full text of the classic FAA guide
We have discussed causes of turbulence, classified it into types, and offered some flight procedures to avoid it or minimize its hazards. Occurrences of turbulence, however, are local in extent and transient in character. A forecast of turbulence specifies a volume of airspace that is small when compared to useable airspace but relatively large compared to the localized extent of the hazard. Although general forecasts of turbulence are quite good, forecasting precise locations is at present impossible.
Generally, when a pilot receives a forecast, he plans his flight to avoid areas of most probable turbulence. Yet the best laid plans can go astray and he may encounter turbulence. Since no instruments are currently available for directly observing turbulence, the man on the ground can only confirm its existence or absence via pilot reports. HELP YOUR FELLOW PILOT AND THE WEATHER SERVICE—SEND PILOT REPORTS.
To make reports and forecasts meaningful, turbulence is classified into intensities based on the effects it has on the aircraft and passengers. Section 16 of AVIATION WEATHER SERVICES (AC 00-45) lists and describes these intensities. Use this guide in reporting your turbulence encounters.