I believe we are dealing with a form of ventilation referred to as:  Buoyancy Driven Stack Ventilation.

The two most significant factors dictating air flow will therefore be as follows:

1.  Temp differential.  Tin-Tout is directly proportional to the flow rate through the top vent.  The larger the difference will be multiplied directly to influence flow rate.

2.  The diameter of the top vent.  Diameter has an influence on flow rate that is approximately Diameter squared.  For our purposes the length of the tube is zero, which would result in a squared effect of diameter.   

The below link will be used to estimate Temp Difference:

https://drive.google.com/file/d/1PqEmqS-_WC6EvCvcyVGV4pUm4kEAZPAB/view

If we search the thermal images of previous threads by Etienne,  we learn that internal temp does not decrease beyond 10C.  This is quite important to point out.   

That's important to keep in mind, because it directly influences heat loss through the upper vent.  That upper vent leaks the heat linearly with low temperatures below 10C or so.

Other References:

https://www.aivc.org/sites/default/files/airbase_4935.pdf

https://www.researchgate.net/publication/281880673_Natural_ventilation_in_Building_Ventilation_the_state_of_the_art

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