Fourier Number
The Fourier number (Fo) is a dimensionless number that is used to describe transient heat conduction. It is the ratio of the conductive/diffusive transport rate to the heat/matter storage rate.
Definition
In general, the Fo is defined as

(1) 
For heat transfer the Fourier number is defined as

(2) 
where
 is the thermal diffusivity
 is time, and is the length through which conduction occurs
For diffusive transient mass transfer the Fourier number is defined as

(3) 
where
 is the diffusivity
 is the time
 is the length of interest
Usage
The Fo can be used with the Biot number (Bi) to determine the unsteady state heating or cooling of an object. If Bi is less than 0.1, then the lumped capacitance model can be used, which can be seen on the Biot number page.
References
^{[1]}
 ↑ Incropera, Frank P.; DeWitt, David P. Fundamentals of Heat and Mass Transfer (5th ed.). Wiley.