The way catalytic converters work is not unlike fire. Once you hit a certain threshold temperature (around 550-600°F) they "light off" and begin burning the combustion byproducts and making a lot of heat. The actual substrate is typically around 800°F to 1800°F, depending on how hard you're working your engine and how much timing is being pulled. The outside surface temperature is usually much lower due to the heat loss to the outside. (The more heat kept in the exhaust, the hotter the converters run, which makes them much more effective.)
If you've ever used one of the Mr. Heater "Buddy" series catalytic propane heaters, those work exactly the same way. It starts with a flame to bring the catalyst up to light-off temperature, and then they light off with a "whomph" and start burning much more efficiently. The only difference is that an automotive cat is lit off by the heat of the engine exhaust rather than a flame.
