Insofar as I know, the Great Lawn and Sheep Meadow are closed in the winter and do not reopen until spring. The reason is to protect the grass.

Grass is dormant in winter and barely grows, so once it gets torn up, it has to be re-seeded, which cannot be done until spring. And then people get frustrated if the Park's authorities keep the lawn closed to protect freshly-seeded grass.

From what little I know about growing grass, I would say there was not a thick enough layer to allow skiing without pulling up the grass underneath, although I noticed tracks on the Great Lawn on Sunday as somebody decided to ignore the fact it was closed. The tracks had clumps of grass in it.

As some of the snow had melted, if there is the weight of people on it, the melt water running into the ground can cause water-logging, which is also not good for grass.

A lot of people seemed to be skiing fairly successfully on the horse track. But if you want to do some serious cross-country skiing, probably best to go out of town where the snow is thicker and stick to cycling on the park loop.

That's a good question, I didn't know they did. Although as they are away from the city, it is just possible they have a thicker layer of snow. I think if you have a minimum of eight inches of compact snow, it's probably ok. I'm not to sure whether the courses would like cross country skiing over the greens though, unless the snow is extremely thick.