Jumping yeast

[brydo]

So my new mangrove Jack's heat pad and thermostat arrived and I've set it 21oc and placed a 5 litre glass demijohn containing an all grain ipa on top of it. It's been fermenting for 13 days now so was hoping to heat it up for the last few days before bottling and small clumps of yeast are now shooting up from the bottom of the jar and then falling back down.
What is this and is it ok ?
Cheers guys

 

[MmmBeer]

If it's shooting up as you say, it's a gas bubble and therefore CO2. It's perfectly normal.

The yeast cells and various large molecules have clumped together (floculated) and the CO2 generated by the increase in temperature gets trapped in the clump by the surface tension of the liquid. Once it reaches the surface it is able to break free and the clump falls back to the bottom.