JC - I'm sure that is a fantastic book you have linked, but here is the definition of 'Stratus' cloud/rain for you. You do not get 'heavy rain' from Stratus cloud as you can see, hence my query
"Stratus clouds are more known for drizzle than for precipitation, however. When heavier rain falls from them, their title is changed to nimbostratus clouds
Nimbostratus clouds form at or below 6,000 feet. They are dark, low level clouds that bring light to moderately heavy prolonged precipitation, such as snow or rain.
Altostratus clouds are incapable of producing heavy precipitation, but they are often the cause of a light drizzle. Following altostratus clouds are nimbostratus clouds, which are the source of heavier precipitation."
I believe '"Nimbus" is 'Rain cloud' in Latin. Hope this helps!