On the basis of analysis of east-west geomagnetic disturbance fields at midlatitudes, we investigated the characteristics of temporal variations of field-aligned currents on the nightside during geomagnetic storms. The results indicate that upward field-aligned currents develop on the nightside, especially in the postmidnight, when the interplanetary magnetic field (IMF) is directed southward. This suggests that the upward field-aligned currents are principally associated with the convection electric field. It is also found that the upward field-aligned currents in the postmidnight can be intensified at substorm expansion, while they principally depend on the southward component of the IMF. The intensification of the upward currents is regarded as a different physical process from the formation of a substorm current wedge at a substorm expansion. We discuss some possible effects of the IMF and substorms on the generation of the upward currents on the nightside, especially in the postmidnight.