If you've ever flown a glider, either a radio controlled model glider or a full-size man-carrying glider, then you'd be aware that cold air do in fact fall in a column. Glider pilots call hot air rising **"thermals"** and cold air falling **"sink"**. Both move in columns, bubbles, sheets etc.

For every "shape" hot air moves in upwards, cold air can also move in the same way downwards. Indeed, as you have noticed, the logic should be applicable to both.

What determines which part of air forms a column is which part is the majority. If a small amount of hot air rises surrounded by cold air then of course that hot air will be a column - simply because of the small amount of air moving up. If the cold air sinks surrounded by hot air then of course that cold air will be a column.

The logic is similar to pouring water. If you pour water out of a bottle into your kitchen sink the water falls in a column. If on the other hand you fill that same bottle submerged in a swimming pool it is air (bubbles) that rise in a column. What determines weather or not water or air becomes a column is which is the majority and which is the minority.

But be aware columns are not the only way hot and cold air can move in.