This is because the stratosphere is the place where plane flies and they fly over the clouds.

Stratosphere:
The layer that is almost free from clouds is the Stratosphere. This layer of the Earth's atmosphere is located above the Troposphere and below the Mesosphere. Here's why the Stratosphere is nearly cloud-free:

1. Stability and lack of moisture:
The Stratosphere is characterized by stable air conditions and very low levels of moisture. Clouds form when water vapor in the air condenses into water droplets or ice crystals. Since the Stratosphere is extremely dry with very little water vapor, cloud formation is rare in this layer.

2. Absence of turbulence:
The Stratosphere experiences minimal vertical air movement compared to the lower layers of the atmosphere. This lack of turbulence means that the air remains relatively stable, making it difficult for clouds to form and persist in this layer.

3. Ozone layer:
One of the defining features of the Stratosphere is the presence of the ozone layer, which absorbs and scatters a significant amount of the incoming ultraviolet radiation from the Sun. This absorption of solar radiation leads to a warming of the Stratosphere, creating a temperature inversion that further inhibits cloud formation.
In conclusion, the Stratosphere is almost free from clouds due to its stable and dry conditions, minimal vertical air movement, and the presence of the ozone layer. This unique combination of factors makes it a cloud-free layer in the Earth's atmosphere.