Friday, November 29, 2013

How large can hailstones grow ?

Giant Hailstones

Andre Willers
29 Nov 2013
“Everybody gets ice . The poor in winter and the rich in summer.”  Bat Masterton

Synopsis :
How large can hailstones grow in a super-cell thunderstorm ?
Discussion :
1.Ball-shaped hailstones grow by accretion as it is bounced up and down by updrafts and downdrafts .
See Appendix I , Appendix II .
2.The updraft velocity must be bigger than the terminal velocity of the hailstone . (Else it simply falls to the ground)
3.The terminal velocity of hailstones is approximated in Appendix III .
V=8.45 * D ^0.553 , where V is terminal velocity m/s and D is diameter in cm .
An upper range updraft of 175 miles per hour is used as per Appendix II = 78.232 m/s
This gives a
D = (V/8.45) ^ (1/0.553)
D = 55.94866 cm diameter hailstone .
This is about the size of a fitness ball

About 80 kg of ice .
A comparative coconut (5 inches=12.7 cm diameter gives an ice mass of about 8 kg .
This must have happened before , but nobody believed the survivors .
4. So , things can be a lot worse .
And will be as global warming drives updraft velocities .
Icily yours
Andre .

Appendix 1
A discussion of how hail forms.

Appendix II
Ever wonder how fast air is rising into the sky during a developing thunderstorm? What about during a Tornado? Here I will try to explain what I know about vertical wind speeds in the centers of natures most violent storms.

Here I will start with the basics. Convection is simply the rising of warm air and the sinking of cooler air. To have clouds, there must be rising, warm and moist air (compared to air surrounding the air "parcel") where the moisture condenses at a certain altitude, forming the cloud. This rising air may be a thermal, from the uneven heating of the earth's surface, or forcing / convergence caused by a front, mountain range, or inflow of air into a low pressure system.

Another important thing is that more heat energy is transferred to this air parcel as the water vapor condenses (or freezes). This makes the air parcel warmer and causes it to rise faster. About 540 calories of energy are released as a single gram of water condenses! When each gram freezes, an additional 80 calories is released. The small fair weather cumulus often have updraft speeds of about 5 MPH.

The really impressive updraft speeds occur in developing thunderstorms and especially in supercells. In a general (non-severe) thunderstorm, the development and early-mature cycle is when the updraft is strongest before downdrafts begin to disrupt the storm. Typical speeds range from about 15 to 30 MPH, or roughly 1,200 to 2,500 feet per minute. At this rate, the relatively "weak" storm reaches a height of about 30,000 feet in 15 minutes, and may last only a half hour.

Severe thunderstorms, require much stronger updraft speeds and depend on the type of storm. Multicell lines generally have weaker updrafts than multicell clusters but are arranged in a "curtain". The updraft speeds in a multicell line storm are a bit stronger than the single cell general storm described earlier. Multicell cluster storms often have updraft speeds around 60 MPH in developing components, or about 5,500 feet per minute. This is quite fast, keeping in mind that most general aviation aircraft can only climb up to 3,000 feet per minute (200 Super King Air).

This is also why pilots should NEVER try to "out climb" the top of a developing thunderstorm. The strongest updraft speeds lie with the most intense kind of thunderstorm, the supercell. A supercell is a "continuous cycle" storm, meaning that it has an updraft side and downdraft side at the same time which are separated from each other allowing the storm to last much longer than 30-45 minutes.

The updraft of a supercell also has a broad low and / or mid-level rotation (mesocyclone) which my further boost its speed. Supercell updrafts generally are stronger than 50 MPH, but 70 or 80 MPH is more typical. In the Great Plains of the United States, supercells often produce baseball and grapefruit sized hail (not to mention tornadoes) because of the extreme speeds of the updrafts within. Such updrafts have been known to reach 150 to 175 MPH, or about 12,000 to 15,000 feet per minute!

No aircraft except for military fighter jets with afterburner power could climb at these rates (for example, the F4 Phantom and Lear 35 Jet both have maximum climb rates less than 8,000 feet per minute). This is why a supercell can literally go from "blue sky to tornado" in a "New York minute". At 15,000 feet per minute, an air parcel will go from ground level to 45,000 feet in only 3 minutes!

An experiment was done via special weather balloon to find out how quickly a supercell updraft will carry it. The device was released into the inflow side of an HP supercell in the Great Plains and ingested into the storm. Only 2 and a half minutes later, the balloon was in the anvil of the storm. It rode the high-velocity core of the storm and gave vital information on the structure of the storm and internal dynamics.

Supercell storms are the most dangerous to aviation. Visibility and wind-shear are the most obvious threats at low levels, however, the updraft and mesocyclone is usually strongest at 20,000 feet. A commercial airliner flying though such a storm will most likely have its wings torn off, and this has happened to planes trying to fly through severe thunderstorms.

Another pilots horror story was an L1011 trying to fly through a "hole" in a multicell cluster of severe thunderstorms. Invisible to the pilot, was that baseball sized hail was falling through that "hole" in the storm, and serious damage to the aircraft was sustained (cracked windows, cratered leading edges of wings, and crushed engine nacelles).

The most amazing stories come from several incidents of people who were unfortunate enough to parachute into a thunderstorm. Imagine a 100 MPH updraft, your parachute is descending at 10 MPH ... Do the math, this means you will go back UP at 90 MPH!

In the book "The Man Who Rode The Wind", a true story of a pilot who ejected into a thunderstorm at 45,000 feet is described. He ejected from an F8 Crusader and descended into the developing storm until his parachute deployed at 10,000 feet. He became caught in the storm updraft and actually re-ascended under his chute to 26,000 feet. Thin air caused him to pass out and the cold caused intense frost bite during his ride up and down the inside of the storm. The water inside the cloud nearly made him drown in mid air!

He was constantly slammed around by the extreme turbulence and at one point his body appeared to be ABOVE his parachute. Finally, the storm weakened and he descended back to earth 30 minutes later. A person found him in a field, severely injured but alive. This storm was not even a severe storm, just a strong summer 30-45 minute long storm. Imagine if this storm was a supercell.

Another incident happened in Germany where 5 parachutists fell victim to a thunderstorm updraft. All landed covered in ice after their wild ride ... yes, they became the "cores of hail stones". Only one of the 5 survived.

Other strange phenomena occur when a tornado picks up debris and it becomes involved with the main updraft of the supercell. This accounts for "rains" of frogs and fish if the tornado passes over water and dumps them far from their point of pickup. Some fish encased in ice occured with one such incident. Other objects such as appliances, roof shingles, insulation, plants, even a computer floppy disk and a desk have landed miles away from a violent
Appendix III
Another discussion about terminal velocities of hailstones .


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