Chapter 9, Verse 23 - Sun Tzu

When there is dust rising in a high column, it is the sign of chariots advancing; when the dust is low, but spread over a wide area, it betokens the approach of infantry. When it branches out in different directions, it shows that parties have been sent to collect firewood. A few clouds of dust moving to and fro signify that the army is encamping.
~ from The Art of War, Giles translation ~

Go here to read the introductory post to the verses of The Art of War.