Abstract: Conditions for rain enhancement operations by aircraft are analyzed,by using NCEP re-analytical meteorological data and GRAPES_CAMS weather-affected models combined with precipitation data.The results show that the main weather situations for rain enhancement operations by aircraft in Yunnan are the south branch trough,the stationary front,the combination of the south branch trough and the stationary front,the 700hPa shear line and the depression or trough in the Bay of Bengal; that humidity of above 60% and vertical upward motion in the low layer (below 550 or 600hPa) are necessary conditions for aircraft rain enhancement,which promote the upward movement of the low-layer water vapor and the uplift condensation growth of the AgI condensation nuclei to form raindrops,thus increasing precipitation.At the same time,only when the macroscopic and microscopic parameters of the cloud system-such as water vapor mixing ratio,vertical accumulated liquid water (super-cooled water),cloud top temperature and cloud top height-reach a certain threshold value,can sufficient cloud water and ice water particles be generated to ensure the condensation growth of AgI.