The stack effect is the airflow pattern in your house from bottom to top. Air pressure also comes into play. Air flows from areas of high pressure to areas of low pressure. When there is a greater difference between indoor and outdoor temperatures, the more significant this airflow becomes. (Source: Fine Homebuilding)
True, you can’t actually see airflow, but it might be helpful to have a way to visualize the stack effect to better understand it. Your home certainly is not located in the northern portion of the country, but a lot of good information can be gained from The Cold Climate Housing Research Center’s YouTube videos, including “Your Nothern Home: Stack Effect” that illustrates the stack effect.