Why are clouds lighter than the sky during the day but darker at night

Question

This is probably a very basic question but I couldn't find a good answer to it, most search results are about rain clouds or clouds appearing red at night (something I've never seen except for during sunset but apparently it's common in bigger cities).

Basically what I'm wondering is why clouds during the day appear lighter than the sky (white vs light blue) while clouds at night and during the evening appear darker than the sky (see image).

Image quality is low because I took it with my phone through my window.

I guess the clouds could be blocking the light and therefore appear darker but in that case, shouldn't the same thing be happening during the day?

Answer 1

As @シャシュワト mentioned, it is simply because light is diffusing/reflecting/refracting/etc from the surroundings and bouncing off the clouds (which are actually coloured white) The same would happen if you looked at a white sheet of paper in a dark room.

At night, the light from the surroundings is not strong enough to bounce off the cloud and reach your eyes, so effectively, the clouds are blocking whatever little ambient lighting there is

Answer 2

There could be quite a few things going on.

Off the bat there's no incoming light for them to scatter: during the day, clouds are white because the water droplets are big enough for all visible light to cause Mie scattering, but if you don't have much light falling on them, you can't observe the scattering and you can't observe light passing through either.

Then you could consider the fact that in some places, it rains more in the evening/night than during the day (if you have hotter surface temperatures during the afternoon, you see cloud formation and precipitation during the late evening, and with the lower temperatures in the night, the air is more likely to become saturated, see Dew Point), and clouds which precede rain are thicker and denser. They don't allow much light pass through.

And lastly, there's less ambient light which they can reflect back towards you.