Continuous and continual are two adjectives that are similar in meaning but are not completely interchangeable.
When something forms an unbroken whole, without interruption, it is continuous:
The road runs parallel to the river for one long, continuous stretch before ending, at Suffragette City, only to pick up again twenty miles later, at Inspiration Point.
“The symphony will be performed in one continuous piece. Please do not applaud until the end.”
Continual can also mean* “continuing without interruption,” but, unlike continuous, continual is used to refer to a condition or action that repeats with some frequency (i.e., there are breaks in between):
The continual power outages this season have led to an increase in sales of generators.
My parents’ continual arguments created a lot of tension in our household.
The unit was subjected to continual attacks over a three-month period.
“These continual interruptions are trying my patience!”
“This continual rain has me feeling as gray as the sky.”
Continuous rain, on the other hand, would never stop.
On the planet Meru, rain is continuous.
Roads, lines, and some events have clear beginnings and endings. We can speak of a stretch of road being continuous—or not—because we can easily measure where it starts and where it stops. However, other situations and events are subject to periods of greater and lesser intensity before the activity fades or disappears completely. (A rainy day may include periods of drizzle or mist between heavy downpours, but one might still remark, “It’s been raining all day.”) Continual covers both kinds of situations. Consider:
Sabrina lives in a continual state of fear. (Who’s to say if fear is present in every moment of Sabrina’s existence? Her fear may or may not be continuous, but the meaning of the sentence is clear: Even if Sabrina has occasional moments where she can forget the real or imagined danger, fear dominates her life.)
Mitch seems to need continual reassurance since the layoffs started. (Mitch may forgets his troubles sometimes—even if only in his sleep!—but to those who witness his agitation, Mitch’s need for reassurance seems insatiable as he worries about being laid off himself.)
The continual public outcry eventually led to the practice being banned. (I doubt the outcry continued 24/7, but it could have. (Strikers, for example, might walk a picket line in shifts, so that the pressure is unrelenting and continuous). Often, the point of this kind of statement is in noting that the demand for change persisted over some period of time, bringing attention to a problem in need of a solution.)
We may disregard a dripping faucet for a time, but we can’t ignore the water gushing from a broken pipe. The actual starting point—whether the subject is a drip or the criticism of a policy or practice—may not have been noticed at the time. Sometimes a movement must build momentum before its significance is appreciated, and often the origins are as unobtrusive as a quietly dripping faucet. Once the dam bursts, action is required!
Continuously is an adverb, as is continually:
The alarm blared continuously until the police arrived.
The city boasts the oldest continually operating hospital in the U.S. (pun intended!)
“Is something wrong with your phone? I’ve continually tried to reach you for the last six hours!”
Takeaway: Continual can mean either unbroken and continuous or regularly recurring. Continuous means unbroken and uninterrupted.
The distinction would matter greatly to a designer or builder. Would you want pipes in your house that are not continuous? If they are disconnected at even one place, you might not have water when you turn on the faucet!
Likewise, a line that is continuous is unbroken—whether the line is drawn on a page or tablet or a family’s lineage is being discussed. If the line is broken at even one place, then it is not continuous.
*As the Merriam-Webster Dictionary (online) entry notes, the word continual is older than continuous and has, since the fourteenth century, meant “continuing without interruption.”