Anyone who tunes in regularly to Action News This Morning or Action News at Noon has probably heard me refer to the term "self-destruct sunshine" on more than one occasion! It seems to have become a favorite weather term among the behind-the-scenes crew at Action News, particularly our morning videographer who I always happen to catch pumping his fist whenever I use the term on air (which I am hopefully correct in assuming is a gesture of excitement rather than anger)! But what exactly is "self-destruct sunshine," and is this something we can expect to see more of as we head deeper into the autumn season?
To be a bit more scientific, self-destruct sunshine results from a process known to meteorologists as "free convection", or "buoyant convection." This is defined by the glossary of the American Meteorological Society as "predominantly vertical motions that are driven by buoyancy forces arising from static instability." Okay, I'll admit that for a non-meteorologist that's about as clear as mud! The definition actually goes on to discuss significant deviations from hydrostatic equilibrium, but that's another entry for another time!
The general process is this. The sun comes out early in the morning and begins to heat the ground. As the ground heats up, it in turn heats the air immediately in contact with it, causing that air to become more buoyant and ultimately begin rising. These air "parcels" (individual units of air with an unspecified size and shape) will continue to ascend higher into the atmosphere, cooling in the process until they ultimately reach their point of saturation (100% relative humidity). Once saturation has been reached, clouds begin to form. Just how extensive the clouds become depends on the amount of water vapor present in the lower atmosphere, and also the temperature difference between the surface and the upper atmosphere. The most vigorous self-destruct sunshine occurs on days with an abundance of moisture in the lower atmosphere and an unusually large temperature difference between the surface and aloft.
We are indeed getting into the time of year when self-destruct sunshine can occur on a semi-regular basis. Typically during the autumn season we see frequent intrusions of chilly continental polar air from Canada, yet the sun angle is high enough that significant heating can still occur at the surface. The result is clear, crisp mornings given way to an abundance of cloud cover and sometimes even showers by afternoon.