An advanced model for melt blowing was used to predict the effects of modified air fields on fiber formation. The model, which was developed previously, involves the simultaneous solution of the momentum, energy, and continuity equations. Crystallization effects were included. The model equations were solved numerically. Simulations were done for two classes of modified air fields. The first class of modified air field has a plateau of constant velocity and temperature. For wide plateaus placed near the die face, the effect of the plateau is substantial. Fiber diameters are reduced (by up to two times) versus diameters for when there is no plateau. The second class of modified air field has a quench (or a plateau-quench). Quenching was simulated as a step drop in air temperature. The simulation showed that quenching can enhance online crystallinity, though fiber attenuation is reduced when quenching is used.