Santiago's location within a watershed is one of the most important factors determining the climate of the city. The coastal mountain range serves as a screen that stops the spread of maritime influence, contributing to the increase in annual and daily thermal oscillation (the difference between the maximum and minimum daily temperatures can reach 14 °C) and maintaining low relative humidity, close to an annual average of 70%. It also prevents the entry of air masses, with the exception of some coastal low clouds that penetrate to the basin through the river valleys.