Top 

- Cirrus (often in bands) and Cirrostratus, or Cirrostratus alone
- progressively invading the sky
- generally growing denser as a whole
- continuous veil extends more than 45 degrees above the horizon
- sky not being totally covered

graphic

graphic

graphic

graphic

from the top downward:
photo by Kruegler
photo by R.K. Pilsbury (Crown copyright)
photo KNMI
photo by R.K. Pilsbury (Crown copyright)