First off, you may have noticed that dogs don't sweat. Their principle means for cooling down is by opening their mouths wide and panting. Some scientists suggest that having cold, wet noses also plays into this cool-down equation. A little moisture on the nose keeps it cooler (hence the cold), and therefore helps to keep the dog cooled down on hot days. A dry nose is not a good sign, and might suggest that the dog is dehydrated, and a warm nose suggests the dog may be running a fever. If this condition persists for more than a day or two, you should head to your vet to have it checked out.

Another reason that dogs have cold, wet noses is because they’re quite fond of licking their noses. Many dogs have long tongues with which they can easily reach their nose &emdash; possibly enhancing coolness and transferring moisture to the nose. Further, after a dog has eaten, you’ll frequently find them using their tongues to clean off their noses, particularly when the meal is messy. This in turn leads to the cold, wet noses we’ve come to expect in dogs.

An alternate explanation as to why dogs have cold, wet noses may have to do with survival skills, and predate domestication of our furry friends. It’s suggested by some scientists that extra moisture on the nose may increase a dog’s olfactory capacity: in other words, the dog’s sense of smell. While the modern pampered pooch may not need to have a particularly good sense of smell, this aspect of the dog is still relied upon. Working dogs especially, need great “smelling” skills to herd, find pests, look for missing people or sniff out illegal substances at airports. Cold, wet noses may simply be better at smelling thing than a dry nose would.

Back in the day, when Noah took all the animals on the Arc, the pair of dogs stuck their noses out the window and got rained on.

Dogs have wet noses for a few reasons. The first reason is inherent to their genetic makeup.

1. Dogs relay intensely on their olfactory senses; the wetness of the nose increases their sensitivity to air flow direction and smell.

2. Dogs also do not sweat so they use their mouth as a way to cool themselves down. Discussions on the inclusion of wet noses to aid in this cooling off process is prominent in the scientific community.

3. Probably the most obvious and least surprising, is that dogs often lick their noses causing them to remain moist.

Dogs have wet noses because of the humid temperature.

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"A wet nose simply means that the air is plenty humid. Just like anything else, when it’s humid outside, your dog’s nose gets wet. If the weather is overly dry, so will be your dog’s nose.

Another misconception is that you can tell how your dog is feeling by touching its nose. If your dog’s nose is hot, then it might have a fever. But if it’s cool, it’s feeling just fine. But this is not accurate. Again, a dog’s nose temperature changes based on the environment. If it’s warm where your dog is, your dog’s nose will be warm. If it’s cold, your dog’s nose will be colder."

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Dogs can discriminate odors at concentrations nearly 100 million times lower than humans can. The dog's wet nose is essential for determining the direction of the air current containing the smell. Cold receptors in the skin are sensitive to the cooling of the skin by evaporation of the moisture by air currents.