Wind in a bowl - convection

21 February 2010


Convection teaser



A bowlA transparent bowl of waterFood colouringFood colouring
Small BottlesA small bottle, ideally quite squatIce cube tray and freezer


Make some coloured ice cubes by mixing 2-3 of drops of food colouring with the water for each cube, then put them in the freezer.

When the cubes are frozen:

Fill your bowl nearly to the top with water, and wait for the water to settle a bit.

Take your small bottle, add 2-3 drops of food colouring and top it up with hot tap water (let the tap run for a while so it is nice and warm).

Put the bottle into your bowl of water. Unless you have found a very squat bottle you may have to lay it on its side. Watch what happens to the coloured water for a few minutes.

If your water becomes too cloudy, change the water in the bowl.

Take one of your coloured ice cubes and place it on the surface and watch for a minute or two.

What happens?


You should find that coloured water rises from the bottle of hot water, and will eventually fall back down again, and that you see coloured water falling around the sides of the ice cube.

Convection in a bowl of water. First a small bottle of hot water is put in the bowl, and then a coloured ice cube is floated on the surface.


When most fluids, including air and water, gets hotter they expand+, this means that the same volume of fluid weighs less, and it will float on the fluid around it. This is the reason that the hot water floated upwards, and cold water from the ice cube is more dense so it sank to the bottom of the bowl.

This produces a circulation, hot fluid rising over a heat source cooling at the top and falling again somewhere else in a process called convection. Air does the same thing, so if you get hot air and smoke coming out of a chimney rises, and air over a particularly hot piece of ground on a sunny day will rise up in a thermal. The cold air rushing in from the sides of the thermal is a wind. In fact the energy to drive all winds comes from convection.

+Water is actually slightly more complex than this, if it is between 0°C and 4°C it actually expands as it cools, which is related to ice being less dense and therefore larger than the same mass of water


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