Amongst the plethora of accessories that Garmin produce for their running watches there is a small pod that will measure temperature, called the "Tempe". It's exactly the same form factor as the Garmin Footpod, and the battery lasts about a year. I put the Tempe under the brim of my Running Hat, attached with a zip tie. I've found that the accuracy of the Tempe is far better than the temperature sensor in some of Garmin's watches, and it's vastly more accurate due to its better position. I think that it's well worth its relatively low price.
The Tempe is compatible with the