When the sky is grey for weeks, sunscreen is the first step people drop. It is an understandable habit, and an unhelpful one.
Clouds are not a filter
A large share of UVA passes straight through cloud, and UVA is the part of sunlight most associated with visible ageing and uneven tone. It also passes through window glass, so a desk by the window counts as sun exposure.
Choose a texture you can bear
Humidity makes heavy creams feel worse. A fluid or a clear gel will sit more comfortably and is less likely to slide off. If your skin is oily, a gel sunscreen can often replace your morning moisturiser altogether.
Apply enough, and reapply
Two finger-lengths cover the face and neck. Reapply every two to three hours outdoors, and again after heavy sweating or being caught in the rain.
The simplest rule is the one that needs no weather forecast: if it is daytime, wear sunscreen.