Very easy for people from uk and usa!
questions : t / f (based on this text : ) )

why do clouds turn gray before it rains?

it is thickness, or height of clouds, that makes them look gray. clouds are formed when water vapor condenses within pockets of rising air. under the right conditions, the air continues to rise, causing the cloud do build higher and higher. the tiny water droplets and ice crystals in clouds are jusy the right size to scatter all the colors of light. when light contains all colors, we perceive it as white. when clouds are thin, they let a large portion of the light through and appear white. but like any object that transmits light, the thicker they are, the less light makes it through. as their thickness increases, the bottoms of clouds look darker but still scatter all the colors. we perceive this as gray. if you look carefully, you will notice that the relatively flat bottoms of clouds are always a little grayer than their sides. the taller the clouds become, the grayer their bottoms look.

questions : t / f (based on this text : ) )

1. clouds are gray because they rise through dirty air.

2. white clouds are thicker than gray clouds.

3. the flat bottoms of clouds always transmit less light than their sides.