It starts to snow. My youngest son has been waiting all year for this. “Ha ha ha!” he shouts, looking out of the window. He’s already in his pyjamas, but slips some trainers on the end of his feet and runs outside. He stands looking at the sky and laughs again, doing a strange jig with his arms outstretched. Then he stares at the floor and his smile vanishes. “Where is it? Why isn’t the snow still there?”

“It’s crap snow,” I tell him.

“It’s not fair. I want to go sledging.”

I bend down to inspect the snow depth. It’s about 0.001 mm. Not quite enough to justify a trip to Hampstead Heath.

“If it’s really deep tomorrow, will you come and get me from school so we can go sledging?”

I look at the sky, then the ground. Finally I sniff the air*.

“OK then – it’s a deal.”

* Meaning – have already checked the BBC weather forecast for the next day.