Eli5: If water is transparent, why are clouds white?
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Light can act in one of three ways when it encounters an interface between two substances (passes from air into a liquid or solid, say): It can reflect, it can transmit (pass through), or it can be absorbed. Which will happen depends on three main things: the wavelength of the light (its energy level, color), the range of energy absorption in the substance(s) – everything has energy to its bonds and electrons and will absorb energy in those ranges, although it might later also re-emit that light, possibly at a different energy level (different color), and the angle that the light is hitting the interface from (at some angle, the light cannot cross the interface and must either absorb or reflect).
It is this last option that makes clouds seem white to us. The surfaces of the water droplets and ice crystals in the clouds provide a lot of surface to the light which is too much of an angle to allow passage, so the light reflects. Scattering of the light usually appears to us as white or gray.
Basically, just like the water in foamy waves seems white to us even though the water is blue in color (lots of light scatter because the angles that the light is hitting are random and in all possible ranges).
Just as an aside, water is not fully transparent, it absorbs color down on the red end of the spectrum more than on the blue end, although the absorbance is weak so the water seems mostly transparent to us except when light passes through a lot of water, which is why there is a depth in water that goes dark, light never gets down that far. This is also why water is a greenhouse gas, a fairly good one actually, because it absorbs infrared energy really well (re-emits it later).
This thing with light scatter is also why sometimes quartz crystals seem clear and sometimes looks white (milky quartz). It is also why glass that is scratched or cracked gets hard to see through, and why mirrors need to be a really smooth surface to work well.
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