I honestly have no idea but my theory it is different for different people. I personally feel the atmosphere around me getting cold or warm. All I can say is it might mean that it is working or a sign that it failed. I hope this helped!
It could be how you feel the energy. When I cast a circle and/or do spells I feel warm. When inside the circle wind seems lessened. If you cast a circle before casting a spell, the breeze might indicate a break in the circle. If you don't cast a circle before casting spells [you don't have to, I've cast spells inside and outside a circle] this could be your body simply reacting to the energy.