Abstract: To explore the key meteorological factors affecting the transformation from heavy haze (HH) to extremely dense fog (EDF), the near-surface circulation patterns of 247 EDF and 96 HH cases in the central area of the Huaihe River Basin (HRB) at 0800 BT (a high-intensity period) for 40 years (1980–2019) were objectively classified. This classification utilized the multi-element oblique rotation principal component analysis method, together with the ERA5 reanalysis data. The classification outcomes facilitated an analysis of the formation mechanisms for EDF under various circulation patterns and enabled a comparison of the meteorological conditions at the surface and boundary layers for HH and EDF cases that share similar large-scale circulation patterns. The results show the following: 1) The circulation patterns of EDF can be divided into five types, each characterized by a cold high-pressure system located either in Northwest or Northeast China, accompanied by a subsynoptic system in the HRB, such as a weak high, frontal, or inverted trough. The study area is located in the northern (33%) or internal (19%) sections of the weak high, in front of the frontal zone (29%) or the invert trough (11%), or beneath the cold high (7%), with prevalent winds from the southwest, south, or east. These winds maintain an average wind below 1.6 m/s at ground level. At 925 hPa, the study area is located in a warm ridge and low-humidity area, with wind speeds averaging 2 m/s. Vertically, relative humidity decreases rapidly with height, dropping below 60% at 925 hPa or 850 hPa, thereby promoting surface radiation cooling. This process leads to an average temperature decrease range exceeding 3°C, forming a deep near-surface inversion, with a temperature difference of 2–4°C between 975 hPa and the ground level. 2) Near-surface circulations for HH are divided into three types, all related to high-pressure systems. The study area is located in the front of (56%), beneath (19%), or behind (26%) the surface high but either lacks a subsynoptic scale system or has only a weak one in the HRB. 3) The factors preventing HH from evolving into EDF include ① an insufficient water vapor source (attributed to northerly ground winds); ② a low cooling range at ground level (average lower than 3.1°C) coupled with relatively high wind speeds (average exceeding 2.2 m/s). 4) The presence of a subsynoptic scale system near the study area is critical in determining whether HH can transition into EDF. This system determines local meteorological conditions, such as the presence of light winds, significant cooling, and sufficient sources of water vapor. The intermonthly variation and generation and disappearance time of EDF under various circulation patterns are also analyzed. The former is related to the monsoon climate in the study area, while the latter is related to the formation mechanism of EDF under different circulation patterns.