Very intresting, ok first of all the clock that runs in systray is not a
normal icon at all, infact if you r-click on the clock you will get the
taskbar ctxt menu. So the best option for you would be to use number
icons to show the temperatue, and guess what someone has already done
that here's the link http://www.codeproject.com/csharp/temperature.asp

hope that helps.

Sijin Joseph
http://www.indiangeek.net
http://weblogs.asp.net/sjoseph