Actually I think the rain/snow equivalency varies depending on the moisture content of the snow. In upstate NY we had Lake Effect snow from Lake Ontario. It could pile up 3 feet overnight, but it would compact into much less very quickly because it was light and fluffy. But during the most recent big snow here, the National Weather Service said to use the 10" snow = 1" rain conversion.
Boyd is correct- as the temperature lowers there is less moisture in the air, so the snow ratio rises. Up in the snowbelts of the Great Lakes you can have ratios of 20, 30 or more to 1. Here in South Jersey it is usually around 10:1 though, which is why people use it as a general rule... wet snowstorms frequently drop as low as 5:1, while "fluffy" storms rise to near 20:1 (like the Presidents Day II storm in 2003- when snow fell at temps near 10-15 degrees). Other factors can influence how well snow piles up too such as dendritic growth in the atmosphere and the amount of clumping that occurs to form larger, smaller, or even the type (dendrite, collumns, needles etc) of flakes.
That said, it is a great chart from the climo office that was posted. Although, you have to remember that is an average in South Jersey which is a very volatile place. Not so much in the summer (except along the coast), but during the winter temperature departures between various areas can vary greatly. The sandy soil of varying degrees combined with lowland swampy areas allows temperatures to radiate rapidly at night leading to large temperature variations over short distances. This is why sometimes a hike into a bog will show that snowcover is still exiting there, while areas around have melted- cool air pools there keeping that area colder. One of the best sites you can check for this variation is located here:
http://climate.rutgers.edu/njwxnet/ run by Rutgers University containing a list of sensors from their own and the DOT. You will notice certain sensors such as Oswego Lake, Berkeley, Hamilton Twp (Atlantic Co), and Woodbine will be consistently lower on nights that feature radiational cooling. “Official” airport readings are even more skewed, since with the exception of Millville (KMIV), and now Miller Airpark (KMJX), temperatures are influenced by either the ocean, urban heat island effect, or simply not having the same soil composition as is found deeper in the Pine Barrens (such as KVAY-Mount Holly and KWRI- Wrightstown).
