Why does condensation form near the window frame first?

That is the area where the two differntiating pressure come in contact

Maybe it’s because this is where the temperature difference is highest?
I’m guessing that the frame can retain more heat than the glass.
If the moisture or water in the frame cools rapidly when it comes into contact with the glass…
(Whereas, the center of the glass is more or less at an even temperature?)

The simple answer is that this is the coolest area of the glass. The complicated part is understanding why that’s the case. The reason is differences in airflow over the surface.

The temperature of a windowpane will be determined by a thermal tug-of-war between the cool air on one side and the warm air on the other. The temperature of the air is one factor, but the velocity at which it moves across the surface is another. Moving air is good at heating and cooling; stagnant air is lousy at it.

In a situation where you have a cool window in a warm room, the inside air that contacts the cool glass will cool and begin to fall downward because it’s now denser than the surrounding air. New warm air takes its place at the top of the window, and you end up with a continuous convection current moving downward across the surface of the glass.

If you could measure the velocity of that airflow at various points on the window, however, you would find that it’s not uniform. It would be greatest away from the window frame, because the frame introduces obstacles to the airflow. For instance, as the air moves down the glass of the upper sash and encounters the ledge formed by the top of the lower sash, it is slowed and piles up there. Velocity measured there would be low. The biggest obstacle would be the windowsill at the bottom of the lower sash; here you’ll have a very low velocity as the airflow is more completely obstructed. Even the sides of the window casing will create eddies along the sides of the window that impede airflow there.

This business of velocity is important when you consider that the cool air on the other side is busily trying to cool the glass down. Back on the warm side, the air that is moving fastest will be doing a better job of keeping the glass warm; where it’s moving slowest, the cool air on the opposite side will have an advantage and the glass will be cooler.

Now to complicate matters even more, consider that the exact opposite dynamic is happening over on the cool side of the window, which has the effect of reinforcing the effects I just described over on the warm side. The end result is that the coldest areas of the glass are the bottom edges of the two sashes (especially of the lower sash), and, to a lesser extent, the lower right and left sides of the two sashes. So this is where condensation forms first.

Harp: So if I read that right, condensation is quicker to form along the bottom of the window (along the window frame) than along the top of the window (also along the window frame)? Neat, I’ll have to take a peek next time.

Right, at least on a typical double-hung window.

Hey! People! Quit GAing my answers when it’s wrong!