When cooking meats, some operators place a pan of water in the bottom of a convection oven to add humidity and prevent the meat from drying out too quickly. In a standard commercial oven, like any home conventional oven, the natural flow of air currents carry heat around the oven cavity. As air heats in the bottom of the oven, from gas burners or electric elements, it becomes lighter and rises to the top of the oven. Once oven walls become hot, radiant heat is transferred from the walls to the food. The air then reheats and the cycle begins again.
Cooking takes place as the temperature of the food mass rises.
When food bakes or roasts in a standard oven, water evaporates from the food surface forming a cooling layer. This thermal barrier surrounds the food and slows the rate of heat transfer to the top surface of the food. Pre-heating is generally only necessary when baking in a standard oven. Cooking in a standard oven is slower and less efficient than cooking directly on a flame or hot surface, but some foods cannot cook any other way because they need dry heat and slow cooking to develop full flavor and tenderness. Since the food cooks quickly, it can easily overcook or dry out.