STORM TRACK: January 31, 1985 (Volume 8 Issue 2)
(This article has been slightly edited to fit this issue of ST. -Editor)
I'd like to suggest one possible source of noise associated with thunderstorms and tornadoes that has been observed by some of your contributors. Chasers have mentioned the 'roaring hailshaft'; but just why should a hailshaft ever roar?
Most of us know that hail production begins in thunderstorms, when updrafts carry condensation high above the freezing level, where it becomss frozen drops. These round, smaller than pea sized, bits manage to be expelled from the sides or top of the updraft and into an existing adjacent downdraft; or just by their own falling mass and drag actually induce a downdraft. Then, depending upon the freezing level and the updrafts/downdrafts, falling hail could pass through the wall of its own spiralling downdraft into an adjacent updraft, carrying supercooled rain. If the draft is strong enough, the hail is again carried upward to cooler altitudes, where the super-cooled rain accretes on the small hail, making it bigger. The process repeats as long as the freezing level, path of hail fall, draft geometry and strength of updrafts continue to cooperate or until the hail pieces eventually become too heavy.
It has been estimated, in various studies, that hail of numerous sizes, shapes, weights and densities fall at a variety of speeds; 15-45 m/s (34-100 mph). These are the approximate terminal velocities or airspeeds of falling hail. An updraft would have to at least equal or exceed these speeds in order to support and vertically circulate hail within a storm. Updrafts of 45 m/s (100 mph) are not uncommon within severe thunderstorms. Nor are downdrafts. Within both the updraft and the downdraft, the hail is still falling downward relative to the air at 15-45 m/s (34-100 mph).
What kind of noise does hail produce by just falling through the air? Let's imagine, for a moment, that we could safely travel inside a thunderstorm in a flying elevator made of sponge one meter thick. Let's travel downward inside a hail laden downdraft at 45 m/s (100 mph). That will give us an airspeed of 0, because we're traveling with the downdraft. Let's open our sponge door and observe what's happening. First of all there is no beating of hail on our roof or walls, because we were careful to use thick sponge in our elevator construction. But golfball hail is falling right past our door at 36 m/s (80 mph), straight down into the foggy abyss. There is a pervasive, continuous hissing or whooshing sound, mixed with the soft clicking sounds of hail colliding with other hail, due to fall speed differences. The total scene would probably sound not much different than a 10 m/s (22 mph) steady wind, passing through large leafed trees on a summer day. You could just hear this soft sound at a distance of 300-500 meters. This falling hail, therefore, just isn't the major noise producer.
Now, our downdraft is 45 m/s (100 mph) and the hail is falling at an additional 36 m/s (80 mph). That means the hail is descending at 81 m/s (180 mph) relative to the ground. In seconds, at this speed, the hail penetrates an extremely turbulent wall between sloping drafts and is thrust by its own momentum into the midst of an updraft, ascending at 45 m/s (100 mph). Suddenly, the hail has achieved a phenomenal momentary airspeed of 125 m/s (280 mph). And so, just like a tornado vortex with high speed winds that howl around poles, through trees and over houses; so -too- the surging updraft would roar over the insurgent hail for several seconds, until its airspeed is reduced to a comfortable terminal velocity of 36 m/s (80 mph) when the hail again rises within the updraft. The high-speed descending hail may also impact ascending hail with speeds of 89 m/s (200 mph), which could be an additional source of noise. The roar could start as the hail alters the updraft and then fade away, as the hail completes its turn-around. Of course, there is lots of hail that passes in a continuous stream from downdraft to updraft; thousands of pieces every second, achieving tremendous airspeeds, colliding with other hail and creating a roaring din.
The altitude at which hail is being turned around will also be a factor in how loudly the roar can be heard on the ground. If the hail is circulating above the freezing level at high altitude, it may not be heard loudly or at all. However, hail that falls into a strong, freezing updraft at lower levels would generate a more audible roar.
The tornado and the roaring hailshaft seem to produce their sound in the same way. High speed airflow over any object, when broken away from that objects surface in a turbulent, non-laminar way, is a source of noise. The higher the speed, the louder the noise. A tornado can produce a roar by subjecting stationary objects at the earth's surface to high speed airflow (it may also have many other associated noises, such as building materials breaking and colliding). A hailshaft produces its roar by passing frozen objects from downdraft into a high speed updraft airflow."
