A small amount of air is pulled in the top of the kiln, circulated throughout the firing chamber and exhausted out the bottom of the kiln. As the hot air moves through the firing chamber, it evens out temperatures, often by as much as two cone numbers. The most reliable way to check heat distribution in your kiln is to use a series of witness cones on each shelf. By examining them after each firing, you can see how firing conditions, including uniformity, affect fired results. Adjustments in switching, loading, etc.
Convection is the first step in the heating process in the kiln.
Air is heated as it passes across the warming kiln elements. As the hot air rises and cool air falls, air currents are created which circulate hot air to cooler places in the kiln. This heat is transferred to the ware, shelves, etc. The kiln will not be uniform in temperature at this early stage of firing unless the hot air is pushed through the kiln by mechanical means. Low cone firings such as 022 and 021 depend more heavily on convection for heat transfer.