# Definition of spin up

**URL:** <https://forum.cmascenter.org/t/definition-of-spin-up/1079>\
**Category:** Run Time Errors and Issues\
**Created:** [October 28, 2019, 5:17pm UTC](https://forum.cmascenter.org/t/definition-of-spin-up/1079 "2019-10-28T17:17:50Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![zyj.jess](https://avatars.discourse-cdn.com/v4/letter/z/8dc957/32.png) [@zyj.jess](https://forum.cmascenter.org/u/zyj.jess)\
**Post date:** [October 28, 2019, 5:17pm UTC](https://forum.cmascenter.org/t/definition-of-spin-up/1079/1 "2019-10-28T17:17:50Z")

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Dear all;

Could you explain to me what is simulation “spin-up”? Appreciate it!

Best;  
Yijia

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**Author:** ![cjcoats](https://avatars.discourse-cdn.com/v4/letter/c/ac8455/32.png) [@cjcoats](https://forum.cmascenter.org/u/cjcoats)\
**Post date:** [October 28, 2019, 6:02pm UTC](https://forum.cmascenter.org/t/definition-of-spin-up/1079/2 "2019-10-28T18:02:57Z")

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One hopes that as a simulation proceeds, its state gets closer and closer to “reality”, even if the model’s initial conditions were quite bad. The simulation-period before the model-state is sufficiently close to reality is the spin-up period.

How long this takes depends upon a lot of things, including the type of model and its resolution (as well as how close the initial conditions were to reality). For meteorology models, this typically takes only a few simulation-hours; for air quality models, a few simulation-days; and for hydrology (ground-water) or river-sediment models it takes a few simulation-years (or decades (!), in bad cases)…