UKH

Good morning, on this lovely cold day. Here is a picture taken on my run to the station this morning - I've seen this circles pattern many times. Why does frost do this?

It's not frost.  It's the footprints from our alien overlords that only go out at night.  It's just when it is cold the footprints stay around a bit longer in the morning.

Not a sciency peep, but I think It'll be the pattern of adhesive mortar /air space under the paving slab and the resulting differential in conductivity and subsequent temperature.

..or maybe the solid core of the slabs staying cold longer than the edges?

Chris

https://www.pavingexpert.com/layflag1#construction

see "spot bedding" and "ring bedding" in the link - it's a sign of local-authority-quality workmanship

DerwentDiluted may have a point but I'd be surprised if they'd been mortared down  on a neat grid. Aren't they usually laid on hardcore and sand for levelling?

It looks like the effect of the slab edges or the gaps, maybe on local temperature and or humidity. Maybe the infill in the gap holds salt better than the slab surface. Maybe evaporation from the damp sub surface is higher from the gaps than through the slab leaving them locally cooler. Maybe the slabs are slightly dished or domed so any salt or water held in the pores of the concrete is unevenly distributed. I'm not sure what the exact mechanism would be.

jk