Well, I’ll bet it has a lot to do with the relative humidity, also, whether a soap bubble pops right away or lasts a long time. Soap bubbles pop when their walls get too thin, and this happens, at least mostly, due to evaporation. Sometimes, if the soap is not very viscous, a little puddle of soap will accumulate at the bottom of a soap bubble, and I’d imagine soap bubbles are almost always thicker on the bottom than on the top.
Water generally evaporates faster when it is warmer, but there are a lot of variables involved -- humidity, air flow, and heat flow.
If your day is really really cold, it may even be possible to freeze a soap bubble!
(republished on 07/25/06)