Brrrrrrrrrrr The Sequel

This begs the question; has the weather gotten warmer here over recent decades? One of my diarists, Brother Ewald Gustav Schaukirk, reports in his entry of February 4, 1780:

This week sleighs have crossed over the ice from Staten Island to this city, which has hardly been known before.

(New York Diaries: 1609-2009. p. 49)

The Upper Bay froze solid enough to support a sleigh drawn by horses? As in clop, clop, clop? How cold would it have to be to freeze the surface of a body of water 50 feet deep, especially one that for half of each day runs salt water from in-coming and out-going oceanic tides? The Lower Hudson, at a depth of a little over thirty feet, would also have frozen. There seems to be a theory afloat that the more impurities water contains, the faster it freezes. Impure water will generally freeze at 32 degrees F or, if it very pure, stay in liquid form until minus 55 degrees F. Impurities in the Hudson? We might be on to something. Was the Hudson filled with more impurities in 1780 than now; hence likely to freeze at a higher temp? This bears further investigation.

According to the New York Times Archive, February 2, 2003, it is more likely for the East River to freeze because it is shallower than the Hudson. That happened in January of 1866-67, bringing ferry service to a standstill. Also briefly during the great blizzard of 1888. On those occasions, some crossed the river on foot, but no mention of sleighs.