It's kind of a gross question, but that doesn't make it any less worth wondering about: In the cold I can see my breath, but how come I can't see my fart?
-- Matt, Vienna
Heck, Matt, it makes the question more worth wondering about. Everybody needs the answer to this, even though they didn't know they needed it until you asked. Consider it a public service.
A fart and a sigh are very different things. They might seem like twin exhalations from opposite ends of the same tube. But no, my friend. When you breathe out, you're ridding your body of the byproducts of respiration. Those byproducts are carbon dioxide and water. When you fart, you're ridding your body of the byproducts of bacterial action on undigestible cellulose and carbohydrates in your lower gut. Those byproducts are nitrogen, oxygen, carbon dioxide, methane, and perhaps hydrogen sulfide. More than 50% of your fart will be CO2 and hydrogen.
So what do you see when you see your breath? It's the moisture that you exhale as it moves from the warmth of your body to the cold air outside. Warm air can hold more moisture than cold air, so outside your body the moisture condenses and becomes visible. A fart doesn't have the proper water content, so we have no similar tell-tale butt cloud.
And while we're on the subject, as another example of our public service dedication, here's some info on one man's crusade to rid the world of another kind of tell-tale butt cloud. Buck Weimer of Pueblo, Colorado, is married to a woman who suffers from a serious bowel disease that makes her unfortunately fragrant. To spare her the embarrassment and him the smell, he invented UnderEase, air-tight underwear with a strategically located charcoal filter that removes the offending gasses (under-tec.com). The Weimers are used to the snickers; but according to users' testimonials, it's a boon to the afflicted.