Why do hikers put on thicker clothes when climbinga high mountain?

Hikers put on thicker clothes when climbing a high mountain due to the decrease in temperature as altitude increases. The higher one goes, the thinner the air becomes, resulting in less insulation and reduced heat retention. As a result, the temperature drops significantly, making it colder at higher altitudes. To combat this, hikers wear thicker clothing to provide additional insulation and retain body heat, helping them stay comfortable and safe during their ascent and descent.