IPA: /ˈsʌdənnəs/
KK: /sʌd(ə)nəs/
The quality or condition of happening quickly and unexpectedly.
The suddenness of the storm caught everyone off guard.
Suddenness → It is formed from "sudden" (from Old English *sudden*, meaning unexpected or abrupt) and "-ness" (meaning the state or quality of). The word "suddenness" refers to the quality of being unexpected or occurring abruptly.
Think of the word 'sudden' which means unexpected, and remember that 'suddenness' describes the state of being unexpected.