Risk factors:
-ambient temperature
- if dog is acclimatised to warm weather
-dog age
-coat type
-fitness level
-breathing ability: flat faced dogs, laryngeal paralysis.
-obesity
-temprament: daft giddy kipper who won't stop or calm and sensible?
Each situation is different. I have seen * heat stroke happen at 18 degrees outside, one case at 8pm when it was evening but still hot and I've seen it happen in the owners house. It is unpredictable, can happen extremely quickly and does kill in a horrible way.
Most cases have not been the stereotypical dog left in hot car.. more often the distraught owner "didn't think it was that hot?" "We were only out for 20 minutes" or "he is usually fine" or "she was just playing ball"
My advice is to pay close attention to the information shared by charities and vets and don't take the risk. As a previous poster said.."no dog ever died from missing a walk"
They also get sun burn, burnt paws, eat dangerous things from barbecues, swallow sand at the beach, get into scraps because the parks are full and get ill from dirty rivers....I love and dread warm weather equally.