大气折射又称蒙气差、折光差（蒙气即行星的大气），是原本直线前进的光或其它电磁波在穿越大气层时，因为空气密度随着高度变化所产生的偏折。这种折射是光通过空气时因为密度的增加使速度降低（折射率增加）。大气折射在近地面时会产生海市蜃楼，让远方的物体出现或荡漾，和非幻觉的升高或降低，伸长或缩短。这个词也适用于声音的折射。无论是天体或地面上物体位置的测量都需要考虑大气折射。

对天文或天体的折射，导致天体在天空中的位置看起来比实际为高。大地折射通常导致物体出现在比实际高的位置上，然而在靠近地面的空气被加热的下午，光线的曲折向上会使物体看似出现在比实际位置低的地方。

Atmospheric refraction is the deviation of light or other electromagnetic wave from a straight line as it passes through the atmosphere due to the variation in air density as a function of altitude. This refraction is due to the velocity of light through air decreasing (the index of refraction increases) with increased density. Atmospheric refraction near the ground produces mirages and can make distant objects appear to shimmer or ripple, elevated or lowered, stretched or shortened with no mirage involved. The term also applies to the refraction of sound. Atmospheric refraction is considered in measuring the position of both astronomical and terrestrial objects.