Forecasting the occurrence and strength of the nocturnal jet is important to aviation, especially to balloonists and small aircraft. Jets also have implications for pollution transportation and the clearance of low cloud and fog. A one-dimensional model showed that the main mechanism for the development of the nocturnal jet was a hybrid between an inertial oscillation in a layer within and above the jet core and a quasi-steady jet within the inversion. The model, supported by observations, suggests that a nocturnal inversion is required to produce a jet and that the strength of the jet maximum is approximately 1.3 times the geostrophic wind. A wind profile minimum is also predicted to occur when the inertial oscillation reaches a minimum in the stress-free layer and this was observed using a tethered balloon flown at Cardington.