Heat and humidity also influence how easily we can become dehydrated. At hotter temperatures, the air can hold more water, speeding up evaporation. This effect is amplified if it is dry outside. That makes for a Catch-22 ― if it is humid out, your sweat evaporates more slowly and you struggle to cool down, but you retain more water. In dry environments, your sweat vanishes quickly, but in doing so depletes the body of moisture. In “dry heat” environments, like across the Desert Southwest, dehydration can hit fast as the atmosphere works to desiccate the body.