Freshly-fallen powder is an inviting sight for skiers. And there was plenty of it that accumulated on city sidewalks and streets this past weekend.Saturday night in Cambridge, after walking our West Highland white terrier Daisy, my girlfriend Laura and I went out for our first cross country skiing this winter. We shook off the cobwebs from our poles, got the boots out of storage and brought the skis downstairs.We skied past bare-branched bushes in front lawns that added a touch of winter scenery, although they occasionally obstructed our path if we came too close. The rhythms of the past winter came back to me, the feeling of bending my knee, gliding over the snow, lifting my back heel and then pushing off.Well, it wasn?t quite so easy to glide. And it was hard to ski side-by-side on the narrow sidewalks. However, we went around the block and then decided to ski down the sidewalks of Massachusetts Avenue, the major urban thoroughfare that leads to Harvard Square and, ultimately, Boston.We just skied a few blocks. There was quite a bit of traffic on Mass Ave., including snowplows. There was also foot traffic – people walking down the sidewalk or waiting at a bus stop. There was more room on the sidewalks, though, and sometimes we were able to ski alongside each other.When we crossed a street, we paid attention to the slope of the curb, and we kept an eye on the traffic to make sure a plow wasn?t headed our way.Although patches of concrete occasionally showed through the powder, for the most part the sidewalks were covered. They felt like a blank page of paper on which to “write” the first tracks of winter.Since then, the shovels and snowblowers have gone to work, and it might be harder to find places to go “sidewalk skiing.” But snow-covered bike paths and football fields remain. And, with two to four inches predicted to fall on Tuesday, maybe we?ll be able to “ski out our front door” again soon.