Arthur Larsen of Centennial, Colorado, writes:
Marilyn: You wrote, "You can see your breath only in winter because the colder the air, the less moisture it can hold (warm air can always hold more water vapor than your breath can)." (February 17, 2013) The season has some relevance, but you can actually see your breath condensate in summer if the temperature and relative humidity are low enough, e.g. at night. An extreme boundary condition of this example is in a desert where the humidity is often very low and the night temperatures can drop below 10 C during summer. Or at high altitudes, like on a Colorado 14er where the same conditions can prevail.
The main reason for the breath vapor condensation is the difference in the relative humidity between the exhaled and surrounding air. The three main factors that play into this are temperature, relative humidity, and pressure. Another factor are the particles in the air that allow the vapor to condensate on them, e.g. dust.
You can even see your breath condensate at room temperature if the conditions are met. All that is required is that the exhaled air has to be saturated with humidity to reach a point of about 5 percent above the relative humidity of the air in the room.
Thank you, Arthur. At room temperature, readers, you can see this by lightly exhaling on a pair of eyeglasses, as if to clean them.