Based on conventional meteorological observations, retrieval data of microwave radiometer, L-band radiosonde observations in Rongcheng, ERA5 data and NOAA (National Oceanic and Atmospheric Administration) data, three typical non-precipitation sea fog cases occurring over the northern Yellow Sea from May to July 2024 are analyzed. Correlation analysis is conducted on data from microwave radiometer and L-band radiosonde, and the average correlation coefficients of temperature and relative humidity below 2 km are 0.83 and 0.64, which belong to significant correlation. On this basis, the characteristics of temperature and humidity structures and the differences in formation mechanisms of sea fog under different synoptic systems in different seasons are investigated. The results are as follows. (1) When sea fog occurs behind cold high in spring, the low-level southerly winds transport warm and humid airflow, the cold air mass behind the cold high and the nocturnal radiation cooling produce cooling effect of the surface, so the formation of heavy fog has humidification and cooling effect. The temperature and humidity elements exhibit distinct characteristics of day and night changes, with a high air-sea temperature difference in the afternoon and a decrease in relative humidity. The liquid water is concentrated at the height of 0.1-0.2 km with a maximum of 0.57 g·m^-3, which is in line with the formation time of high humidity layer. The increase in the surface temperature destroys the temperature inversion layer, which is the main reason for the dissipation of heavy fog. (2) The weak cold front type of heavy fog in early summer occurs in the northwesterly flow behind the trough with poor water vapor conditions. The microwave radiometer detects a thinner high humidity layer than the advection cooling fog, and there is a dry layer in the humid layer. The cooling effect is obvious near the surface layer, and a strong temperature difference (5.3 ℃) in the inversion layer forms between the surface and the height of 300 m. The temperature rises in the afternoon and the heavy fog dissipates. (3) The prevailing advection cooling fog in summer is closely related to the stable subtropical high. The low-level easterly winds continue to transport water vapor, and heavy fog can last for several days. During the heavy fog, the day and night changes of temperature and humidity are not dramatic, with a maximum temperature difference of only 3.81 ℃ in the inversion layer. The liquid water content is abundant, and the large values last for a long time with a maximum exceeding 1.0 g·m^-3, which is mainly due to the humidification effect. When the previous stable synoptic situation changes, the stable layer is destroyed and the heavy fog dissipates.