system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does warm air blow down on you when you exit some buildings through automatic doors in the winter?
gpt
They are air curtains. The concept is that they are using airflow to prevent the warm inside air from escaping out and the cold outside air from coming inn. And just in case some outside air does get inn they make sure the air that forms the inside part of the air curtain is kept at a higher temperature then the rest of the store so that it will heat up any of the cold outside air coming inn to room temperature.