The sky is blue and the white clouds are low. What happened?

Due to the turbulence effect, low clouds are generally generated at the top of the atmospheric boundary layer. During the day, solar radiation heats near the ground, and the upper air above the ground is evenly mixed by turbulence, which is called atmospheric boundary layer (or planetary boundary layer). After the water vapor in the near-surface layer is taken to high altitude, it gradually reaches saturation due to adiabatic cooling, thus condensing into clouds.

The height of cloud formation, called uplift condensation height, is the height of cloud you see, which is determined by two factors: temperature (vertical profile) and water vapor content.

Simply put, it is actually only related to the relative humidity of the ground (the vertical temperature profile is determined by the dry adiabatic cooling rate). The higher the relative humidity on the ground (the closer the water vapor is to saturation), the lower the height of the cloud bottom, and the closer the cloud you see. When the relative humidity reaches 100%, the cloud bottom touches the ground and becomes fog.