They are actually Cirrus Clouds.
a thunderhead cloud is a thin wispy white cloud
Cirrus clouds
A cirrus cloud can be described as wispy and feathery.
cirrus
If you mean above 20,000 feet, it's probably a cirrus cloud.
A wispy cloud is very fragile, very slight. If even a small breeze touched a wispy cloud, the cloud would disappear.
thin and wispy
a thunderhead cloud is a thin wispy white cloud
Cirrus clouds
Cirrus clouds
A cirrus cloud can be described as wispy and feathery.
Cirrus clouds
cirrus
Wispy, high cloud formations
Cirrus
Altostratus cloud
Cirrus. Cirrus clouds are wispy or curly. Cirrus means "wispy."