It's a base word because it stands alone as a complete word with its own meaning. "Howling" is simply a verb form of the word "howl", which means to make a long, loud, mournful cry.
It's a base word because it stands alone as a complete word with its own meaning. "Howling" is simply a verb form of the word "howl", which means to make a long, loud, mournful cry.