How to tell a cooling tower under construction from a finished one in satellite imagery
A natural-draft cooling tower looks like a simple shape from above: a ring, tapering to a waist, flaring back out at the lip. But that silhouette means different things depending on where the build sits in the sequence, and at 0.3-0.5 m GSD the difference between "foundation poured last month" and "operational since spring" is legible if you know what to look for.
The build sequence, and what each stage leaves behind
Most hyperboloid shells go up the same way, and each phase has a visible signature.
Foundation and ring wall. You'll see a circular excavation, a ring of formwork, and usually a cluster of rebar bundles staged nearby. No vertical structure yet. The ground around the ring is disturbed, with wheel ruts from concrete trucks and a batch plant or mixing pad often sitting within a few hundred meters.
Shell climbing. This is the stage most people misread. Slipform or jump-form construction climbs the shell in lifts, and from directly overhead a partially built tower can look nearly finished, just shorter. The tell is the rim: an unfinished shell has a flat or stepped top, not the smooth outward flare of a completed lip. You'll also see a scaffolding collar or climbing form ring sitting right at the current working height, plus a tower crane or two parked beside it. Shadow length off the top edge is a rough proxy for how much height is still missing versus the design height you'd expect for that reactor class.
Topped-out, pre-commissioning. Full height reached, but the shell is bare grey concrete with no streaking. Air inlet louvers around the base, if the design has them, are often still boarded or scaffolded rather than open. The basin at grade may be dry or under liner work. No plume, obviously, because there's no cooling load yet.
Operational. This is where the imagery stops looking like a construction site and starts looking like infrastructure. Weathering streaks run down the shell from mineral deposits at the lip. The base is clear of scaffolding and cranes. In cool, humid conditions you'll catch a visible vapor plume rising off the tower, which is the single most reliable finished-tower signature when the pass happens to catch the right weather. Access roads show wear patterns from routine traffic rather than heavy equipment.
Mechanical draft towers, the long rectangular cell type, go through an analogous sequence: foundation slab grid, steel frame erection, fan deck and fan stack installation, fill media and casing last. A half-built mechanical tower reads as a bare steel skeleton with open cells; a finished one has uniform casing panels and fan stacks with consistent shadow geometry across every cell.
Why this matters next to a reactor building
Cooling tower progress rarely moves in isolation. On a reactor-adjacent site, tower construction tends to track alongside containment dome closure, turbine hall roofing, and switchyard buildout. If a shell has topped out but the transmission corridor into the site is still unimproved and the switchyard footprint is bare dirt, that's a different schedule read than a shell topping out next to a switchyard that already has transformers and bus work installed. Reading the cooling tower on its own gives you one data point. Reading it against what's happening in the rest of the footprint tells you where the whole facility actually sits in its build sequence, not just the tower.
That's the harder part of this work: a single frame of a shell under scaffolding is only useful next to last month's frame of the same corner of the site, and the one before that. Doing that comparison by hand, across a whole facility, every week, is where most analysts lose time. Facility Expansion flags construction, excavation and hardening on a named site as they appear week over week, so the change is already marked on the map instead of buried in two archive pulls you have to line up yourself.
If you're tracking a specific site on a weekly cadence, it's worth seeing what that flagged view looks like for your facility.