Oxidation - the breakdown of rock by oxygen and water, often giving iron-rich rocks a rusty-coloured weathered surface.
Because it's easier to scale hustle then relative humidity. and high relative humidity and high precipitation rate are the associates. I should know because I grew up in both greater New Orleans, which is subtropical, and the Oklahoma panhandle, which is semi-arid 5% relative humidity or higher is normal in New Orleans. While 6% relative humidity is relatively familiar in the Oklahoma and Texas panhandles.
