The practice of Pictorial Photography cannot be taught by a Kindergarten system; the mere comparison with typical examples must be accompanied with a… - Alfred Horsley Hinton

...strictly speaking, tone is the relative lightness and darkness due to the effect of light governed by atmosphere, and has nothing to do with the relative lightness and darkness or relative value with which various colours appear when compared with each other.

… nature often produces combinations and effects which on paper appear incorrect.

As a general rule, therefore, the lights and shadows of clouds near the horizon are less vivid than in clouds higher up. Exceptions to this rough rule will be found when the source of light is near the horizon, as in sunset and sunrise, also when there is a gathering of local stormclouds which may hover over the distance as a dark pall, when there is fair weather and light, transparent clouds near to us.