What do the different cloud icons mean on the iPhone Weather app?

In the app, there are many different icons which are really quite self explanatory. However, there are two icons that are just clouds - no rain or anything along with it - but different shades of grey. One is dark grey, the other light grey.
What difference(s) do these signify?

They both mean "Mostly Cloudy", the dark grey one is at night, the light grey during the day.

There's a full list of the symbols and their meanings here: What do the different cloud icons mean on the iPhone Weather app