1995 Toyota Corolla Engine heats up while idling

While sitting in heavy stop & go traffic for over an hour tonight, I noticed the temperature guage go from its normal cold reading to 1/2 way up. I also noticed a lot of steam coming off the outside of the hood (although it was raining heavily so I don't know if that was a factor). Should I be concerned about a radiator problem?
The guage never went above the midline, or anywhere close to the red, and hasn't had any previous overheating problems even during the hottest days last summer.
Thank you!