I got on the trail late last nite and not only did I find it more enjoyable, I found that without the sun beating down on me, my HR ave was 11 BPM less then when I ride right after work when its still hot or early morning when the sun starts to beat down on me.

Is this a coincidence or is there a relation to outside temp and HR?

HR goes up with ambient temperatures and with fever. Keeping cool requires more work and the heart responds. Steve

Heart rate increases about one beat per minute for every degree above 75 F.