During winter, you need to wear a sweater to stay warm. But when I asked how a sweater works, people told me that sweaters have air pockets that keep you warm. Since air is an insulator, these air pockets keep you warm.
But aren't you already surrounded by an atmosphere in the first place? Why do you specially need a sweater for that? What is the difference between air around you and the air inside a sweater?
Although air is a very good insulator it can transport heat via convection.
To prevent the air moving and hence reduce heat loss by convection the air is trapped in the voids of a sweater.
Here is a picture of a very coarsely 'knitted' heavy string vest which has lots of voids in it which contain trapped air.
The air is trapped between the string, the body of the person wearing the vest and the outer garments.
This type of vest was issued to some aircrews not only to keep them warm but as an escape aid. If needed the vest could be unravelled and plaited to form a strong rope to aid escape!
The air inside the sweater does not move around and get replaced by the cold one from the outside. It remains warm after the heat from your body warmed it.
The point of the sweater is to hinder the process of convection, which is the dominant cooling process that the insulating air is taking heat away from you.
Air indeed is an insulator. But that does mean it will not get cooled. It takes time. It's for the same reason sweaters and woolen clothes, quilts trap air and avoid exchange of heat.
