This is because temperature is complicated. If you want to measure the air temperature you need to isolate from other systems like the 5770K fusion explosion in the sky and the 70-80C ground. (The Sun is constantly trying to reach equilibrium with the earth and would happily burn us to a cinder except we are re-radiating that energy back into space.) Typically measurements are taken in shaded, well ventilated areas, 5 feet off of the ground.
If you want your Garmin to take a valid measurement, place it on a string, 5 feet from the ground, in a shaded area for 5-10 minutes.
It's just physics unfortunately. You might be able to use an IR camera to take a "Picture" of the ambient temperature and kind of take a stab at guessing, but a simple thermocouple is never going to be able to do what you are asking of it. However, that won't stop Garmin from selling one to you!
If you want your Garmin to take a valid measurement, place it on a string, 5 feet from the ground, in a shaded area for 5-10 minutes.