Location: anytime involving the "big freeze"

Throughout the movie, supercold temperatures - those capable of freezing people alive - are dramatized by frost creeping across various surfaces (and sometimes people). This is most notable when Sam runs through the library with medicine from the ship, heading for the room with the fireplace, closely pursued by frost creeping across the stone floor and walls. Frost is, basically, ice; ice is just water, and frost occurs when moisture in the air freezes onto a very cold surface. I don't know if there are many winters in Hollywood, but trust a Michigander - winters here are dry. If it were already snowing in New York, the buildings - their windows, most noticeably - would already be frosted and there would be minimal moisture in the air. Thus, when the big freeze arrived, there would be no dramatic, yet incorrect, frost.

Example 2: flag freezes during Jack's hike to New York. How did that flag get wet enough to turn into a sheet of ice in the space of a few seconds? Was it raining earlier?