This winter I conducted an Experiment on my feet, specifically: how long into the winter and in what conditions can I (comfortably; this wasn't intended as an experiment in foot-related suffering) continue to wear sandals?
Now it's spring again (at least here: there are daffodils in full bloom and I was cycling in a T-shirt again yesterday) I can probably declare the experiment finished and draw appropriate conclusions. Which are:
- Anything above 10oC and sandals are perfectly comfortable.
- Anything below about 2-3oC mark is definitely too cold, even if dry. This only happened on a very small number of days this year, though it was an unusually mild winter.
- 3-10oC depends largely on how wet it is; if it's wet underfoot or actively raining then it's likely to be chilly, although at the top end of that range might be OK anyway, depending on what sort of mood I'm in and how long I'm likely to be out.
- There is a difference between 'walking the dog' (or other walking-around-outside activity), 'going somewhere indoors by public transport', and 'standing around in a playground'. If I'm mostly going to be on a bus/tube and then indoors, sandals are fine even getting down towards the zero mark. If I'm standing around in a playground (especially if wet), boots might be wiser even if it's closer to 10. Walking the dog I'm only out for 30 minutes at a time anyway so even if it's chilly I'll probably cope. (When it's freezing that's still long enough to be properly uncomfortable, though.)
- If cycling rather than walking then your feet don't move enough to keep warm; toe-coverings required. But in fact I've been wearing bike sandals all year round for about 8 years now and just wear waterproof socks with them. This is obviously a fashion disaster but if I'm going somewhere where I can't just take my shoes off on arrival (which in general I prefer if at all possible) I carry proper shoes with me.
- Fewer people than I would have expected appeared to either notice or comment on my footwear choices, even in the middle of December.
So it has been very informative!
There was a purpose to this beyond scientific experiment; my knees are happier when I wear sandals than when I wear boots, so the less time I spend wearing boots the better. I am pleased to discover just how feasible it is to minimise boot-weeks.  I just don't much like shoes! The RFH don't mind you wandering around without shoes on. I got told off in the British Museum once, also on a train when walking between carriages.