Solid materials suspended in the atmosphere in the form of small irregular particles, many of which are microscopic in size. It imparts a tannish or grayish hue to distant objects. The sun's disk is pale or colorless or has a yellowish tinge at all periods of the day. Dust cannot be a stable component of the atmosphere because it must eventually fall back to the earth's surface when winds and turbulence become too weak to bear it aloft. Dust is due to many natural and artificial sources, for example, volcanic eruptions, salt spray from the seas, blowing solid particles, plant pollen, bacteria, and smoke and ashes from forest fires and industrial combustion processes. It was once thought that dust particles were a main source of condensation nuclei; this is no longer regarded as probable as most dusts are not sufficiently hygroscopic. Compare smoke, haze; see duststorm, dust devil.