Homework activity: Convection – brewing up a storm!

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Convection is one of the major driving forces behind our weather. Convection causes warm air to rise above cold air and is the principle mechanism of cloud formation. Some of these clouds are quite innocent, fluffy looking little things whilst others grow into menacing thunder clouds. But how and why does this happen? A simple experiment using household equipment can help us investigate. Give these instructions to your students so they can find out more for themselves.

 

A STORM IN A TEA CUP?

Not quite. We are going to set up a model of our atmosphere using the following things:

 

  1. A transparent clear plastic or glass container or dish about the size of a shoe box.
  2. Some red food colouring
  3. Some ice cubes made from water with blue food colouring already in it.
  4. Red and blue pencils or pens and some plain paper.

 

METHOD

  1. Fill up the container with tap water at room temperature until it is about ¾ full.
  2. Gently, place a few blue ice cubes together at one end of the container.
  3. Add a few drops of red food colouring at the opposite end of the container.
  4. Have a rest for up to 10 seconds.

 

OBSERVATIONS

Enjoy your rest? Using the red and blue pencils, sketch what you see happening in the water. Carefully observe how the convection current moves the blue and red liquid from one place to another. Try and show this as a series of diagrams and try to explain what is happening.

 

CONCLUSIONS

  1. What happens to the blue coloured liquid?
  2. What happens to the red coloured liquid?
  3. How does this behaviour model what is happening in our atmosphere as clouds are formed? Use the internet to find out!
  4. Check your observations by reading the explanation below!

 

EXPLANATION

Heat is being transferred through the process of convection in the container. The cold, blue water sinks, while the warmer, red water rises and then stays higher up. This is exactly what happens to the air in the atmosphere when a cold front encounters a warm front.

Thunderstorms are formed when the air in the atmosphere is unstable. The warm air is forced to rise by the approaching cold front, which leads to a cumulus cloud being formed if the air is moist. The cloud will continue to grow as long as warm air below it continues to rise. An updraft is created – these can reach speeds of up to 145 km/h!

As the warm air rises it cools and condenses, forming small droplets of water. Eventually the droplets become too heavy to be supported by the updraft of air and they fall as hail. Down drafts are initiated by the downward drag of falling precipitation. Electrostatic charging occurs and thunder, lightning and heavy rain begin!

 

INVESTIGATE FURTHER

Repeat the experiment, but try it using different temperatures of water in the container. How does the temperature of the water change the behaviour of the convection currents? Can you use this information to explain why thunderstorms tend to happen more regularly in places with hot climates?

 

By Mark Prince for Smart Learning