Rising temperature is becoming an increasingly important driver of hydroclimatic stress in semi-arid environments, yet its local expression and hydrological implications remain insufficiently documented across much of southern Africa. This study examined long-term maximum temperature (Tmax), minimum temperature (Tmin), and temperature-driven atmospheric water demand at three meteorological stations-Choma, Livingstone, and Sesheke—representing the Ngwezi River Basin hydroclimatic zone of southern Zambia from 1973 to 2022. Annual Tmax and Tmin trends were evaluated using the Mann–Kendall test and Sen’s slope estimator, while monthly, annual, and seasonal distributions were analyzed to assess intra-annual variability, spatial contrasts, and temporal shifts in the thermal regime. Climatological shifts were examined by comparing 1973–1995 and 1996–2022, and Thornthwaite-based potential evapotranspiration (PET) was estimated as a conservative indicator of temperature-driven atmospheric moisture demand. All stations exhibited statistically significant warming. Livingstone showed the strongest warming rates (+ 0.25 °C decade−1 for Tmax and + 0.35 °C decade−1 for Tmin), followed by Choma (+ 0.20 and +0.25 °C decade−1) and Sesheke (+ 0.15 and + 0.30 °C decade−1). Minimum temperature increased faster than maximum temperature at all stations, indicating a clear asymmetric warming pattern characterized by reduced nocturnal cooling and elevated baseline thermal conditions. This asymmetry is hydroclimatically important because warmer nights can sustain higher atmospheric moisture demand, increase crop and ecosystem stress, and reduce recovery from daytime heat exposure. Monthly and seasonal analyses revealed strong spatial contrasts in baseline thermal regimes, while the sub-period comparison showed pronounced warming during late winter and early spring, especially August–October. This suggests an earlier onset of high-temperature conditions, compression of the cooler season, and intensification of pre-rainy-season evaporative stress. PET increased markedly, especially from the mid-2000s onward, closely tracking positive temperature anomalies and indicating rising atmospheric water demand. The findings show that observed warming in southern Zambia is not merely a background climatic tendency, but a structurally important thermal shift with direct implications for evapotranspiration, soil-moisture depletion, agricultural stress, and long-term water security.