Arid regions worldwide, and particularly in Saudi Arabia, have been experiencing intensifying atmospheric thermal stress in recent decades, with profound implications for public health and urban sustainability. This study systematically analyzed the variability and trends of thermal discomfort index (TDI) across six major Saudi Arabian cities from 1984 to 2024, employing POWER data and robust statistical techniques, including descriptive analysis, Mann-Kendall Test (MKT), Sen's Slope (SS), Theil-Sen Slope (TSS), Pearson's Correlation and Lagged Correlation analysis. Monthly TDI values showed notable seasonal variations, ranging from winter minimum of 10.9 degrees C to summer maximum of 29.4 degrees C, with inland cities such as Riyadh (13.9-25.4 degrees C), and coastal cities like Dammam (16.2-29.4 degrees C), experienced distinct patterns of thermal stress. Trend analysis confirmed significant TDI increases during summer and transitional months across all cities, with the strongest warming trends in Dammam (June: SS = 0.044 degrees C/year) and Madinah (August: SS = 0.039 degrees C/year). Correlation analysis highlighted temperature as the primary driver of discomfort, while humidity emerged as a critical amplifying factor, particularly in coastal cities. These findings reveal an escalating patterns of atmospheric thermal stress across arid urban landscapes, exacerbated by synergistic effects of warming and humidity. Importantly, this study highlights the necessity for targeted urban heat mitigation strategies and the formulation of evidence-based environmental health policies to enhance resilience in arid environments. By quantifying both immediate and delayed climatic influences on diurnal thermal discomfort, this study establishes a foundation for more adaptive and climate-resilient urban planning in regions at the forefront of global warming.