Clouds block sunlight, not UV rays. Your skin is still exposed on overcast days. Skipping SPF because it looks grey outside is where a lot of daily damage adds up.
To get the SPF protection listed on a product, you need to apply about a teaspoon to your face. Nobody applies that much foundation. SPF in makeup is a bonus, not a replacement for sunscreen.
SPF 30 blocks about 97% of UVB rays. SPF 50 blocks about 98%. The difference is smaller than most people think. What matters more is applying enough and reapplying throughout the day.
MOISTURIZING RICH SUNSCREEN SPF 30
$12 USD
Broad spectrum mineral zinc oxide protects from UVA and UVB rays. Shea butter and hyaluronic acid hydrate while it protects. Minimal white cast, no greasy residue.