Climate system dynamics are driven by both internal and external forcing, such as volcanic eruptions, solar variations, or human-induced modifications to the planetary radiative balance, for instance via anthropogenic emissions of greenhouse gases and/or land-use changes.
The RESIN project  gives the following definition:
Other International Definitions
The U.S. National Weather Service specifies zero to three hours, though up to six hours may be used by some.
- European Climate Adaptation Platform (CLIMATE-ADAPT) Glossary
- Mach, K.J., S. Planton and C. von Stechow (eds.). Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Annex II: Glossary. [Core Writing Team, R.K. Pachauri and L.A. Meyer (eds.)]. IPCC, Geneva, Switzerland, pp. 117-130.
- American Meteorological Society (AMS) - glossary