Abstract: Based on the data of temperature, humidity, cloud height, visibility and all-sky view imaging from the automatic meteorological system of Hefei Xinqiao Airport, China, the infrared characteristics of all kinds of clouds as well as the evolution of clouds and fog over the airport from July 2019 to April 2020, are analyzed. The results show that: (1) the higher the cloud bottom height is, the lower the temperature is, which means that the higher clouds have the weaker thermal radiation resulting in the darker infrared cloud image, and vice versa. And different types of clouds have their own unique infrared characteristics. (2) As for the average shortest diameter of the largest cloud block of wave clouds at various heights, that of stratocumulus is 2–4 times that of altocumulus, and that of altocumulus is 2–4 times that of cirrocumulus. Generally, fractostratus shows fluffy-cotton alike structures, while fractonimbus are usually in the form of smoke band or scattered sheet. (3) There is a clear transformation process between cloud and fog. Radiation fog generally occurs during the period when clouds just disperse and at least half of the sky is clear, and the fog gradually rises to clouds and continues to dissipate. When a breach is formed in the sky, it indicates that the fog begins to dissipate, which can be used as an important indicator for the rapid recovery of visibility. The evolution of the non-convective precipitation cloud layer shows that the cloud base height changes from high to low, and the cloud layer changes from dispersed texture to uniform layer at the same time. The uniform cloud layer always appears in the early stage of the precipitation, and the relative humidity of ground presents a sharp increase.

Key words: infrared detection, aviation meteorology, cloud image recognition, cloud evolution