Why do some air-conditioned stores blast you with jets of air as you enter? I went to a grocery store on a hot day that was very well air-conditioned, and I noticed as I went through the open entrance that there seemed to be a very powerful downward air current right at the doorway.  After crossing the invisible threshold, the temperature immediately dropped a good 15 degrees or so.
How does this process work?
 A: This is called an air curtain or air door and it actually keeps flying insects from being able to enter the store.  It also helps trap the colder air inside.
Edit: see the link for how it works.
A: Most shopping malls have this kind of air door that blast you with high velocity air flow when you enter. As you might have noticed, shopping malls are always the cleanest places of them all. And I am not just talking about cleanliness of the floor area but the entire atmosphere in a mall. The reason they keep it clean is obvious. The method which they use to keep it clean is the answer to your question. The cleaning process in a shopping mall is modelled on the same process which is used in an IC fabrication facility, only that the constraints are less stringent. The pressure inside the mall is relatively higher than outside. Because of this airflow is always outward. The air comes from the ceilings & is collected at the floor. There is something called ACH(Air Changes Per Hour). In a fabrication facility, its about 100ACH. If you ever happen to enter a Fab facility, you always have to go through a series of doors, in each you are blasted with high velocity clean air to remove dust particles on your clothes & body. The constraint is less stringent in malls, purpose is same i.e. to aid the system set up in the mall to keep it clean. Also it acts as an air door since it would be irritating for a customer to have an actual door.
