Distribute black bin bags, and ask students to tape over desk.

Distribute shallow tub and bottle containing very warm water. Add dry ice (2 pellets).

The following activities can be done with more or less emphasis on the weather phenomenon they are modelling.

Ask what weather phenomenon they have made. Cloud. Water vapour in the air cools to form water droplets.
(How dry ice makes this: super cold, becomes a gas at -80°C. The water vapour in the air (from the warm water in the tub), cools and condenses among the cold CO2 gas.)

Add more dry ice and hot water to the tub, then demonstrate making tornados: hold hand flat and upright within the cloud spilling out of the tub, then lift up fast. (Too tricky for most Ks, but they like to watch.)
The spinning air mass of a tornado is called a vortex. The air rotates around the funnel shape.
A real tornadoes is one of the most powerful forces on earth - winds up to 500km/hr.
They form when warm, moist air meets cool air and they start to spin. The air spirals up into a thundercloud. The low pressure inside (from fast winds) cools the air which condenses into a cloud.

Another weather feature is also a vortex but spins in a ring: called a microburst.
A microburst is formed when cold air descends to the ground fairly fast, then hits the ground and spins away in a ring.
This kind of vortex can be modelled with a pellet of dry ice inside a little warm water in a recycled water bottle. Squeeze and release the water bottle quickly and gently. It seems to work better as the dry ice bubbling subsides a little.