Why do you get a cold draught under a door?

You get a cold draught under your door because the temperature outside of your door is colder then the inside (your room) Draughts are a naturally cold because the room temperature inside a house is mostly always warm or warmish..a draught filters through the bottom of your door and cools the temperature of the room. You get them because 15% of cold air enters though your floor boards..simple solution for this is to get insulation.