In reply to Flinticus:
It's complicated and not very intuitive to teach, but something you get used to if you spend a lot of time outside, especially in the mountains. I think most climbers have had days cragging in the winter when they only need a tee shirt as the sun is shining and they are sheltered from the wind. Then you top out and you need a duvet.
My other half, who is from a warm part of the world and not very outdoorsy, is constantly baffled by picking the right clothing for the weather, but usually only looks at forecast temperature. I make my judgement based on temperature, windspeed, cloud cover, humidity and how active I'm going to be.
The same is true indoors, once air movement is taken out the equation, the biggest impact on thermal comfort is radiative heat loss (or whatever you want to call it). Tackling this means people can be comfortable in cooler air temperatures, but most people get obsessed with their thermometer rather than how they feel.