[Scruffywolf]

In regular cold tap water the effect won't last very long. What you can do is put a small crock pot in the bottom of the barrel keeping the water hot then dropping in the dry ice as the effect wains. Unfortunately the effect lasts only about 20 minutes or so then you have to drop in more dry ice. I once bought a large block and kept it in the crock pot in a cauldron and it worked pretty good. ..... cheers Scruff'

 

[Hooked_on_Scares]

Warmer water = more fog, but also eats your ice quicker. The ice will also quickly cool down the water, so you'll have to keep heating up the water to maintain the degree of the effect.

- Hook

 

[Judge]

dry ice doesnt last long at all.
a 2" x 2" will last like a few minutes

 

[Wolfman]

Put a candle under it, swap the water over for fresh hot water, but do something. The dry ice will freeze the water...

 

[dheppel]

This is too late for this year but next year forget about the dry ice and use an ultrasonic transducer. this is what is in most cool humidifiers now. you can get one pretty cheep through ebay. it will give you long lasting fog effect and all you have to do is replace the water time to time