A simulation study asking how the water cycle of a tidally-locked, Earth-like planet depends on its total, finite inventory of water. Using nine idealised general circulation model runs (Isca) with a 'bucket' land hydrology — initial global water depths of 1, 10 and 100 m and three evaporation thresholds — the authors find that every simulated world evolves toward an 'eyeball' state with a barren day side and water cold-trapped on the night side, but that a lake just east of the substellar point can form first and persist for 150–350 years (1 m), 1000–1300 years (10 m) or about 8000 years (100

Simulated climate · tidally-locked worlds

Many rocky planets around small stars keep one face to their sun forever. Give such a world a limited supply of water and its fate looks sealed: the day side bakes dry and the water piles up in the dark. A set of long climate simulations finds a surprising holdout — a single lake beside the hottest point on the planet, kept alive for millennia by its own rainstorm.

Based on: The Hydrological Cycle and Circulation of Tidally-Locked Planets with a Finite Moisture Inventory — Daniel A. Williams, Geoffrey K. Vallis, Stephen I. Thomson & William J. M. Seviour. University of Exeter · arXiv:2610.13517 (2026) · CC BY 4.0

1 Run the planet’s clock

2 Cloud-base lab

3 Two worlds, mapped

4 What it means

Run the planet’s clock

Every simulation starts the same way: a planet the size of Earth, turning once every ten days so one face always sees the star, covered by a uniform layer of water 1, 10 or 100 metres deep. Then the clock runs for thousands of years. Pick a starting depth and watch where the water goes.

Nine simulated worlds. Traced from Fig. 6 of the paper (10-year rolling means over the lake region, 5–15° E, 5° S–5° N). Colour = starting water z₀; line style = ocean threshold z_oc. The 1 m and 10 m runs are shown to year 2,000. Click or drag on the chart to move through time.

Starting water z₀ uniform global layer

Lake lifespan in the simulations: ≈ 8,000 years

Ocean threshold z_oc evaporation throttle

Time since the start logarithmic

Look at or drag the globe

Show

The bucket rule E / E₀ vs. depth

Each patch of model ground is a bucket. Deeper than z_oc it evaporates like open ocean; shallower, evaporation falls in proportion to its depth.

Field notes 0 / 5 found

Watch a lake dry out

Find the lake that outlives 2,000 years

See the 30 K oasis: switch to temperature while a lake is alive

Throttle test: same 1 m of water, lower threshold, shorter life

Visit the cold traps on the night side

What is simulated and what is drawn. Lake depth and lake temperature follow the paper's Fig. 6. The globe's patterns are a teaching reconstruction anchored to the paper's year-2,000 maps (Figs 1, 3 and 4); the frames in between are interpolated, and the storm, vapour and rain are drawn schematically. The paper maps the 10 m and 100 m worlds; the 1 m world is drawn with the 10 m pattern. The planet does not spin in this view because it is tidally locked: the star never moves in its sky.

Why the lake feeds itself

Air leaving the surface cools as it rises. The height where its vapour first condenses into cloud — the lifting condensation level, or cloud base — depends on how warm and how humid the air is at the ground. Over the lake the air is cool and moist, so cloud forms low and the rain falls straight back in. Over a dried lake bed the air is hot and parched: it must climb most of the way up the atmosphere before condensing, and by then the planet-wide westerly wind has carried it off toward the night side.

Air at the surface

Challenge

Find the tipping point: how dry must the air get before the rain misses the lake?

The same contrast in the simulations

These are the paper's own soundings at the lake's location after 2,000 years; the two presets above switch between them. Over the living lake, air at 23 °C with a dew point of 16 °C needs only a short lift before it condenses, and the rain returns to the water directly below — a way for the lake to replenish itself. Over the dried bed, air at 42 °C with a dew point of −23 °C is so arid that the cloud base sits most of the way up the atmosphere.

About this lab. Cloud base is computed from the two sliders with a standard textbook approximation (Bolton 1980). The presets are the surface conditions in the paper's soundings, where the cloud base sits near 900 hPa and 375 hPa. The share of rain that returns and the lake-level gauge are illustrative: the paper reports that the majority of the lake's evaporation is recycled as rain and a very small portion escapes to be cold-trapped on the night side.

Two worlds at year 2,000

Same planet, same star, same 2,000 years. The only difference is the starting water: 10 m or 100 m. Drag the divider across the paper's own maps.

Surface temperature with mid-level winds. The drier world peaks at 343 K just east of the substellar point; the wetter world has a 304 K cool spot in the same place — the lake. Night-side cold spots are 165 K (drier) and 205 K (wetter): a moister atmosphere traps more heat on the night side.

Reading the maps. AS = antistellar point, T = terminator, SS = substellar point (✱). The star is overhead at SS; everything left of T is permanent night.

Maps: Figs 1, 3, 4, 5 and 11 of Williams et al. (2026), arXiv:2610.13517, CC BY 4.0. Panels cropped and overlaid for comparison.

A long goodbye, not a steady state

On a tidally-locked planet with a finite water supply, how much water there is decides not just how wet the world is, but how long its day side stays wet — and whether a visitor would find a lake under the noon star or a salt pan.

Water inventory sets the clock

Every simulated world ends as an “eyeball”: a barren day side and a wet night side. How long that takes varies by orders of magnitude with the starting inventory.

A lake can hold out

Before the eyeball state, a lake forms just east of the substellar point. Its lifetime rises non-linearly with the inventory: 150–350 years from 1 m, 1,000–1,300 years from 10 m, about 8,000 years from 100 m.

The lake makes its own weather

Evaporative cooling holds the lake surface about 30 K below its surroundings, and a convectively unstable column of air above it rains most of the evaporated water straight back.

A slow leak ends it

A very small share of the moisture escapes into the planet-wide circulation and is cold-trapped on the night side. Even at year 2,000 the wetter lake's surface energy budget is about 2 W m⁻² out of balance: long-lived, not permanent.

No way back

Once the lake is gone, air above the hot, dry lake bed must rise to roughly 375 hPa before it can condense. Any moisture that drifts back is swept away before it can rain.

What could change the story

Because the leak is so slow, the authors suggest that even a weak return flow of water along or beneath the ground could maintain the lake indefinitely — a process the model's independent buckets do not include. A continent sitting at the substellar point might prevent a lake from forming at all, and ice on the night side could lock water away for good.

For scale, Earth's water would make a global layer about 2,700 m deep. The inventories explored here are far smaller — closer to those predicted for Titan, which is tidally locked to Saturn.

The simulated world

Sources & credits

Paper. Williams, D. A., Vallis, G. K., Thomson, S. I. & Seviour, W. J. M. (2026). The Hydrological Cycle and Circulation of Tidally-Locked Planets with a Finite Moisture Inventory. arXiv:2610.13517 [astro-ph.EP]. https://arxiv.org/abs/2610.13517

Figures. Figures 1, 3, 4, 5, 9, 10 and 11 are reproduced from the paper under the Creative Commons Attribution 4.0 licence; panels were cropped and re-arranged for side-by-side comparison. The lake time series were traced from Fig. 6 and are approximate.

Illustration and models. The hero image is an AI-generated conceptual illustration. The interactive globe and the cloud-base lab are illustrative teaching models built on the paper's reported values; they are not the authors' simulation. Cloud-base formula: Bolton, D. (1980), Monthly Weather Review 108, 1046–1053.

Type. Fraunces, Space Grotesk and JetBrains Mono, used under the SIL Open Font License.