Is that Soap or Sap? It's Both!

I have never seen anything like this weather; it's RIDICULOUS. Today it's 63 degrees outside and raining, and on Christmas day, it's supposed to be 70.

It turns out even the trees are confused. Walking in the forest today, I notice several with foaming white stuff oozing off the bark -- it looked like soap bubbles. I thought it might be sap running as thin and bubbly as soapy water, but it was not sweet. Folks on Facebook thought it was disgorging phloem (basically sap), but it turns out that it is what it looks like: soap.
The folks at the Au Sable Institute for Environmental Studies explain:

When it is dry, particles from the air deposit themselves on the surface of tree bark (like dust). This dust includes bits of salts, acids and other particles in the air. When it rains, these particles dissolve into a liquid solution. Soap is essentially a chemical mixture of salts and acids formed into a molecule. The primitive soap then foams as it moves down the trunk and hits a rough patch.

It turns out that it is also a soap made from sap. Pine tars are used to make soap, and perhaps a few other tree tars can be used for the same purposes. In any case, I now know what it is, and why, but in truth I have never seen it before that I can recall, despite a lot of time in the woods.