Drought is usually described as missing rain, but the other side of the ledger is how aggressively the atmosphere removes water. That side has its own measurements.
Demand is the atmosphere's pulling power
Evaporative demand describes how much water the air could remove from a well-watered surface over a given period, independent of whether that water is available.
It rises with temperature, wind speed and sunshine, and falls as humidity increases. All four act together rather than any one dominating.
Two summers with identical rainfall can leave soils in very different states if one was hot, windy and clear while the other was cloudy and humid.
Reference surfaces make the number comparable
Because real surfaces differ, demand is calculated for a standardized reference, conventionally a short green crop with unlimited water supply.
That convention lets a value from one state be compared to another without the result depending on which crop or soil happens to be present.
Actual evaporation from a real field is then estimated by scaling the reference value according to crop type, growth stage and available soil moisture.
Pans and instruments provide the direct check
The oldest direct measurement is the evaporation pan, a standardized open water container whose daily loss is recorded and refilled.
Pans overstate what a vegetated surface loses, because water in metal absorbs heat differently, so their readings are scaled before being used agriculturally.
Modern networks calculate demand instead from automated observations of temperature, humidity, wind and solar radiation, which are available continuously and everywhere stations exist.
High demand can create drought without a rainfall deficit
When demand runs far above normal, soils dry and vegetation stresses even in a season whose rainfall totals look ordinary on paper.
This is the mechanism behind rapid drying during hot, windy spells, when weeks of accumulated soil moisture can be stripped away quickly.
Because rainfall-only indices miss these episodes entirely, drought assessment now weighs demand alongside precipitation rather than treating rain as the whole story.
The same numbers drive fire and irrigation decisions
Fire agencies use demand-related measures to judge how quickly fine fuels such as grass and needles will dry after rain.
Irrigation scheduling uses the same calculations in reverse, estimating how much water a field has lost so replacement can be timed rather than guessed.
In both cases the value of the number is that it describes conditions in advance of visible stress, when the response still has time to work.