The technique of aerial or tonal perspective shows how things far away in the landscape normally appear paler (except when clouds or sun affect them). The artist Cornelius Varley studied the effects of light and atmosphere. This watercolour he painted in north Wales around 1805 has about six different tones. If you describe tone as proportions from 100% for black to 0% for white, the sky would be 10%, the dark rocks on the right 50%, and the hills 20% to 40%.
Kyffin Williams also painted the mountains of north Wales. He called this painting, Meadows, Mist and Mountains. The mountainside in the distance is so pale it is easy to miss it. The two rocky hills in the middle are darker because they are in silhouette. The foreground is much darker.