My day job is air pollution control. Today was my re-certification day for EPA Method 9 visible emissions testing. The certification process is known as "Smoke School" to those in the business.
A smoke generator (It takes smoke to know smoke.) is set up in a large parking lot and the prospective smoke readers gather around. Someone watching the operation would notice that all of the readers are gathered on the side of the generator towards the sun. That's because to read smoke consistently the sun has to be at the back of the observer. (Try looking up sun and down sun at the smoke from a camp fire or even the haze on a hazy morning or evening.) Today was cloudy, but we still had the sun at our backs, even though it was behind the clouds.
First we do some practice runs. Black or white smoke (We do both.) is generated at various known-to-the-operator opacities from zero (No smoke) to 100 percent (You can't see through it at all) at 5 percent increments. We all make our best guesses as to the opacity. Then the operator announces the actual value. If we all guess within 15% it is time for the real test: Accurately estimate fifty smoke readings at randomly varying opacities, 25 white and 25 black. To pass the smoke reader has to average less than 7.5% error each in both black and white. No single reading can be off by more than 15%. This takes about an hour. The answers aren't given out until all 50 readings are finished. You can take the test as many times as they offer it, usually 4 times a day over 2 days. If you pass the test once, you are a certified method nine visible emissions reader - for six months. After six months your certification expires and you have to re-certify.
Our current testers give out marvelously tacky hats to the readers who pass on the first test. Today I won my eighth hat.