a persons birthday on the day of a leap year
An event that occurs every 18 months is often referred to as a "biennial" event, although "biennial" typically means every two years. In a more specific context, it may be described as an "18-month cycle" or "semiannual" depending on the nature of the event. In scientific or astronomical contexts, specific terms might be used to denote cycles of that length, but there isn't a widely recognized single term for an 18-month event.
Every 200 years
bidecadal
Every 100 years is called a century, made up of 10 decades.
The phrase "once every 3 years" can be expressed as "triennial." This term refers to an event or occurrence that happens every three years. Alternatively, you could also say "every three years" for clarity.
An event that occurs once every three years.
it is a a centry
People born on a double leap year
Biennial (biannual is twice a yesr)
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 )
Every 4 years
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.
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.
An event that occurs every 18 months is often referred to as a "biennial" event, although "biennial" typically means every two years. In a more specific context, it may be described as an "18-month cycle" or "semiannual" depending on the nature of the event. In scientific or astronomical contexts, specific terms might be used to denote cycles of that length, but there isn't a widely recognized single term for an 18-month event.