Back in the 70's and 80's, the baby boomer Italian's from NYC used "yo" like an equivalent of whistling out loud to get someone's attention-- usually someone at a distance or if you wanted to immediately get attention up close. The word was also preceded with a "hey" in many cases, as in "hey, yo." This was the old-Italian guard usage. When African Americans got a hold of it, "yo" was not only used with the "Italian usage," but also became an abbreviation for the possessive, "your."
Otherwise, see Lili2344 for a history lesson on where the slang originated.
Hey yo, Joey, wutsamatta. I ask'ja ta trow me the ball tree times already.
African American colloquialism: (now used by all ethnicities despite class, education, and geography)
The effect on Long Island whereby the western regions of the island are generally warmer due to the closer proximity of the mainland (where there is more land mass heat being generated) and the population gets denser (which creates more radial heat from electricity, heaters, cars, etc...).
As you move from Montauk to Brentwood you can really notice a difference in the air temperature due to the urban heat island effect.