It is well known that the ionosphere varies markedly with altitude, latitude, longitude, universal time, season, solar cycle, geomagnetic activity, and shows significant day-to-day variability. Total electron content (TEC) measurements and ionospheric model simulations have been used extensively to study the general morphology and variability of the ionosphere during both geomagnetically quiet and disturbed conditions. In this talk, the global day-to-day ionospheric TEC variability will be discussed. Of particular interest are spatial correlations, which describe a statistical relationship between ionospheric variables at different locations. These correlations are not only important for a better understanding of the physical mechanisms that cause ionospheric weather variability, but are also a crucial component of modern-day data assimilation systems. In order to better understand the observed TEC variability and its causes, the relative importance of the neutral wind and the electric field for the TEC variations will be discussed. Model simulations using a physics-based numerical Ionosphere/Plasmasphere Model (IPM) are used to study the effects of the neutral wind and vertical drift on TEC.