Cloud seeding to enhance precipitation (unproven at decision-relevant scale)

Mechanism

Aircraft or ground generators disperse silver-iodide particles (for cold clouds) or hygroscopic salts (for warm clouds) to give cloud moisture nuclei to condense or freeze on, aiming to increase rain or snowfall over a target area. Programs run at state or national scale: nine US states operate them, and the UAE flew 311 seeding missions in 2023.

The evidence, stated plainly

This approach is unproven at decision-relevant scale. The US GAO review (December 2024/January 2025) found estimated effects are often statistically indistinguishable from zero: natural precipitation variability is so large that program-scale claims (such as Utah's 5–15%) cannot be verified. The SNOWIE experiment did quantify real but modest snowfall from seeding at the level of individual storm events. No program anywhere has demonstrated a drought-scale water yield. A city facing recurring Day Zero risk cannot bank on seeded precipitation.

Where it might fit

As a marginal supplement in mountainous regions with suitable winter storm tracks, funded at low levels and evaluated honestly, alongside—never instead of—demand management and firm supplies.

Implementation path, if pursued

Treat it as a research-grade intervention: require independent statistical evaluation designed before operations begin.
Cap spending relative to measures with proven yield; the opportunity cost against demand management is the central policy question.
Prepare for political volatility in both directions: seeding is attractive to announce in drought, and it attracts conspiracy backlash (Tennessee banned it in 2024 after the post-Dubai-flood wave).

Trade-offs

Effects, if real, are modest and event-level, not drought-scale.
Claimed program yields are unverifiable with current evaluation methods.
Spending here diverts money and political attention from measures with demonstrated yield.
Cross-border and liability questions arise when weather is deliberately modified.