This study analyzed how climate variability affects lactation yield and reproduction in Holstein cows in a continental climate. It specifically compared Wet-Bulb Temperature (Twb) with the standard Temperature–Humidity Index (THI). We conducted a retrospective study on a dairy farm in Konya, Türkiye, analyzing a total of 144 complete lactation records from a herd of 90 cows calving between 2022 and 2023. To rule out nutritional factors, a consistent TMR diet without pasture access was maintained in both years. Average Daily Milk Yield (ADMY) was calculated to adjust for lactation length. Climatic data showed a distinct contrast. Ambient temperatures and THI were similar between years (p > 0.05). However, 2022 was characterized by “humid heat” (high Twb), while 2023 presented a “dry heat” profile with significantly lower Twb (p < 0.001). This difference significantly impacted performance. Cows in the 2023 group produced much higher milk yields (50.55 ± 3.01 kg) than the 2022 group (30.74 ± 0.81 kg) (p < 0.001). Unexpectedly, milk yield peaked during the Autumn and Summer seasons of the low-humidity year. In contrast, fertility declined with thermal load. Poor winter fertility suggested a “carry-over” effect of previous heat stress. These findings show that Twb is a better indicator of thermal comfort than THI in continental climates. Furthermore, low humidity can significantly reduce the negative impact of heat on milk production.