One question that we are all bound to have asked as children is ‘Why is the sky blue?’ which may have led to some interesting responses from adults close by! This may also have been followed up with ‘How are clouds made?’ to further satisfy our curiosity about the world around us. Read on to find out some clues as to how you can explain and demonstrate these concepts to inquisitive young ones.
Why is the sky blue?
To explain why the sky appears blue on a clear, sunny day, we first need to know a little about the visible light spectrum. In scientific terms, light is a form of radiation. The light that our eye can see is just a small part of a larger range of radiation types, called the electromagnetic spectrum, that have different wavelengths, energies (frequencies) and properties. As the name suggests, visible light is the only part of the electromagnetic spectrum that we can see (with the naked eye). The diagram below shows where visible light fits in to the electromagnetic spectrum.
The light we see from the Sun appears as white light and we know that white light is made from a mix of colours. We see this when a rainbow appears in the sky. A rainbow is visible when white light from the Sun passes through tiny water droplets in the sky. The water droplets cause the white light to bend (refract) and split into its component colours – red, orange, yellow, green, blue, indigo and violet. Each of these colours of light have different energies so they come out of the water droplet at different angles, causing them to become visible.
- More information about visible light and its colours can be found on this Science Learning Hub webpage.
We now know a little more about the spectrum of colours in visible light and how rainbows form, so let’s move on to our initial question about why the sky appears blue. Sunlight entering Earth’s atmosphere is scattered and bent by gases in the atmosphere. Clean, dry air scatters blue light the most. Therefore, on a fine and sunny day, the sky will look blue. On rainy days, the sky looks grey because the water droplets in the air scatter all colours of light, not just blue, and the mix of light colours appears grey to our eyes.
- The Blue Skies activity in this PALMS demonstration video shows how you can easily model why the sky is blue with just some water, milk and a torch. The accompanying Teacher Notes can be found here.
- This NASA webpage explains the concept further and includes a short animation explaining why the colour of the sky changes as the sun sets.
How are clouds made?
Clouds are collections of lots of tiny water droplets or ice crystals. They form due to a process known as condensation, as part of the continuous water cycle. You can find out more about the water cycle by watching The Water Cycle animation from WASP.
Water can exist in three states – solid, liquid or gas (often called vapour). Clouds form when warm air containing water vapour gets cooler and changes to droplets of liquid water or small crystals of ice which collect together.
Clouds appear white or grey because the water droplets or ice crystals are large enough to scatter all the colours that make up white light, so the light doesn’t split into its component colours. When the droplets or ice crystals in clouds become heavier, by collecting on particles in the atmosphere, they will fall to the Earth’s surface as a form of precipitation (rain, snow, sleet, hail).