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Tomorrow morning I am taking a hot air balloon flight over the Gold Coast Hinterland, I will be taking my D700 with 24-120 f4. Would you recommend the use of a CPL?

I personally would not. Early morning sky is fairly atmospheric....CPL would lose you a couple of stops of light and you are going to need them....skys can be easily manipulated in Photoshop anyway. I'm presuming very early flight as is usual for hot air baloons. I would also consider taking a prime lens if you have one...50mm would be useful but any prime should also go for a flight. I would maybe under expose by 1/3 of a stop to give the sky that intense blue that seems to make baloon shots so attractive.
Remember...the only thing keeping you alive is a lot of hot air. Don't think about that too much.  PS...no reason why you can't take the CPL and give it a tryout for a few shots. Experimentation is a great way to learn new tricks. Also, rear LCD and instant playback may just indicate the best way to go.

Thanks Matt, I will take the 50mm also. It is an early flight, we meet at 4.45am! I am looking forward to it but the wife is somewhat apprehensive.

I think you'll be looking to get some wide panoramic so a CPL would not be the best option for that.... using the 50 or similar prime to stitch a set of shots is the way to go for that.
Apart from that... just relax and enjoy the ride... a photograph is nice but not as memorable as the experience itself !!!

I am looking forward to seeing the pictures!