Why 90°F Feels Different: Six Factors Behind Regional Humidity

A 90-degree day in Phoenix and Houston are identical by thermometer, yet feel vastly different due to humidity. Humidity is arguably more significant than temperature in determining heat's physical experience and danger, as high humidity impairs sweat evaporation. Six factors determine regional humidity: proximity to large bodies of water, mountain rain shadows, prevailing wind patterns, vegetation (evapotranspiration), urban heat islands, and ocean current temperatures. Proximity to warm water, like the Gulf of Mexico, raises humidity in coastal areas like Houston and New Orleans. Mountain ranges create rain shadows, stripping moisture and creating arid conditions, as seen east of the Cascades and in the Tibetan Plateau. Prevailing winds from oceans bring moisture, while continental winds bring dryness, exemplified by the Indian monsoon's seasonal reversal. Dense vegetation, such as the Amazon rainforest, generates its own humidity through evapotranspiration, sustaining local rainfall. Urban areas experience lower relative humidity than surrounding rural areas due to the urban heat island effect and reduced vegetation. Ocean currents, like the cold California Current versus the warm Gulf Stream, influence coastal humidity, explaining why California is drier than the Gulf Coast and why coastal deserts like the Atacama and Namib exist. These factors combine to explain why some regions are consistently humid and others dry, affecting how heat is perceived and its health risks.