This is caused by the temperature of the water in the atmosphere is low and the water does not become sluggish as the ice immediately and gradually take shape as a clear seal under the clouds.
This series of events caused by the temperature of the water in the atmosphere is low. And water may freeze immediately. And gradually form a seal beneath the clouds clear.
This situation caused by the temperature of the water in the atmosphere at a low level. And water could not become the ice immediately. And gradually forms a sealed transparent under the cloud layer.