Biennial (biannual is twice a yesr)
An event that occurs once every three years.
a persons birthday on the day of a leap year
People born on a double leap year
An event that occurs every thirty years is called a "triennial" event. However, this term can be misleading since "triennial" technically refers to something happening every three years. For events specifically occurring every thirty years, the term "tricenary" can be used, although it is less common.
El Nino.
Eventual world climate change
Eventual world climate change
When used as an adjective, it means occurring every, or last for, four years. As a noun, it references an event that occurs every four years. (see: http://dictionary.reference.com/browse/quadrennially )
An event that occurs every second year is called a "biennial" event. The term comes from the Latin word "biennis," meaning "every two years." Examples of biennial events include certain festivals, conferences, and art exhibitions.
Biannual can be used in a sentence as an adjective to denote an event that occurs twice a year, or every two years. So, "It's a biannual event" is a rather ambiguous statement.
Every 4 years
An example of an occasional climate event that occurs every 3-8 years is the El Niño Southern Oscillation (ENSO). It is a climate pattern that involves the periodic warming of sea surface temperatures in the central and eastern tropical Pacific Ocean, leading to significant impacts on weather patterns worldwide.