On what?
Local conditions, I would have thought. Like, season, weather, ground type, indoors or outdoors.

That hammock looks cool. I want one.

I reckon it will be warmer than a tent in wet conditions. Not much, but a little. It's not that the air is warmer than the ground (though it might be), it more that gas is much less dense than solid. Water at 5 degrees feels much colder than air at 5 degrees, that's because you're losing more heat thanks to your skin being in contact with a much higher concentration of colder matter. It'll be the same logic behind air and ground. The more air you're surrounded by, the less heat loss there will be.

Also, the fact water can drain off quickly, rather than collecting in pools, will help matters.