The Sun in May Is Not the Same as the Sun in December

The UV radiation reaching Earth’s surface in mid-May is dramatically more intense than it was in December — not because the sun itself has changed, but because Earth’s orientation to it has. The sun is high enough in the sky in May that UV index values on clear days regularly reach 7, 8, or higher across most of the country — values the World Health Organization classifies as high to very high, requiring sun protection for all outdoor activities.

People don’t treat May sun this way. They treat it like spring sun — pleasant, warming, something to enjoy after a long winter. The temperature feels mild. The air doesn’t feel hot. And yet the UV radiation coming down on a 65°F May afternoon is functionally identical to the UV radiation on a 90°F July afternoon. Temperature and UV intensity are unrelated. The burn potential is the same regardless of how the day feels.

This disconnect — between how May feels and what May’s sun is actually doing — is responsible for the early-season sunburns that dermatologists see every year as predictably as the calendar turns. Understanding what UV radiation actually is, how it damages skin, and what protection measures actually accomplish is the foundation of sun safety that most people never receive in a systematic way.

UVA and UVB: Two Different Problems

Ultraviolet radiation reaches Earth’s surface in two relevant forms — UVA and UVB — and they cause different types of damage through different mechanisms.

UVB radiation has shorter wavelengths and higher energy. It is absorbed primarily by the outer layer of the skin — the epidermis — where it directly damages DNA in skin cells, causing the mutations that can lead to skin cancer. UVB is also the primary cause of sunburn: the redness, pain, and peeling of a burn are inflammatory responses to UVB-induced DNA damage. UVB intensity varies significantly with sun angle, season, and time of day — it is most intense when the sun is highest in the sky, making midday the highest-risk period and summer the highest-risk season. Cloud cover reduces UVB modestly but not dramatically.

UVA radiation has longer wavelengths and lower energy per photon, but penetrates more deeply into the skin — reaching the dermis, where collagen and elastin fibers provide the skin’s structural support. UVA causes oxidative damage to these structural proteins, producing the photoaging effects — wrinkles, loss of elasticity, uneven pigmentation — that accumulate over years of sun exposure. UVA also contributes to skin cancer risk, though through different mechanisms than UVB. Critically, UVA intensity is relatively constant throughout the day, throughout the seasons, and through cloud cover — meaning UVA exposure occurs even on overcast days and through windows.

This distinction matters for sunscreen selection. Products labeled “broad spectrum” block both UVA and UVB. Products that are not labeled broad spectrum may block UVB — preventing sunburn — while allowing UVA penetration that causes photoaging and contributes to cancer risk without the immediate feedback of a burn.

What SPF Actually Means — and What It Doesn’t

SPF — Sun Protection Factor — is one of the most widely misunderstood numbers in consumer products. Most people interpret higher SPF as simply meaning more protection, which is true, but the relationship between SPF number and actual protection is not linear in the way the numbers suggest.

SPF measures how much longer protected skin takes to burn compared to unprotected skin, under controlled laboratory conditions using a specific amount of sunscreen applied at a specific thickness. SPF 30 means that skin takes 30 times longer to reach the same level of UVB-induced redness as unprotected skin. SPF 50 means 50 times longer.

The protection percentages implied by these numbers are where the non-linearity becomes important. SPF 30 blocks approximately 97 percent of UVB radiation. SPF 50 blocks approximately 98 percent. SPF 100 blocks approximately 99 percent. The jump from SPF 30 to SPF 50 provides only about 1 percent additional UVB blocking, not the 67 percent improvement the numbers might suggest. Beyond SPF 50, the incremental benefit of higher SPF values is modest.

The more significant variable than SPF number is application amount and frequency. The SPF rating is determined using 2 milligrams of sunscreen per square centimeter of skin — a quantity most people dramatically under-apply in practice. Studies consistently show that people apply roughly 25 to 50 percent of the amount used in SPF testing, which means the effective SPF of a correctly labeled SPF 50 product applied at half the test amount is roughly equivalent to SPF 17. Using enough sunscreen — most adults need about an ounce (a shot glass full) to cover exposed skin — matters far more than choosing SPF 100 over SPF 50.

Reapplication is the other critical variable. Sunscreen degrades with sun exposure and is removed by sweating and wiping. Most dermatological guidelines recommend reapplication every two hours during outdoor activity, and immediately after swimming or heavy sweating regardless of the elapsed time. Water-resistant sunscreens maintain their rating for 40 or 80 minutes of water exposure — not indefinitely.

The UV Index: The Forecast Number That Actually Matters

The UV index — displayed in most weather apps alongside temperature and precipitation — is a standardized measure of the intensity of UV radiation reaching Earth’s surface at solar noon, scaled from 0 to 11+ with higher numbers indicating greater radiation intensity and faster burn potential.

The World Health Organization’s guidance for UV index values:

At UV index 1-2 (low): Minimal sun protection needed for most people in typical conditions.

At UV index 3-5 (moderate): Protection recommended — sunscreen, hat, and seeking shade during midday.

At UV index 6-7 (high): Protection essential. Reduce time in direct sun between 10 a.m. and 4 p.m.

At UV index 8-10 (very high): Extra protection needed. Avoid sun during midday hours. Unprotected skin can burn in less than 15 minutes.

At UV index 11+ (extreme): Maximum protection required. Unprotected skin burns in minutes.

In mid-May across most of the United States, UV index values on clear days regularly reach 8 to 10 — the very high range — at solar noon. These are the same values that prompt sun safety warnings at beach destinations. They occur on perfectly ordinary May weekdays when people are gardening, walking, having lunch outside, or otherwise going about their lives without beach-level sun awareness.

Skin Type and Individual Risk

UV damage accumulates differently depending on skin type, which is formally classified on the Fitzpatrick scale from Type I (always burns, never tans, very fair skin) through Type VI (never burns, deeply pigmented skin). People with Type I and II skin — fair skin that burns easily — are at the highest risk of acute sunburn and have the highest lifetime skin cancer risk from UV exposure.

However, the common belief that people with darker skin don’t need sun protection is incorrect and harmful. People with Type IV through VI skin do have significantly more melanin — the pigment that absorbs UV radiation and provides some natural protection — and are less likely to burn acutely. But they are not immune to UV damage. Photoaging, certain types of skin cancer, and UV-induced immune suppression occur across all skin types. The SPF equivalent of deeply pigmented skin is estimated at roughly SPF 13 — meaningful protection but not complete protection, particularly for the UVA exposure that causes cumulative damage without burning.

Window Glass and UV: The Gap in Most People’s Protection

One UV exposure source that most people don’t account for: windows. Standard window glass — in homes, offices, and vehicles — blocks most UVB radiation but allows UVA radiation to pass through relatively freely. This means that sitting near a window for hours each day, driving with the sun coming through the side windows, or working in an office with significant window exposure all involve meaningful UVA exposure without any UVB exposure and therefore without the sunburn signal that would otherwise prompt sun protection behavior.

The cumulative UVA exposure through windows contributes to photoaging and some skin cancer risk over years. Window UV films that block both UVA and UVB are available for home and vehicle windows. For people who spend significant time in sun-exposed indoor positions — next to office windows, driving long distances — broad-spectrum sunscreen on exposed areas even indoors is a reasonable practice.

A Simple Framework

The practical sun safety framework that dermatologists recommend is straightforward: check the UV index each morning as part of the weather check, apply broad-spectrum SPF 30 or higher generously to all exposed skin any time the UV index reaches 3 or higher, reapply every two hours during outdoor time, wear a broad-brimmed hat and UV-blocking sunglasses, and seek shade during the peak UV hours of 10 a.m. to 4 p.m. when possible.

May’s UV index makes this framework relevant every day, in weather that doesn’t feel dangerous enough to prompt it. That gap — between how the day feels and what the sun is doing — is where most sun damage accumulates. Closing it requires knowing what the UV index is and treating it as the relevant health metric it is.