BmB said:

I haven't actually read the script, I simply assumed this was true:

Tobar said:

I had always thought it was his hand. But according to the script:

There is an ominous cracking sound from the base of the weather vane and a piece breaks off, falling into the clouds far below.

It is, here's the whole bit:

EXTERIOR: BOTTOM OF CLOUD CITY -- WEATHER VANE -- DUSK

Unable to hang onto the pipe, Luke tumbles out, emerging at
the undermost part of Cloud City. Reaching out desperately, he
manages to grab onto on electronic weather vane.

LUKE: Ben...Ben, please!

Luke tries to pull himself up on the weather vane but slips
back down. He hooks one of his legs around the fragile
instruments. All the while, a powerful current of air rushes
out at him from the exhaust pipe.

LUKE: Ben. Leia!

There is an ominous cracking sound from the base of the
weather vane and a piece breaks off, falling into the clouds
far below.

LUKE: Hear me! Leia!

Got it from here.