When we think of inertia, we think of a mass or object and the amount of force it takes to either stop it or start it in motion. Effort you might say. Inertia though takes many forms. The shortest day and longest night of the year falls on either the 21 or 22 of December depending ion the year. This is the day that gets the least amount of sunlight to the Northern latitudes. Sunlight/sun heat energy is the lowest of the year on that day.
I don't know about you, but I've always thought that when you have the least of something, it can only get better after that. Nope, in the case of Winter, Winter itself doesn't even start until the Winter Solstice occurs. This is all due to that darn pesky thing called inertia. Inertia you see can take on many forms and having winter happen after the days start getting longer is due to the cooling off cycle we have been in since mid Fall. Now that we've lost the heat we had stored in the Northern hemisphere (There are two distinctive weather patterns located on the Northern and Southern hemispheres.), it needs to build back up again. This is why it takes so long for Winter to give up it's cold embrace when our days get longer and sun energy increases from Solstice onward.
Inertia can take on many forms. From mass, to heat, to even politics and public opinion. We won't go there except to point out it was inertia in effect during the 60s, the McCarthy era, and even now. Rest assured the pendulum will eventually move the other way, but for a period we'll enter Winter or Summer (depending on your point of view). America is kind of like Wisconsin weather. If you don't like the weather, stick around. It's bound to change.