Clouds roll in. The glare eases. People take the hat off.

That is the wrong cue. Thick overcast that hides the sun does cut UV. Broken, puffy cloud that still leaves a sharp shadow can do the opposite: cloud walls bounce extra ultraviolet down, and the index at the ground can sit above the same hour on a cloudless day.

How the boost works

The sun’s disc is still in the open. Direct UV keeps arriving. Bright cloud sides then scatter more down on top of it. WHO notes that light or thin cloud has little effect and may even enhance UV because of that scattering.

NIWA’s public UV work puts a size on the spike:

• Puffy cloud, sun still visible — about 20% or more above the clear-sky index; nearby cloud can push it to about 30%
• Clear-sky peak of 10 — the real-world peak can occasionally exceed 12
• Sun fully hidden (no shadow you can pick out) — typically about half the clear-sky index, with a wide range

Field measurements of these bursts most often last a few minutes. Some run 20–30 minutes. A long stretch outdoors still adds up.

The EPA’s average cloud factors used in U.S. forecasts go the other way—scattered cloud about 89% of clear-sky UV, broken about 73%, overcast about 31%. Those are average cuts across a sky. They do not describe the minutes when the disc is open and the cloud walls are bright. Your eye sees “cloud.” The ground can be taking more than a blue-sky noon.

Why you drop cover

Three traps:

1. Cooler air, softer glare. Clouds block heat and visible light better than they block UV. Comfort is not the index.
2. “It’s not full sun.” If you still have a sharp shadow, the disc is not covered. That is the enhancement setup, not shelter.
3. A peak you already checked. A clear-sky peak of 8 can behave like about 10 in a bright gap. Keep cover on for the break, not for the grey in between.

Open shade still leaves scattered UV. That is the shade problem. Cloud walls add a different extra: they can raise the open-sky number itself.

When you still have a shadow

Use the live UV index first, then treat the sky as a switch, not a shield:

1. Check today’s peak. If it is High (6–7) or above, keep cover on through the bright gaps. A 20–30% bump can move a High hour into Very High.
2. Use the shadow. Sharp shadow: treat it as at least full sun, maybe more. No shadow you can pick out: UV is usually down, often toward half—still not zero on a High-peak day.
3. Do not wait for the next break to reapply. The burst is the gap. Hat, shirt, and sunscreen stay on between clouds.
4. Dark storm cloud is the exception. Heavy cover that kills the shadow is a real cut. Puffy fair-weather cloud is not.
5. Recheck when the sky opens. The live reading moves faster than the look of the afternoon.

Sources

Short-term cloud enhancement (often ~20–30%, a peak 10 briefly over 12) is NIWA UV guidance. WHO notes that light cloud can scatter extra UV. EPA transmission percentages are forecast averages, not the size of one bright gap.