Why is the Sky Blue?

Why is the Sky Blue?

Learn about the scattering of light!


◊ A large straight sided glass

◊ Water

◊ Skimmed milk

◊ Torch

◊ A teaspoon

◊ A dark roommilk, glass and a torch


1. Fill the glass about 2/3 full of water.

2. Add about half a teaspoon of milk and give it a stir.

3. In a dark room, shine the torch down on the top of the water and look through the side of the glass. The light should appear blue.a torch shining through water in a glass

4. Then shine the torch through the bottom of the glass and look down from the top of the glass. The light should appear red.shining torch through the bottom of the glass


So why does it happen and how does this relate to the atmosphere?

The milk particles in the water scatter the light from the torch, just like gas molecules in the atmosphere scatter light coming from the sun. Blue light is scattered more than red light, which is why the sky is blue. The further the light has to travel through water the more of the blue light is scattered. The scattering of the light is known as Rayleigh scattering. This is why when the torch light is shone up from the bottom of the glass through a large volume of milk and water most of the blue light is scattered out and only red light is left to see.

This is a similar process to the one which leaves red skies at sunset; because when the sun sets it has to travel through a larger amount of the atmosphere before reaching our eyes. This means a large amount of the blue light is scattered, meaning only the longer red wavelength of light reaches our eyes.

Find out More

Red Sky at Night, Shepherd’s Delight

Why isn’t there any pink in rainbows?a rainbow

More experiments and demonstrations

IoP Snowflake

Find more Marvin and Milo ‘Do try this at home’ ideas on the Institute of Physics’ website

Hot Air Rises

Hot Air Rises

Learn about convection


 A large, wide mouthed jar (maybe a jam jar)

 A baby food jar



 2 different colours of food colouringa jam jar and string


1. Tie the string around the top of the smaller jar, making sure the end is long enough to lower the smaller jar to the bottom of the larger jar.

2. 2/3 fill the larger jar with cold water and some yellow food colouring.

3. Please get an adult to help with this step. Fill the small jar with some red food colouring and boiling hot water.Balls of string

4. Please get an adult to help with this step. Carefully lower the smaller jar using the piece of string into the larger jar.

5. Watch what happens!

What should have happened

The hot red water will have risen through the colder yellow water. This will have left a red-orange layer on the top of the yellow. The hot water is less dense than the cold water, so it rises above it.

Try this:

Try the experiment the other way round. Fill the larger jar with boiling water and the smaller with cold water. The colder, more dense water will be trapped at the bottom of the jar.

How does this relate to the atmosphere?

In the same way that the hot water rose through the cold water, in the atmosphere warm air rises through colder air. This is the process by which clouds can form and is known as convection. Air near the ground is heated by the sun throughout the day and rises through the cold air. This is the same mechanism that hot air balloons rely on.a hot air balloon

Where can I find more information?

More experiments and demonstrations including a teabag rocket.

Teabag Rocket

Teabag Rocket

Hot air rises


◊ Teabag (the sort with a string attached – cheaper tea bags tend to be lighter and work better)

◊ Matches

◊ Saucer or platea teabag rocket

This experiment needs still air conditions – no air conditioning or draughts from people moving around. It works very nicely – but very quickly!


1. Open up the teabag at both ends, removing the staple and string, such that you have a long cylinder of dry paper. You could always make a cup of tea with the tea leaves to have later, as long as you have a strainer!

2. Stand the open teabag upright on the saucer, then light the top of the teabag.

3. Watch what happens next – whose rocket goes highest?

The burning teabag heats up the air immediately above it. Hot air rises, so the warmer air will gradually rise up above the teabag, creating a rising current of warmair, also known as a thermal. When the teabag has burnt down far enough, the lift from the thermal will balance the weight of the teabag, and so it will lift up – it may even reach the ceiling!

So how does this relate to the Atmosphere?

The motion of the Earth’s atmosphere and oceans is driven by thermals – hot air rising. The large scale circulation of the atmosphere redistributes heat from the sunny Tropics to the less sunny polar regions. All sorts of weather systems, from land and sea breezes to hurricanes and tornadoes, are the result of warm air rising. You can sometimes see thermals in the atmosphere if you watch birds or gliders being carried upwards, or by watching a puffy cumulus cloud billow out.

Related Experiments

Alternatively, watch Science from the Sporran’s toaster hot air balloon:

Solubility of Carbon Dioxide

Solubility of Carbon Dioxide

SOLUBILITY= the property of a solid, liquid or gas to dissolve in another liquid.


 Two large bottles of identical fizzy drink

 A fridge or freezerfizzy drinks


1. Keep one bottle out of the fridge or freezer and put the other in the fridge/freezer for about an hour.

2. Take the bottle out of the fridge/freezer.

3. Immediately open both bottles SEPARATELY and listen to the fizzing noise made by each. The solubility of the carbon dioxide in water increases as the temperature decreases. So the water in the bottle put in the freezer will have absorbed more CO2 from the air above it, and hence there will be less pressure of CO2. This is very important in the atmosphere system.

How does this affect the atmosphere?

By the same token, the solubility of carbon dioxide in water decreases as the temperature of the water increases. This is important for the climate system because as the oceans warm, carbon dioxide, a greenhouse gas, is released into the atmosphere. This is known as a positive feedback mechanism. The release of CO2 into the atmosphere could lead to increased trapping of heat from the Earth.the atmosphere

More experiments and demonstrations

The Greenhouse Effect

Learn about carbon dioxide as a greenhouse gas

There are several ways to demonstrate the greenhouse effect in the classroom:

A good lab based experiment is described on the RSC website

Iain Stewart used a candle with an Infrared camera and carbon dioxide in a BBC documentary. Unfortunately, it is very difficult to recreate this in the classroom.

The following experiment, which demonstrates that increasing the greenhouse gas concentration leads to an increase in temperature, is commonly shown. However, it has a number of flaws – it is very hard to make sure that both jars receive the same amount of energy, that the colour of the jars and their surroundings isn’t having an effect on the experiment and most importantly, that the chemical reaction producing the CO2 doesn’t affect the temperature recorded.


◊ A desk lamp with a normal lightbulb (not energy saving or fluorescent)

◊ Two jam jars with lids

◊ Vinegar

◊ Bicarbonate of soda

◊ Two thermometers small enough to fit into the jam jars


1. Pour 2cm depth of vinegar into each jam jar

2. Add a thermometer to each jar, put the lid back on and place underneath a lamp for about 10 minutes.

3. Record the temperature of both thermometers.

4. Add about half a teaspoon of bicarbonate of soda to one of the jars and quickly screw the lid back on.

5. Put both of the jars underneath the lamp again for about 10 minutes.

6. Record the temperature of both thermometers again. The jar with the vinegar and bicarbonate of soda should have heated up most. This is because the reaction between the vinegar and the bicarbonate of soda releases carbon dioxide, a greenhouse gas, which absorbs infrared radiation (in this case heat from the lamp).
NB heat is also released by the reaction, so make sure the reaction has finished before you look at the temperature!

So how does this relate to atmosphere?

The sun is the external heat source for the Earth. If the Earth didn’t have an atmosphere with naturally occurring greenhouse gases it would be much colder than it is now: in fact it would be nearly -18°C, 33°C colder than the average temperature of the Earth today. Greenhouse gases exist naturally in the air and trap heat energy coming from the Earth, in a process known as the greenhouse effect. Greenhouse gases include water vapour, carbon dioxide, methane and nitrous oxide.

In the last couple of centuries, humans have increased the greenhouse gases in the atmosphere and this has enhanced the greenhouse effect leading to a greater amount of heat being trapped in the atmosphere. There is compelling evidence that most of the recent climate change is caused by man-made greenhouse gases.

Make a Lava Lamp

Make your own Lava Lamp

Learn about densitya lava lamp


◊ Plastic drinks bottle

◊ Vegetable oil

◊ Water

◊ Food colouring

◊ Alka-Seltzer tablets


1. Fill the plastic bottle ¾ full with vegetable oil.

2. Fill the rest of the bottle (almost to the top) with water.

3. Add approximately 10 drops of food colouring.a bottle filled with oil, water and food colouring

4. Add about half an Alka-Seltzer tablet and watch the reaction! Just add more of the Alka-Seltzer tablets when you need to. When the Alka-Seltzer tablet dissolves carbon dioxide is released. This nucleates (attaches to) the food colouring and water particles and causes them to float to the top of the soil. Upon reaching the surface the bubbles burst and the coloured water sinks back down.alkaseltzer in oil, water and food colouring

Note on Lava Lamps

Although this might look a little like a lava lamp, lava lamps usually work with heat. Lava lamps contain two liquids, one of which is more dense than the other when they are both the same temperature and so sinks below it. However when it is heated it becomes less dense than the other when it is heated and bubbles up to the surface.a lava lamp

How does this relate to the Atmosphere?

Density differences drive the circulation of the atmosphere and oceans, ultimately leading to all our weather.

Where can I find more information?

Find out how a lava lamp works

More experiments and demonstrations

Water Cycle in a Bowl

Our rain originally came from oceans, seas and lakes. It became clouds and eventually fell on us as rain.

What you will need:

  • A large bowl
  • A small yogurt pot or plastic cup
  • Clingfilm
  • Water
  • Small weight or a few coins
  • Sunny window sill

What to do:

  1. Take the large bowl and fill it with several centimetres of water.
  2. Place your small pot in the centre of the bowl of water, making sure not to get any water inside it.
  3. Cover the large bowl with clingfilm and fasten this down securely to the side of the bowl.
  4. Put a weight on top of the clingfilm, over the centre of the small pot to push the clingfilm down into it slightly.
  5. Place your experiment on a warm sunny window sill and leave for a few days.

    You should find that the heat of the sun evaporates the water, which rises, condenses on the cool plastic, and falls into the small container. This is a small version of what happens in the real water cycle.

You will need

a bowl
a small weight

Web page reproduced with the kind permission of the Met Office.

Tornado in a Jar

Make a tornado in a jar

A tornado is air which is spinning very fast. It is made by a special type of thunderstorm called a supercell.

What you will need:

  • A clear jam jar or similar see-through container with a screw-on lid
  • Washing-up liquid or liquid soap
  • Food colouring

What to do:

  1. Fill up the container with water then add a few drops of the washing-up liquid and a few drops of the food colouring. Tightly screw on the lid.
  2. Swirl the container around in a circle lots of times and then stop. Inside you should see what looks like a tornado. It will slowly disappear as it reaches the top of the container.
    Tornadoes in the real world are made in the same way.

You will needtornado in a jar

Web page reproduced with the kind permission of the Met Office

There is an alternative version here.

DIY Weather Fronts

Weather fronts

We get different sorts of weather because of the way warm air and cold air move around us in the atmosphere.

What you will need:

A large, clear tank (an empty fish tank would be ideal)
Warm and cold water
Two plastic cups
Small stones (pebbles)
Food colouring

What to do:

  1. Fill the tank with normal water and leave this for a few hours to come to room temperature.
  2. Place a few small pebbles in each of the two cups to act as weights.
  3. Pour some hot water and a few drops of red food colouring in one of the cups.
  4. Fill the other cup with very cold water and add some blue food colouring.
  5. Place the two cups into the water at the same time, one at either end of the tank – the pebbles should hold the cups at the bottom.
  6. Carefully watch how the two different coloured waters move. You should see that the warmer red water should rise to the top, and the cooler blue water should sink to the bottom.

Warm air (shown by your warm red water) is less dense than cold air (cold blue water), so warm air rises and pushes down with less pressure than cold air. As air cools, it becomes denser, so it sinks and also pushes down with greater pressure.

You will need

an empty fish tank

Web page reproduced with the kind permission of the Met Office