Evapotranspiration — How the Sun and Wind Pull Water Back Into the Sky
Rain falls, but it doesn't stay put. A huge share of it — globally, somewhere between three-fifths and three-quarters of all the rain and snow that falls on land — is returned straight back to the atmosphere rather than running off into rivers or soaking down to groundwater. The process responsible is evapotranspiration (ET): the combined effect of water evaporating directly from soil, puddles and wet surfaces, plus water pulled up through plant roots and released as vapour through their leaves (transpiration). Meteorologists and hydrologists treat the two as one combined number because, from a weather station's point of view, there's no practical way to tell which share came from bare ground and which came from a leaf.
What Controls the Rate?
- Energy. Turning liquid water into vapour takes heat — about 2,260 joules per gram. More sunshine and warmth available at the surface means more evapotranspiration.
- Humidity. Dry air can absorb far more moisture than air that's already close to saturated, so ET runs faster when the air above the surface is dry.
- Wind. Still air quickly becomes locally humid right above the surface, which slows further evaporation. Wind sweeps that damp layer away and replaces it with drier air, so a breezy day evaporates noticeably more than a still one.
- Water availability. Obvious, but decisive — none of the above matters if there's no water left to evaporate, which is exactly what happens during a drought.
- Vegetation. Deep-rooted, evergreen plants (conifers, for instance) can keep transpiring for more of the year than shallow-rooted or deciduous ones. Leaf area, and how tightly a plant closes its stomata to conserve water, both matter too.
Potential vs Actual Evapotranspiration
Two related numbers get used side by side, and it's worth keeping them straight. Potential evapotranspiration (PET) is a theoretical ceiling: how much water a well-watered, short green surface (grass is the usual reference) would lose to the atmosphere given the day's sunshine, temperature, humidity and wind, assuming water is never in short supply. Actual evapotranspiration (AET) is what really happens, which can never exceed PET and falls below it as soon as the soil starts running dry or a plant closes its stomata to conserve water. The gap between the two — PET minus rainfall — is what irrigation has to make up, which is why PET is the number behind most irrigation-scheduling and crop water-demand calculations.
The Seasonal Pattern
PET follows the sun far more closely than it follows temperature alone. The illustrative shape below is typical of the UK: a summer peak roughly ten to twenty times the winter minimum, driven mainly by day length and sun angle rather than by how warm any particular day feels.
Relative monthly potential evapotranspiration — illustrative shape, not measured data for any single site.
Evapotranspiration and Southampton's Climate
Southern England is where Howard Penman himself worked out the original Penman evaporation equation, at Rothamsted in Hertfordshire in the 1940s, and MORECS — the Met Office's own rainfall-and-evaporation system built on that work — puts a typical southern England location's total potential evapotranspiration at roughly 500–550mm a year. That's comfortably below Southampton's typical annual rainfall, which is why the region isn't usually short of water over a full year. The catch is timing: PET peaks hard in June and July, exactly when rainfall tends to be at its least reliable, so a garden or lawn can run a real short-term moisture deficit in a dry summer spell even in a part of the country that's wet overall across the year. It's the same reason a lawn browns off in a July dry spell and then greens up again within days of the first proper autumn rain — the annual water budget was never really in doubt, only the week-to-week one.
For the real numbers driving all of this day to day — solar radiation, wind run, and humidity at this station — see our own Trend Statistics and Highs, Lows and Averages pages.
How It's Actually Calculated
The standard reference method is the Penman–Monteith equation, recommended by the UN Food and Agriculture Organization (FAO) as the default way to calculate reference evapotranspiration (ET₀) from ordinary weather-station data: temperature, humidity, wind speed and solar radiation. To go from a generic grass-reference figure to the water use of an actual crop or plant, that reference value is multiplied by a crop coefficient (Kc), which varies through the growing season as the plant's size and leaf cover change. Older, simpler methods — Blaney–Criddle, Hargreaves, Makkink — are still used where full weather data isn't available, trading some accuracy for needing less input data.
Background based on standard hydrometeorological references: the FAO Penman–Monteith methodology and the Met Office's MORECS rainfall-and-evaporation system.