"Upwardly billowing clouds" is not a standard meteorological term. It's a more descriptive phrase that could refer to a few different types of clouds, depending on the context. Here's a breakdown:

1. Cumulus Clouds: These are the classic cotton-ball shaped clouds that often have a rounded top. They are formed by rising thermals of warm air and can indeed have a "billowing" appearance.

2. Cumulonimbus Clouds: These are the towering thunderheads that are associated with severe weather. They often have a massive, "billowing" structure with a flat, anvil-shaped top.

3. Pileus Clouds: These are cap-shaped clouds that often form above cumulus or cumulonimbus clouds. They can appear as a billowing, wispy cloud above the main cloud body.

4. Mammatus Clouds: These are rounded, pouch-like formations that often hang underneath cumulonimbus clouds. While not necessarily "billowing" upward, they can give the overall cloud formation a more undulating, "billowing" appearance.

To understand the specific type of cloud being described, you need more information. Consider:

* The size and shape of the cloud: Are they small and fluffy, or large and towering?

* The surrounding weather conditions: Are there any storms, or is it a clear day?

* The direction the cloud is moving: Are they moving upward or sideways?

By providing more context, it will be easier to identify the specific type of cloud being described.