This article is from the Hiking/Backpacking With Canines, by Terri Watson with numerous contributions by others.
Warmer temperatures on a hike call for additional precautions to guard
your dog against heat exhaustion or heat stroke. As mentioned above,
always make sure that you carry enough water for your hike and give
give your dog (and yourself) frequent drinks. When I am backpacking, I
carry a plastic cup hooked to the outside of the pack to make it
easier to get to. If it's too difficult to get to a cup or bowl, you
may be tempted not to water your dog frequently enough. Some people
teach their dogs to drink directly from a water bottle. I prefer the
cup because I lose less water onto the ground.
Watch your dog for signs of heat exhaustion or stroke. Particularly,
unusually rapid panting, and/or a bright red tougue or mucous
membranes. The dog's primary mechanism for cooling off is through
panting. Since this cooling process uses evaporation the dog will
require more water when he is panting heavily. I have read that the
shorter-nosed breeds (eg, Bulldogs, Pugs) may have a less efficient
heat exchange rate, so should be watched especially closely.
If you determine that your dog is overheating, your should stop
immediately and get her into the shade. You should check with your vet
for the best ways to cool down an overheated dog. Since I've never run
into an extreme case, I've just kept my dog cool using the same
approach they use for endurance riding in horses. I put cool water on
her belly and groin area. The logic here is that you want to cool down
areas where there is a large blood supply, and allow evaporation off
the skin to provide heat exchange. If you're near running water you
might put the dog into that, but I don't know what the dangers of a
sudden temperature change might be. The main idea you should take from
this is that heat stroke is a life threatening condition and your
should be able to recognize the warning signs and know how to prevent
it. Even on a cooler day, if it is very sunny, and your dog is working
hard and is a dark coated breed, they can get overheated.
Cold weather poses its own set of problems, although I am less
familiar with these. The main thing I have run into is my dog getting
cold in our tent during early season hikes. She is an indoor dog, and
not heavily coated. If it's a cold night, once she has come into the
tent and been still for a couple of hours, she will sometimes start
shivering. My solution has been to invite her onto my sleeping mat to
get her off of the cold ground and give her the additional insulation
of curling up against me. If you routinely hike in colder climates or
have a short coated dog, you might consider having a pad to get them
off of the ground, and/or a blanket or towel to throw over them.
Don't be fooled by cold weather. Adequate fluid levels are essential
for heat maintainance in both temperature extremes. Drink plenty of
water and make your dog do the same.