If the upper atmosphere is is very thin, why do space vehicles heat up as they enter the atmosphere?

relative humidity, temperature and dew point -- are bound together in the mathematical relationship below. relative humidity changes when temperatures change. because warm air can hold more water vapor than cool air, relative humidity falls when the temperature rises if no moisture is added to the air. so i would say a

i believe it would cause an increase of percipitation, though i could be wrong. so i would say b is the answer.