An elementary problem asks,
A book is at rest on a table top. In what direction is the net force of air pressure on the book?
Is this a meaningful question, and if so, what's the answer?
If we imagine that the book and table are completely smooth, there's no air between the book and table. Then the air pressure from above the book creates a large net downward force of hundreds of pounds. (This will be countered by an upward force from the table.)
On the other hand, if we imagine that due to the rough nature of the book and table, there is some air between the book and the table at most places, maybe there is enough air underneath the book to provide a net pressure force that is upwards. The scale height of the atmosphere is about 10^4m, so a 1cm book needs to have less than 1 part in 10^6 in contact with the table to have net upward force from air pressure.
How realistic are these approaches? Do we need a molecular view of the book, table, and air to understand the situation? For a typical, everyday book and table, is it meaningful to ask what direction the net force of air pressure pushes?