Recently, it has also become known that, even when no kosa is observed on the ground level because no visibility is affected there, thin dust is observed in the high-altitude atmospheric layer called the free atmosphere (also known as free troposphere).
また近年、地上では視程も低下しないため黄砂として観測されない時に、自由大気（自由対流圏）と呼ばれる高層で薄い砂塵が観測されることがわかってきた。