Curse of the wet lawnmower seat

What is supposed to happen in this arid high desert of the Snake River Valley in lovely Eastern Idaho when the sun comes out? Anything watery is supposed to dry out. Our average humidity here is something like .oo3%, which is why my skin is aging at a rate triple that of my midwestern family members. I could easily pass for a member of the AARP thanks to our climate. So why is it that when the baby is properly fed, diaper changed, appropriately clothed for mid-morning in mid-spring (waaay easier said than done thanks again to the aforementioned climate), the sun is out, the big sisters are engaged in something with a mess-making factor of less than average, the gas tank on my Craftsman is full, the Baby Bjorn carrier is properly fitted, the sunscreen is applied to all applicable areas, and the planets are aligned--why is it that I do not see the huge pond of water on the seat of my favorite suburban toy? Unfortunately I know from experience that when the sun has warmed that pool of water to the temperature of the black vinyl seat it occupies, you don't necessarily feel it right at first. It is comfortable. Then after a couple of mowing minutes, you realize that something is wrong. Your pants are wet.