The top is no more than 5 meters higher than the bottom. How is this small difference in height able to make such a remarkable difference in the amount of frost formed?! I have noticed this phenomenon for a couple of days now and am desperate to explain it!

The picture attached shows what I am describing.

Please see: http://imgur.com/zzbEN.jpg

I noticed this frost pattern on a manhole cover in Bern, Switzerland. It was in the middle of the city's Bear Pit enclosure. What could have caused this?

It if it is the 'easy' answer (something star shaped was covering the area), what could it be?

Thanks

p.s. I remember years ago a question similar to this asked on the last word. It was about a spotty melting pattern on paving slabs caused by (it emerged) a technique of adhering the paving slabs using dollops of mortar in each corner of the slab to minimise waste. (Anyone have a link?; I cant find it)