How do these cloud forms grow?
Looks like Cirrus Fibratus (Ci fib) to me.

See e.g. Wolkenatlas or Clouds Online.

Regarding the formation of Ci fib, Name of Clouds says:


  Cirrus fibratus clouds are formed when winds at high altitudes are strong and flow over the cold air from below. The combination of the warm air and the moisture is usually unevenly distributed hence the trail of fine fibers at the ends of such clouds.