Mr Smith, 43, originally from Inverurie was reported missing after failing to return home yesterday afternoon after going hill running in Perth and Kinross.
His family described him as an experienced fell runner as he went out to run up Meall nan Aighean, Carn Mairg, Meall Garbh and Carn Gorm and was expected back yesterday at 5pm.
His sister in-law, Elaine Smith said this evening: “It's been a pretty horrendous day, or horrendous 24 hours really.
“He's such a fit guy, he's got a sensible head on him. We're keeping all of that in our minds and just staying as positive as we can.
“None of that takes away that it's a really upsetting situation, this is something that happens to other people, isn't it? It doesn't happen to you."
A search and rescue operation got underway this morning in the Glenlyon area amid fears that Mr Smith may have become ‘disorientated’ while out on his run.
Ms Smith continued: “We shared the social media post with some local hill walking groups and running groups to try and target people who might be in the area and they've been
fantastic as well. We've had a lot of coverage on that side of things but we don't want it to slow down.
"We don't want anyone to forget Chris's face.”
Ms Smith also mentioned that she has seen some rumours flying around social media that her brother-in-law has been found and wanted to make it clear that this isn’t the case, he is absolutely still missing.
A Police Scotland spokesperson said: "Officers in Perthshire and mountain rescue teams are carrying out searches in the Glenlyon area following a report of a missing 43-year-old man.
“Emergency services are being assisted in the searches by the Coastguard helicopter. Anyone with information is urged to contact police on 101, quoting incident 2775 of 27
Ms Smith added: “On behalf of the family, we want to express our thanks to all of the emergency services who have been involved with the search because it's been tireless and
we're really appreciative of it."