UV Safety

What Time of Day Has the Lowest UV?

How solar elevation, your shadow, and the 'shadow rule' translate UV physics into a simple daily plan.

Roughly 60% of the day's UV arrives between 10am and 4pm — when the sun is highest and its rays travel the shortest path through the atmosphere. The rest of the day has meaningfully lower UV.

The shadow rule

A simple, equipment-free check: if your shadow is shorter than you are tall, the sun is high enough that UV is strong (typically UV 6+). If your shadow is longer than you, UV is usually low to moderate.

Estimated UV by hour (clear summer day, mid-latitudes)

  • 8am: UV 1–2
  • 10am: UV 4–6
  • Noon: UV 8–10
  • 2pm: UV 7–9
  • 4pm: UV 3–5
  • 6pm: UV 0–2

Why early/late sun still matters

UVA penetrates clouds and is steadier across the day than UVB, so cumulative photoaging happens even outside the peak window. Daily SPF still pays off.

Key takeaways

  • 10am–4pm is the high-UV window.
  • Use the shadow rule for a free UV estimate.
  • Daily SPF covers off-peak UVA exposure.

Related DermUV pages

Context behind What Time of Day Has the Lowest UV?

Burn-time figures come from minimal erythemal dose (MED) values published for each Fitzpatrick skin type, divided by the erythemal irradiance at a given UV index. Personal MED varies by roughly 20 percent, so treat every number as a planning range.

Sunscreen labels are tested at 2 mg/cm². Typical real-world application is closer to half that, so DermUV assumes about half the labelled SPF when it estimates protected time.

Cloud cover is not protection. Thin or broken cloud can still pass most incoming UV, and light cloud edges can briefly scatter more UV toward the ground than a clear sky.

Plan outdoor blocks around the peak UV window, usually between 10am and 4pm local time. Shade, UPF fabric, a wide-brim hat, and UV-blocking sunglasses reduce dose more reliably than sunscreen alone.

Reapplication matters more than the SPF number. Reapply roughly every two hours outdoors and immediately after swimming, heavy sweating, or towelling, regardless of what the bottle claims.

UV scale reference

The WHO scale groups UV 1-2 as low, 3-5 as moderate, 6-7 as high, 8-10 as very high, and 11 or above as extreme. Protection is recommended from UV 3 upward, and shade during the middle of the day becomes the priority from UV 8 upward.

A practical routine at moderate-to-high UV is roughly one ounce of broad-spectrum sunscreen for full-body coverage, applied 15 minutes before going out, then reapplied on schedule. Clothing, hats, and shade carry the rest of the load, especially for children and anyone with a history of easy burning.

Keep exploring DermUV

These pages give more depth around what time of day has the lowest uv?, from UV-scale reference material to calculators and city forecasts.

Numbers are modelled, not measured at your exact location, so treat them as guidance for planning rather than exact thresholds.