This is one of those questions for which I should have an answer - but I don't.

I know that dogs do better in high heat when the humidity is lower.  Does anyone have any idea what the trade off between heat and humidity is?

I have the opportunity to get an indoor swamp cooler (for the cost of going to get it)  If I can drop the indoor temperature to, let's say, 80° and the cooler takes the humidity to, again for sake of argument, 50% is that going to adversely affect their ability to cool themselves?

References appreciated - just for my knowledge - or practical experience.

I think that will depend on what your starting temperature and humidity is. For the most part your dogs are going to acclimate to whatever is their normal as long as that is within a safe range of temperature/humidity combination. High heat and low humidity traded for slightly lower heat but higher humidity you may only break even in comfort.

You'll just have to do the math from your starting point and take a stab at what your ending point with the swamp cooler could be.

http://www.sleddogcentral.com/forum/pop_printer_friendly.asp?TOPIC_ID=3427

https://www.aphis.usda.gov/animal_welfare/content/printable_version/Tech%20note%20-%20Temperature%20and%20Humidity%20in%20Dog%20Kennels-QA%20%20April%202013.pdf

80 degrees but at 50% humidity is high and they probably aren't going to be very comfortable. But 100 degrees at 30% humidity is going to be equal and you've gone nowhere. For the cost of going and getting one I would think it's worth a try depending on where in the numbers your house sits now.

The swamp cooler belongs to a friend of mine who lives about 5 miles away - he doesn't like them and hasn't used it (it was given to him by a fellow leaving the area)

I've seen the HCCT <??> chart but it goes by dog body weight and temperature with no reference to humidity - mine are almost always in the "5" category.

I had to laugh at the 120 rule mentioned on SD, gee, that means I can run mine today, it's only 110 with 5% humidity - that's only 115 so I should be fine (NOT!!)

Okay, let's see it's 110 outside with 5% humidity and 100 with 25% humidity inside - and my crew are all crashed.  Two on tile and Sasha on the rug - none are looking hot (not panting, come when I call them) (( that's about normal, but they tend to rotate to see who gets the tile ... ))

UPS just came by and Avalanche and Sky were at the glass door to see who'd pulled in.  I think that as far as being acclimatized goes my crew are pretty used to the heat and know how to handle it.  Of course they always have plenty of fresh water available inside and out - and it get refilled at least twice during the day / evening.

I've tried ice in the water and chilled water from the fridge and they want none of it, they'll wait till it gets up to room temp and then they'll drink.  I'll have to look for it but there's an interesting article about how our bodies handle the different temp liquids at high and low temps ... lukewarm seems to be the best temp for our mammalian bodies regardless of the temp.  

I may give Ron a ring and see about 'borrowing' (if I take it, it's mine!) the evap cooler and see what it does.  I go by the line that if it's uncomfortable for me then it's going to be uncomfortable for the pups (even if their temp is about 5 degrees higher than a human)

I'm rambling ...

I don't think you are going to find anything about humidity and dogs being comfortable since comfortable is relative. It seems everything is going to be geared toward when it's dangerous. Every dog is different and they acclimate to different things. Under 120 rule and two of mine can go out and run and be just fine. Not as much for poor Kye though. It was 93 degrees yesterday with humidity in the 60% range and all of mine went out in it and did fine, but they don't live full-time in that. Full-time at that level and there would be long term stress issues more than likely. Mine live in a house that is 73 with an internal humidity level of around 25% and they are comfortable. I imagine if you could get your combined down to around the 100 mark then you will have comfortable dogs.

This does not really help but we have to deal with humidity here so just a little diverse info. The 120 rule is usually applied for humid areas that being if you have 60% and 60F it is not good. It is standard to have 70% + humidity here now and all day 90% + come end of summer. Taking the humidity out of the house is SO beneficial here. You do feel cooler with a lower humidity. My house humidity is in the 60s now but the house temp is in the 70s the dogs would like both to be lower but the air movement is the best help. Box fan to the rescue. But what I wanted to mention was that my basement has a dehumidifier that brings the humidity to 60% in the basement and the temp is read at about 5 degrees colder than the main floor however the real feel is 8-10F colder. So that is all the info I can give. As I said it does not totally help but it might be better to get some dry climate weather people who understand the swamp cooler better. Now that I think about it I am questioning why I replied. RAMBLING here too

Third time's the charm, right.  I'm hitting strange keys on the kb and have lost this message twice already! << argh!! >>

I'm really less concerned with comfort, more concerned with safety.  Our 10 day prediction has us at well over 100° until Friday a week where it's 98°  Nighttime lows have been - and are predicted to stay - in the mid 70's but then our humidity goes way up overnight - 50-75% We're looking at 105-110° for the next four days when we are predicted to come down into the 100-105° range - but as I've said before our local temp tends to run 5° higher than predicted.

My pups got up this morning about 7:00, went out to take care of necessaries, played around some and then by 8:30 we were pushing 80 with 53% humidity outside.  They wanted back in shortly after that and are now crashed around the living room.

I've lived here - without air - for 8 years and the various dog for 5 years or so.  At the moment Sky is on the tile under my chair, Avalanche on the tile in one corner and Sasha on the tile in the other corner - none are seemingly stressed at all, no panting, not even breathing hard.  

My concern comes at 110° when I'm heading to town to spend some time in the restaurant - there's no place for the dogs to get out of the heat - they, of course have shade and plenty of water, but it's still hot! Unlike being trapped in a vehicle, mine do have the ability to move around to the cooler (well, less hot) spots and since the insulation under the trailer is pretty well gone and it is well skirted, the floor remains quite a bit cooler.

Picked up the swamp cooler last evening - it'd been in "storage" for a "few" years and the rats have chewed the plug off the cord so I have some fixin' to do before I can even see if it works.

The question that comes up from time to time "Can Huskies live where it's hot?" is pretty well answered by my crew. Yes, they can but don't expect much activity when it's really hot.

For some reason Al I read this as swamp vests and didn't reply because, well never used a cooling vest before, lol.

Anyway, our temps are almost as bad as where you are. We have been in a heat advisory for a week and still in it until maybe Sunday when it will drop down to a nice and cool 95, haha. I can't regulate the humidity well in my house. AC keeps stuff cool in the house, about 76 in the bedroom (YUCK) and anywhere between 72-74 in the main part of the house. Humidity according to my gauge runs around 70% in the house, yep I know I need a better AC window unit, just ain't happening atm. Outside humidity is running right at 50% give or take, and actual temps 103-105, over night mid 70s. Real feel is about 105-107 outside, but it doesn't feel terrible outside in the shade, wind steady at 10 mph gusts of 21. OK Got that out of the way Outside I am dying after about a half hour outside, the kids are doing better than myself, they will stay in the shade, get in their pool, Sofie will play fetch about 4 or 5 runs, and Miya will chase Sofie a tad. I do short trips outside with them, 15 minutes to a half hour, shorter in the after noon longer in the morning and evening. I do this several times a day. To the swamp cooler question.....I have never had success with one. Living in the camper when we first moved here, and AC went in the camper a neighbor let us borrow one and it didn't do squat, unless you were directly in front of it, now maybe it was defective IDK, but, we bought a used one the following year for the shop and again it did nothing unless directly in front of it, ymmv I guess. I would try it, if for anything else one dog could lay in front of it and feel some comfort You may find it may be more beneficial to just get fans and blow the heat around? Let us know how it works, maybe I just used to broken swamp coolers.