Why does an open ended column reflect sound?

[BigMacnFries]

I am reading about how sound waves travelling down a column that is open at one end (clarinet, vocal tract etc). Apparently when they reach the open end some are reflected and interfere constructively or destructivly with subsequent waves in the column. My question is why are the sound waves reflected at all at the end of the column, its not like there is a thick solid to bounce against.
Thanks

[Tide]

In effect there is an impedance mismatch at the end of the column. Basically, it means that the pressure variations at the end of the column end up pushing and pulling a noticeably larger mass of air.

[BigMacnFries]

Can you please explain more, I don't understand. Thanks

[Tide]

Try this: http://www.geocities.com/Vienna/3941/oetubes.html

[Antiphon]

Can you please explain more, I don't understand. Thanks

Whenever the pipe changes shape or ends, the pressue and velocity ratio
of the moving gass changes. At such a change there will always be
some energy reflected backward. In free space this is how radars work,
and in pipes this is why you hear stange echoes from very long pipes.

The only way to prevent a reflection is to have the open end presented
with the same conditions as occurr when you place another identical pipe
to the end, i.e. extending the pipe.

[BigMacnFries]

Link was excellent, thanks Tide