This is a lesson learned that I just remembered so I'm passing it on. Many years ago I was in the PA mountains on a bear hunt. November weather is tricky here. I didn't know the forecast and the day started out pleasantly warm so I dressed for that. As the day went on it started getting colder. The temps were really dropping.The sky opened up an it poured rain for only a min or two but enough to get me wet before I could get my poncho on. I found a great solution to get warm really fast. When I plan on hunting all day I carry a small can of Sterno with me to heat up an MRE or Mountain House meal. I got my poncho and Sterno out of my pack and put on the poncho and sat on a rock that was low enough that the poncho touched the ground. I opened and lit the Sterno and placed it under the poncho between my feet.
Within in minutes I could feel the sterno doing its magic and getting me warm. I had my arms inside the poncho (not in the poncho arms) so I could open my jacket and let the heat in.
There was a lot of steam coming off the poncho but at that time I didn't care. Now I make sure that at least my light weight jacket is waterproof because you never know for sure what the weather will bring.

I hunt out of a tree stand with a tarp around it and use a similar method. I have a small alcohol stove which is silent and scent free that I heat up some water for a cup of tea about 9:00. The stove sits on the floor of the stand so I can harvest some of the heat while my water is heated. I'll then use the hot tea in the cup to warm my hands while I sip the hot tea with a cookie or candy bar. I have had deer under my stand while the process is happening so it doesn't bother them. That little ritual will get me through a cold morning.

Smart trick with the Sterno and poncho. Good reminder to always pack a waterproof layer and a way to make heat fast. Weather can turn anytime. Stay warm.