If you're out with others when it's cold, be aware of the "umbles":

Stumbles, fumbles, mumbles and grumbles.

These can be relatively early external signs of altered mentation and potentially of the on-set of hypothermia.

Hypothermia can occur even in fairly warm temperatures, too; you don't need freezing temperatures.

When carrying food, remember you're going to be eating a whole lot more when it's cold and you're exercising.

Avoid cotton clothing, as that gets and stays brutally cold when it gets wet. Wool or synthetics work much better.