Winter mornings on Division Street begin before the shops do — the brick plaza dark, the air sharp enough to sting. B. Hobby Lawn Care is out in it, pushing driveways open and salting walks so the day can start on time. By April the same routes turn green and the work becomes mowing, edging and hauling off what the storms left behind. Somerville sits low between the ridge and the river, and weather here can change twice before noon. Clearing a path or cutting a clean line across a front yard — both are the same promise kept in different weather.