This summer your winter night temps are our summer temps. We have only seen a few days with temps around 20°C and that was in April. For the two last winters our temps have been down to -15°C (5F) which is cold for our area. Further inland they got -30° to -40° (-22 to -40F). I think your son should count himself lucky he's in Ireland and not in Norway.
Droopy, those temps are just not something I can imagine or relate to. Having never lived in a cold place ever, I just can't imagine minus 30 degrees!! I couldn't imagine my son living there. He'd be on the first plane home again I think!
