Division of Geological and Planetary Sciences
Cal Tech

Dear Click and Clack,

Regarding the enigma recently posed by a Seattle caller, and surmised by you to plague mankind for years if not decades, of why birds poop preferentially on white cars, I have sent the problem through the Atmospheric Sciences Supercomputer (ASS) and have reached the following conclusions:

1. A white car will reflect more light than the ground and other cars around it.
2. Consequently the white car, and the air directly above it, will be slightly cooler than the surrounding air.
3. This temperature contrast will induce a downdraft of air centered directly over the white car.
4. As birds fly through this downdraft, they will, on average, be pulled almost imperceptibly lower to the ground.
5. Normally, if a bird flies lower, the air pressure on its body will increase, causing it preferentially to poop upon descent (known in the parlance as PUD).
6. Of course, in the case of the white-car dilemma, the small descent of the bird will not change its external atmospheric pressure, because the bird will follow an isobaric path through the downdraft.
7. However, because the bird has experienced so many PUDs in its lifetime, its bird brain is CONDITIONED to produce a PUD whether or not there is an actual pressure increase.
8. Hence, the white car is the preferential target of falling PUDs.

As you can imagine, I was thrilled to obtain these insights from the supercomputer and therefore solve the mystery outright. I will soon submit these findings to the appropriate journal.

Scientifically at your service,

David Evans
Graduate student in Geology

[ Previous Follow Up | How it Started | Next Follow Up ]