"Upward billowing clouds" is not a standard meteorological term. It's likely a descriptive phrase used to describe certain types of clouds, but without more context, it's impossible to say for sure which ones.

Here are some possibilities based on what "billowing" might suggest:

* Cumulus clouds: These are the classic puffy white clouds that look like cotton balls. They are often described as "billowing" because they can grow tall and round, like a mushroom cloud.

* Cumulonimbus clouds: These are thunderstorm clouds. They can be massive and powerful, with towering, billowing tops that reach high into the atmosphere.

* Pileus clouds: These are cap-shaped clouds that form above a cumulus or cumulonimbus cloud. They can look like a billowing smoke plume above the main cloud.

* Lenticular clouds: These are lens-shaped clouds that can form over mountains. While not strictly "billowing", they can appear as a series of stacked, rounded clouds.

To help me understand what you're looking for, please provide more information:

* What does the cloud look like? Describe its shape, color, and size.

* Where did you see it? What was the weather like?

* What context did you encounter the phrase? Was it in a book, article, or conversation?

With more information, I can give you a more precise answer.