biannual
The BI word that means twice every year is "biannual." It refers to events or occurrences that happen two times within a year, typically spaced six months apart. It's important not to confuse it with "biennial," which refers to something that happens every two years.
Any two events in which one cannot happen without the other.
Tony Greig
"Once a year" is an adverb (like "sometimes" or "often") and therefore can't be made plural. It stay the same even when describing multiple actions, people, or events. (for example, "Christmas and Easter both happen once a year," or "John and I go to the mountains once a year." If you would like to describe something that happens more often, try "twice a year", "three times a year", "ten times a year", "many times a year", or "once a month". For something less often try "once every two years" or "once every three years".
A hundred year (insert any phenomena) can and do happen multiple times in a locality in a one year period. The "hundred year" notation is a human estimate on how often we average the phenomena to occur. So, if two happen the same year, to maintain the average, we would suspect another wouldn't happen for roughly another 200 years. As things occur, our databases are revised and averages recalculated. Enjoy being rezoned a flood plain...
Meteor showers occur several times each year, and there will be two or more solar eclipses every year from now through 2020.
Once in a blue moon! Well that is what the second full moon in a month is called, a blue moon. It is very irregular, sometimes it can happen multiple times a year or even two months in a row. Other times it can be years between them.
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One of a pair of complementary events.
When two events cannot occur simultaneously but one may happen if the other does not, they are referred to as "mutually exclusive events." In probability theory, this means that the occurrence of one event precludes the occurrence of the other. For example, when flipping a coin, the outcomes of heads and tails are mutually exclusive.
No, it is going to happen in Ethiopia. Just kidding, no idea when that will happen. The two types of events are not connected.
Hiroshima and Nagasaki were the location of the dropping of two atomic bombs from the Allied forces.