I have been wanting to ask this question for several days but didn't have a place to put it!! In general, how old of a scent can SAR dogs track outdoors, and how does weather/ temperature affect it?
Being that you said "track" I assume we are talking about a living person using a trailing dog? If so....
Training places a HUGE factor in this question. What a dog trains for is what they are going to be able to accurately work.
I think it's important to know the certification requirements used by NASAR (National Association of Search and Rescue), which is the industry standard.
sartech 3 which is the most basic
trail ½ to 1 miles in length, do it within 1 hour, including breaks. Track is aged between 2-4 hours
sartech 2
trail 1 to 1 1/2 miles in length, in the wilderness, do it within 2 hours. Track is aged between 8-12 hours
sartech 1-the highest level in NASAR
trail is 1 to 1 1/2 miles in length, do it within 3 hours Track is aged between 24-30 hours. In sartech 1 you have to do this in both wilderness and urban settings.
There are other requirements and training, but this gives you an idea of what the dogs have to train for to pass.
The problem with this is very seldom does real life SAR actually coincide with these testing requirements (and because people are training to pass these tests, this is what their dogs can do).
Most of the time a person isn't even reported as missing until they've missed dinner, and the family has looked for them, and, and.... This means that the person has often already been truly missing/lost for 8-12 hours before the call is made to LE. Then, as you well know it can be hours if not days before a dog is actually called. As you can see, that doesn't fit so well with the ages of the trails in the tests.
There are a lot of great handlers and dogs out there though that do train on more real life time trails. There are some variances on what the comfort level is for an "old" trail, but here IMO is a general "timeline" of sorts
0-1 days hot trail, if it's an area where the person is in everyday, it can be difficult for some dogs as there are so many trails laid over one another and nothing has had a chance to settle.
2-5 days Excellent results with weather playing only a small role (barring hurricanes, tornadoes, and other catastrophic weather).
6-9 days still very reliable results but weather plays a bigger factor.
10-14 days variables really start to play in and training on these ages is crucial.
Past 14 days to 21 days the results in my experience are very mixed.
After that I am not comfortable at all.
Yes, weather and terrain play a huge factor as does what kind of weather and terrain you train in. Extremes are bad. Dogs that are not used to working in certain climates are going to have reduced success. We can explore weather on another post as this one is already getting very long!
Mind you, other handlers may have different times, these are just mine. I will say though that given the science behind "scent" I am really not into the whole "trailing a year later" thing. Again, for another time.