In the Southern Hemisphere, where the June solstice is known as the shortest day of the year, it marks the first day of astronomical winter, but the middle of winter in meteorological terms.
Backfill refers to products or materials used to fill excavations. The most common form is composed of a mixture of gypsum and calcium bentonite. Apart from filling holes, one of the primary purposes is to mantle sacrificial anodes under the ground.
Unburnt air/fuel mixture passing through the exhaust and igniting also results in backfiring. This is usually caused by a temporary loss of ignition or rich idle mixtures in combination with exhaust air leaks. If there is only moderate backfiring it may be because the exhaust valves are leaking or the tappets are tight.