Many have mentioned the wind... How much cheaper can it get, it's free?
That said, it also pays to understand thermal (air currents rising)and inverse (air currents being pushed down to the earth) conditions. A smoker can detect either of these conditions by simply watching their cigarette smoke. If you hunt in a camp with a wood stove, or have one at home, you no doubt have seen both of these conditions revealed by the chimney smoke. Also a simple campfire will reveal these conditions.
Just some food for thought.