I've read articles about cows parsnip yearly, for decades.  And every one, this one included, doesn't tell the whole story.  Yes, sun causes this plant to emit toxic sap to the surface areas of the plant, where it is easy to come into contact with exposed skin.  But people often assume that if it is cloudy and rainy they will not get a cow's parsnip rash.  This is definitely not always the case.  Pick a day that it's raining hard and mountain bike Johnson Pass with bare hands.  As your hands whack into the parsnip, the stalks will break and the sap inside the stalk will get on your skin.  And the following days and weeks will be just as much hell as if you did the same on a sunny day.  Trust me.  This I know from very painful experience.