If you start to get cold and your tanks are low the downward spiral can be rapid. Your partner is responsible and knows what to do. Be more conscious of all this when the weather is bad. Occasionally I have seen hypothermia happen when in sunshine. Very often runners prefer to have hoods down even in quite bad conditions.
Problems begin to assert themselves after a few hours.
Remember that the body has a brilliant computerised, yes computerised, system for spreading heat around the body, better than anything the heating world has come up with. Our brain says, look, body, make sure my brains get plenty of heat otherwise your navigation and thinking will suffer. After a while if no joy we ll take some from his guts and stop him metabolising things as long as MY brain is Ok. A dangerous spiral starts after this. Head cover, remember the head is effectively one sixth of the body surface.