4. Dense sheets of middle-level nonfibrous clouds, heretofore called 

 altostratus, have been changed to altocumulus opacus. This change 

 restricts the dense altostratus forms to the dense fibrous sheets of 

 rain (sometimes snow) that usually succeed cirrostratus by lowering, 

 a development that is of much importance to the forecasters. If a 

 dense sheet of middle-level clouds shows definite relief on the lower 

 surface it is classed as altocumulus and not altostratus. 



5. The name "nimbostratus" has been introduced and the name 

 "nimbus" has been eliminated. The observer should read the 

 definitions carefully and note particularly that the name "nimbo- 

 stratus" should not be applied to the lower parts of a cumulonimbus 

 cloud. When precipitation falls from nimbostratus clouds it is con- 

 tinuous in character. Nimbostratus is evolved by lowering of a 

 sheet of altostratus or in exceptional cases from a sheet of strato- 

 cumulus but not from cumulonimbus. 



6. The hyphens in cloud names have been eliminated, e. g., cirro- 

 stratus has been changed to cirrostratus. 



IV 



