The ground absorbs a lot of heat during the day, and at night the heat is released by radiation, this is a process called "radiative cooling". When we see many stars at night, it means that there are very few clouds in the sky that night. The heat radiation can be effectively released into space, so the temperature drops more quickly. Conversely, if we can't see the stars because of clouds, the clouds reflect the radiation back to the ground, making the ground relatively hot.