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).
- The Make Clouds demonstrations in this PALMS video show three different ways to make your own clouds in a bottle. The accompanying Teacher Notes can be found here.
- The Year 1 PALMS Package contains a lot more fun, hands-on activities related to weather including Teacher Notes and Student Worksheets.