Warm air melts ice, yet models project more 30-mm hail. What wins?

Answer: More fuel for updrafts

More fuel for updrafts ✓ — Correct. Warmer low air can hold more water vapour, so storms can feed stronger updrafts and more supercooled water into the hail-growth zone. The twist is that melting also increases; the model result is a size filter, not a simple more-ice story. Nature projects 30-mm-or-larger hail rising by 37.9-51.8%, while smaller hail declines.

Colder clouds everywhere — Almost the intuitive guess, but warming does not make storm columns colder everywhere. The key growth zone can be better supplied even while the lower fall path gets warmer. That is why the same climate signal can both melt small hail and allow the largest stones to survive.

Less rain below clouds — Not quite. Less rain below the cloud is not the main mechanism; hail grows before it falls, inside the storm's updraft. The useful clue is that the air feeding the storm carries more moisture, so the updraft has more material and energy to build large stones before melting starts.