.

• Dry topsoil may absorb the first rainfall before much water reaches streams.
• Intense rain can exceed the soil’s infiltration rate and become runoff.
• Water-repellent soil after fire or extreme drying may produce rapid runoff.
• Warm weather can return part of the new moisture to the atmosphere.
• Rain falling outside a reservoir’s watershed may not improve its storage.
• Groundwater recovery may lag behind surface conditions.

A drought can end meteorologically while hydrological drought continues. Likewise, a river may rise after a storm even though root-zone soil remains dry across nearby farmland.

Dry Spell, Aridity, Drought, and Desertification

These terms describe different conditions and should not be treated as synonyms.

Dry spell

A short period with little or no precipitation. It may end before water shortages develop.

Aridity

The usual long-term dryness of a climate, such as that of a desert.

Drought

A temporary departure from the expected water conditions of a region.

Desertification

Persistent land degradation in dry regions caused by climatic variation, human activity, or both.

A desert can experience drought when it receives less water than its already low norm. A humid region can also enter drought after an unusually dry season.

What Determines Drought Severity?

Drought severity reflects more than the total amount of missing precipitation. Scientists examine duration, intensity, geographic extent, timing, temperature, stored water, and water demand.

A moderate rainfall deficit can cause major agricultural losses if it occurs during germination or flowering. A larger deficit may have fewer immediate effects when reservoirs are full, soils begin the season moist, and water demand is low.

Seasonal forecasts can identify conditions that favor drought, including ENSO phases and persistent pressure patterns. Exact onset, local rainfall, and recovery remain difficult to predict because drought develops through interacting parts of the atmosphere, land, vegetation, and managed water system.

Drought begins in the weather, but its course is written through the entire water cycle. Rain and snow determine the initial supply; heat, wind, soil, plants, storage, and water use determine how quickly that supply disappears and how long the land takes to recover.