MHT is very sensitive to less than optimum conditions; when conditions are not optimum (due to closed runways, bad weather etc.) the acceptance rate is reduced significantly.
Because the tower must provide visual separation, ILS RWY 15R circle to RWY 4L operations require the weather to be approximately 1500/4-5 (this may vary slightly).
There is no instrument approach to RWY 4L or RWY 22R. These runways have strict noise abatement procedures.
On Fridays and Sundays, during the summer months, MHT has a large volume of VFR traffic to and from Cape Cod.
When ZBW is in holding for MHT or N90, much of the holding traffic drops off of the ASD.