Chabad's article explains the “halachic hour” like this:
The hour has a special meaning in Jewish law. "The third hour of the day" doesn't mean 3:00 a.m., or three sixty-minute hours after sunrise. Rather, an hour in halacha is calculated by taking the total time of daylight of a particular day, from sunrise until sunset, (or According the other opinions, from dawn ("alot hashachar") until three stars appear in the sky ("tzeit hakochavim")) and dividing it into twelve equal parts. A halachic hour is thus known as a sha'ah zemanit, or proportional hour, and varies by the season and even by the day.
My questions are: What is the shortest possible halachic hour in New York City?
What is the longest possible halachic hour in New York City?
Would it be the same every year or one year may have a shorter “shortest hour” than a different year?
