# Why do thermal currents affect light but wind does not?

I notice if I put heat under a laser beam it will cause the beam to shimmer. The heated air is causing a disturbance in the air which affects the light. Yet, I can blast a stream of air from a compressor across the beam and there is no effect on the light. What’s going on?

As @niels nielsen mentioned, the thermal effects are more prominent, since the average velocity of the air molecules is larger in comparison to the velocity of air out of the compressor, thus the compressor is just a slight perturbation to regular state. You have to compare the velocity of the air out of compressor to the the RMS velocity of "air particles" -the speed of sound, which is about $$340 [\frac{m}{s}]$$. Only when the velocity out of the compressor becomes comparable to the speed of sound you will start to notice effects on light trajectories. On the other hand, even the use of a crude estimation found in Wikipedia speed of sound shows that for oven at $$T\approx 200[C^\circ]$$ the speed of sound changes by more than $$30\%$$, thus disturbing more the light trajectories.