I NEVER had a problem with my Air Zound working in 20s and 30s temperatures. It's not small, but it's very lightweight and VERY loud. I tried an eletronic horn first (this was years ago, sorry I don't remember which one) and found it not very effective due to the high pitch. The AZ gets you noticed, believe me.

FWIW, I haven't used any kind of horn for the last 2-3 years. My current routes have a very low incidence of bike-car conflict and it's not worth the trouble anymore.