To think there's no such thing as bad weather, just bad clothing?

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It's chilly out there. Yes.

But is it "too cold" to take a 3 week old out for a walk, if suitably wrapped up? Eg in a snow suit and hat and/or layers and blankets?

MIL of my niece thinks it's far too cold for a baby to be outside. Sister says "pah!" And takes baby for walks anyway.

I didn’t have a car when my DC were babies and always went out daily, would’ve gone mad if not. I still can’t spend a whole day inside. The babies were always warm and dry, it was me that got wet trying to push a pram with one hand and hold an umbrella with the other,

There's sometimes too bad weather even in the UK to go do particular activities - eg, go on a boat, climb a mountain.
But 95% of the time the weather is fine to do normal activities like take a baby for a walk.

When I was a baby, DF worked ski seasons in Switzerland, DM would take me out in my (lined with sheep skin) pram in weather as low as -15.
She did hear a few stories though where people had gone cross-country skiing with babies in slings and they died of the cold

Yabu.
Whatever clothing you wear it'll still be unpleasant to go for a walk in very hot humid weather or on a cold wet day.

If you want or need to take a baby out it will be fine suitably wrapped.

But ..... if you don't want to, at that age it won't make a difference, so don't feel like it's a necessity!