Suddenly Bill became aware of the fact that he could not see very far ahead. There did not seem to be any clouds in the sky, but the visibility became extremely limited. Gradually the haze grew thicker around them. It had the distinct odor of burning timber. The Flight had entered the dense smoke pall which covers the Northwest section of our country during the forest fire season. It has not the appearance of clouds, but is just as impenetrable to the eye. Clouds vary in color from white, through gray to black. At times they have a distinctive greenish tinge which always is indicative of hail being present. The smoke pall has a dirty, grayish brown color which tends toward a golden hue when the sun shines on it.