Unmanned aerial vehicles (UAVs) have attracted attention in several industries. The channels of UAV communications have different characteristics compared to the terrestrial channels. In this paper, probabilistic line-of-sight (LoS) air-to-ground (A2G) channel models are studied. The LoS probability is a key factor to characterize the LoS A2G channel model. Despite the importance of channel modeling to maximize UAV utility, few survey studies are available. We provide several existing studies about LoS probability. Then, we show numerical results and provide insights into the LoS probability models.