The key to staying warm and dry is making use of your own body heat. You're already running at about 100 degrees doing basic tasks. A good Marmot or Carhartt or any tight rain coat made of next to nothing will keep moisture away. The key is to trap the heat and not get wet. My rain gear folds up to about the size of a half of a basketball. You don't have to go all out. It's really what type of rain repellent you apply and how well zippers work. Look for something that has velcro over the zippers. When it's 30 degrees and you're an oven but dry, you're doing it right.
 
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