Dogs. Do they smell great? No. Is that their only bad quality? Yes. Dogs are objectively great, but they aren't lauded for their scent. A scent which is exponentially worse when they're wet. But why is that?
It's fairly complex, so I'll let Brunning explain:
"When you're taking your dog for a walk, you're actually taking a whole host of microorganisms for a stroll too. These bacteria and yeast dwell quite happily within your dog's hair... and in the process producing a range of volatile organic compounds. This is the genesis of what will become the olfactory assault of wet dog smell.
If these volatile organic compounds are being produced all the time, you might wonder why wet dog smells so much worse than a dry dog. Water itself has a hand in this; it helps to break down the micro-excreta of the bacteria and yeast in the dog's hair. As the water evaporates from the dog's hair, it can carry with it some of the volatile compounds, allowing them to reach your nose. As the water evaporates, it also increases the humidity of the air surrounding the dog, and as humid air is able to accommodate a higher concentration of smelly molecules, the effect is further intensified."
Okay, that all makes sense - dogs smell because their hair is home to a host of microorganisms whose smells are exacerbated when they become wet. But what exactly are these microorganisms? Brunning says this, too, is complex, as it's not a single molecule, but rather a "cornucopia of molecules that contribute, which individually can have wildly varying smells, but combined make a soggy dog a smelly dog."
Brunning cites a study on wet dog smell (perhaps the only one of its kind - there hasn't been much research on our damp canine companions). The study found that "the concentrations of benzaldehyde, phenylacetaldehyde, acetaldehyde, phenol, and 2-methylbutanal...increased by a greater factor compared to dry dog hair than some of the other compounds."
Brunning himself says there's a lot more to explore when it comes to our beloved but smelly friends. But for now, don't blame the dog, blame their microorganisms. And, for your own sake, keep your dog and her microorganism friends out of the rain if at all possible.