It doesn't matter who it is or where, when people look up to a darkening sky they know what it means. The darker a cloud, the harder the precipitation that comes with it; it's just one of those simple facts of nature. But what about the added moisture changes the cloud from a white to a darker color?
Like snow, clouds are made up of many small ice crystals that reflect the visible light from the sun without absorbing any color wavelengths and giving them their white appearance. Or in basic language, clouds are white because their complex makeup reflects light. Yet in an airplane looking down from a window one would see with clarity that all clouds are white, even if those clouds are dark from the ground. So why is that?
First, it's better to explain the cloud itself. Clouds and the moisture within it are lifted on subtle warm air currents from the ground. When a cloud becomes too heavy for these air currents it sinks towards the earth, or will release its moisture in the form of rain or snow depending on the temperature. Understanding the thermal currents and the supporting of clouds explain why the cloud ceiling in the heat abundant summer is so much higher than one in a colder winter. This also explains why rain clouds get darker than snow clouds, because warmer upward currents can support a greater moisture load.
Going back to the colors and visible light, what makes a rain cloud dark? Now understanding that a cloud in warmer weather holds more moisture - more rain - it is easier to see that as the level of moisture increases in a cloud, fewer colors will be scattered around by the cloud as the water begins to take color. With the more light absorbed by the water, the darker the cloud will appear from below, while the ice crystals on the top part of the cloud will reflect the same quantity of light regardless of the heavier water-laden portion below. This is why clouds are dark and darker clouds are more likely to rain, because the moisture count in the cloud is much more saturated and collected in the bottom portion of the cloud.
So next time you have a summer picnic and the skies grow dark, instead of rushing along and scrambling for shelter, take a moment to reflect and observe. If possible, make a game of it to determine how the warmth around you will delay or impede the storm's progress. If nothing else, instead of just being wet, you may have learned something.