It typically needs to be at or below 45 degrees Fahrenheit for people to see their breath. When individuals exhale, they release moisture into the air. That moisture is then cooled down by the air, which creates a type of fog.
Sometimes the temperature can be warmer and people still see their breath. The level of humidity in the air is also a determining factor for when individuals can see their breath. It's possible to see a person's breath at 60 degrees, but only if the relative humidity is higher than 80 percent. Conversely, individuals who are breathing in and out quickly may not see their breath even in colder weather.