Thermal Updrafts

Thermal updrafts are a natural phenomenon produced by localised heating of air. This causes air to rise and fall in relatively small localised areas.

We can make a simple experiment to demonstrate this effect. The diagram on the left shows how the top half of a bottle can be used to trap heat from the sunlight. As the air warms up inside it rises and exits the top of the bottle. If a simple propeller is places above, it will turn due to the air passing through it. The bottle must be placed on some small blocks so that the air can flow in at the bottom.

The propeller can be made by cutting out a shape like shown from some stiff card. If the blades are all twisted the same way this will spin when air passes through it. It can be easily mounted by attaching a stiff wire to the bottle neck and inserting it into a hole in the propeller.