This exercise shows us the difference between shots taken in direct sunlight and under cloud.

These pictures were taken a few minutes apart on a day when the clouds were moving quickly across the sky. They vary between 1/160th of a second and 1/500th, a range of one and two-thirds stops.

The sun acts as a small, bright light source meaning that in direct sunlight, shadows (the parts of the scene that the light does not strike) are deep. In the third example, shot in direct sunlight, there is still some detail in the shadows, due to the nature of the subject, but far lass than in the cloudy version.
When cloud blocks the path of light between the sun and the subject, it scatters the light causing it to strike the subject from many angles at the same time, meaning the sun becomes a low-contrast light source. The clouds effectively become a huge diffuser to break up the direct lighting that the sun provides. While some sources will tell you that this soft light is not ‘interesting’ enough to make good pictures, if you want to show the maximum detail in a scene, this it is often ideal.

Overcast conditions

While cloudy conditions and soft light can make a scene flat because of the lack of shadows, it also allows us to show details that wouldn’t be visible in sunlight, as in the pictures below.
Here, the flowers and leaves are all an even colour and brightness because of the diffused lighting. With overhead, direct sunshine, the flower on the left would have been shaded by the nettle leaf above it and may have appeared darker than the one on the right – in reality, they are the same.

Rain

As I sat down to write this post, the heavens opened and I spent 5 minutes outside with my camera wrapped in a towel, laughing to myself about how daft it was to be outside in the rain. Rain is usually enough to keep us indoors, especially with expensive camera gear. It can however give some great shooting conditions with interesting reflections and even, diffused lighting.

While I didn’t take this picture tonight, it’s another example of why shooting in rain should not be discounted.

References

Hunter, F et al. (2012) Light – Science and magic. An introduction to photographic lighting. Oxford; Elsevier Inc.