Why does this bit steam up?

Ok so it’s a damp moist day out there, approx 15 degrees centigrade

But weirdly I end up with this patch of condensation…on the outside of the windscreen. What’s causing it?

Ok so it’s a damp moist day out there, approx 15 degrees centigrade

But weirdly I end up with this patch of condensation…on the outside of the windscreen. What’s causing it?