Engines Are Also Affected by Wind Chill
Wind chill factors to 60 below! We often hear such pronouncements by the weather bureau during the winter. The conditions under which a wind chill factor of -60oF exists might vary from an outdoor temperature of -30oF and slow winds of 10 mph to a temperature as high as -5oF and strong winds of 35 mph. (For comparison, the wind chill factor at -60oF and a wind of 40 mph is a nippy -150oF.)
These factors are computed based on the exposure of dry human flesh to the elements. The question is often raised if the same effect applies to engines and other inanimate objects. The answer, which surprises many people, is yes--if the engine is warmer than the surrounding air.
Heat is transferred in one of three manners. Radiant heat, such as that which we receive from the sun, is in the form of electromagnetic waves that travel with the velocity of light and are transmitted through a vacuum, as well as through air (better, in fact, since they are absorbed by air to some extent.) The second manner in which heat is transferred is by conduction, or actual contact. This is the form that does the most damage if you lean against a hot stove or spill boiling water on yourself. The third means of heat transfer is by convection. This term is applied to the transfer of heat from one place to another by the actual movement of hot material. Hot-air furnaces and automobile radiators are examples.
In some cases, such as a steam radiator, all three factors are in operation. If you hold your hand close to, but not touching, the side of the radiator, you can feel the heat being emitted by radiation. If you place your hand on top, you may be burned by conduction. If you hold your hand over the radiator, you will feel warm upward-moving convection currents in the air.
It is mostly because of convection that wind chill affects machines as well as people. If you were to take an engine block and install it on mounts in the middle of a field for the winter, it would make no difference in the temperature of that engine block if the wind blew or not. The block would remain at the ambient temperature of the air surrounding it, whether the air was moving or still. But if you were to start the engine and let it warm up, you would find that its "skin" temperature would be lowered significantly in a stiff wind.
Any object that creates its own internal heat will have heat removed from it faster if the air around it is moving--assuming, of course, that the air is colder than the object. The moving air simply snatches the heat away from it much faster than does still air. So wind chill does affect machines, but only if they are at a temperature above that of the surrounding air.
