The first time rain falls on Asgore's face–real rain, not the false drip-drop of endless condensation of Waterfall–he stands in it. It soaks through his clothes, his fur; he bathes in it. It cleanses him. Hundreds of years of grit, ingrained into his skin his fur his soul, from the Underground wash away. He inhales and exhales years of must.
It's warmer than rain beneath the earth.