This is something you see in the sky, you'll need more hints though so I'll explain, it's sometimes fluffy and sometimes gray, and can release snow, hair or rain.
Cloud
If you look up during the day, then you will see this lit, because it is Earth's closest star, around which we orbit.
Sun
What is weather?
It's what the sky is like outside.
How's the weather?
It's sunny.
Why do we need to measure weather?
To prevent accidents and help us decide what to wear outside.
How's the weather?
It's snowy.
Which instrument measures rainfall?
A rain gauge
Read the riddle and guess the weather: I'm a breeze or a gust, a cyclone or gale, I am what is measured, on the Beaufort scale.
Wind.
Which animal is usually on a wind vane?
A rooster
Read the riddle and guess the weather: Dropping from the sky, more beautiful than rain, there are no two pieces, that'll ever be the same.
Snow
What does a thermometer measure?
Temperature
Read the riddle and guess the weather: This natural phenomenon, cannot be seen when it's dark, for it is cause by light hitting rain, to create a colourful arch.
A rainbow.
Read the riddle and guess the weather: This is something you see, when outside there's a storm, it comes down from the clouds, and appears in droplet form.
Rain