If you have any snowmelt, temp for the snow is going to be at or around 32F, 0C. If there is no snowmelt and it's cold enough outside, you'll have temps going below 32F - given this, supposed an air temp of about -10F, the snow will eventually get to -10F if there is nothing else affecting it, but there is ground heat, wind chill, etc. If you've got a large base, ground heat is only going to affect the bottom layers, which you'll never contact.
Lastly to consider, if the slope is a high traffic area, the friction of skis/boards/bodies is going to be heating and melting significant amounts of snow, so your snow temp will be higher than your -10F air temp.
Based on these factors, I'd say given a -10F air temp, your snow is probably around 0 to 10F with the sun out. But that's a total WAG (wild a** guess), and the best thing you can do is just walk outside and stick a thermometer in the snow - much easier, much more accurate