Mojave air

In August, Las Vegas humidity typically ranges 35% AM → 17% mid-PM. San Francisco: 86% → 61%. Chattanooga: 92% → 56%.

At <30% RH, stratum-corneum hydration falls and dry skin becomes rougher and less elastic, enough to make fine lines look more established than they are.

3–4 hours outdoors

A long cycling or climbing session gives you hours of accumulated UV exposure. August Las Vegas reaches UV Index ~10.

UVB acts primarily in the epidermis; UVA penetrates more deeply into the dermis and contributes heavily to photoaging. Duration matters: several lower-dose hours still accumulate.

The desk window

Ordinary glass blocks almost all UVB but transmits meaningful UVA.

Repeated low-dose UVA through the same window, on the same side of the face, is the kind of cumulative exposure that can produce asymmetric photoaging over years.

AC, day + night

On top of August outdoor humidity in the 17–35% range, air-conditioned home, office and bedroom environments can extend dry exposure across most of the day and night.

Squint + smile

Desert brightness comes from direct sun plus reflected light from pale pavement, rock and other bright surfaces.

Crow’s-feet result from structural photoaging underneath and repeated orbicularis oculi movement on top.

36° North

Las Vegas sits at about 36°N. In August–September, midday UVB readily supports cutaneous vitamin-D synthesis.

Excellent vitamin-D conditions, to be balanced with sun protection.