Different precipitation seasonality regimes, although produced by different surface and atmospheric conditions, can be understood from the basic atmospheric fields. By comparing the change of water vapor from winter to summer with the change of temperature using the NARR reanalysis, three seasonality cases over the United States and Mexico are analyzed. In the western coast of the U.S., the change of temperature from winter to summer is much greater than the change of water vapor. So, relative to summer, the coldness of the winter air is much more significant than the dryness, which makes the winter have a large saturation extent and thus precipitation. In contrast, over South Mexico, the much more significant moistness of the summer air than its warmness is important to the summer monsoon precipitation. In the southeastern U.S. where precipitation occurs throughout the year, the changes of water vapor and temperature are roughly equivalent.