The weather in San Francisco is known for its variability. One minute it's summer and the next, winter. Variability means the tendency to shift or change — of being "variable."
There are many words that contain vari-, and they almost all have to do with change or difference. Variability is the noun version of this idea. The variability of the weather means that you never know exactly what to wear. T-shirt or sweater: the choices vary along with the weather. Many things are known for their variability: the taste of teenagers, the talent of the singers on "American Idol," Frank Sinatra's moods. These are just some of the things that exhibit real variability.