Hail Hazards

The way to not get hailed on is to avoid places where it’s hailing. Talk about a ruined day! It happens a dozen or so times each year. Most of the time it’s small hail, so no damage is done except chipped paint and an apology to your laundress. But, occasionally, hail is a big problem. For example, the following is from an NTSB report in reference to a hail encounter in a DC-9 during climb-out from Atlanta:

“They were examining the line of thunderstorms on their radar and requested a turn to 330 for weather, which was approved. On that heading they could see a 10 nm-wide gap between two cells. They could also see there was no weather on the other side of the line. Suddenly, as they entered the gap, an egg-sized hailstone smashed into the center windshield, shattering the outer pane. Turbulence began and lasted about 10 seconds. Large hail continued for about four seconds. It took out all outer panes of the windshield and the entire radome departed, going through the right engine. The pitot and static systems went with the radome, leaving the crew with no airspeed and unreliable altitude information. Of course, there was extensive damage to all leading edges and the right engine, which did continue to put out power.

They exited the weather on the north side of the line of thunderstorms and the crew successfully diverted into Chattanooga, using airspeed/altitude relays from ATC.”

An experience like that tends to make you wonder if perhaps being shot out of a cannon at the circus wouldn’t be a safer occupation.

How to avoid hail encounters is the question. An obvious answer is to stay out of thunderstorms. That’s only a partial answer, however. Hail encounters have occurred 10, 15 even 20 miles entirely outside thunderstorms. In addition, hail damage has occurred in areas where there were only rain showers 15 minutes earlier. Sudden, explosive hail storms like that will only increase with global climate change, we are told.

What is the answer, then, to avoiding a hail strike incident? You must begin before flight by discovering where hail is likely to be encountered during the day. For the USA that’s simple; NWS has made it easy for us to begin that investigation. It’s where you should begin first thing each morning, whether you’ll be flying that day or not.

Call up spc.noaa.gov. You’ll be rewarded with a USA map. Select “Convective Outlook” on the right side and you’ll get another USA map with colors indicating where thunderstorms are likely to occur within the next few hours. At top left, you’ll find three tabs. Select “HAIL” and you’ll get a revised USA map showing where hail is likely to occur this day.

What you get is not a simple forecast of hail; it’s a “Probabilistic Forecast” of where hail is likely to be encountered. “Probabilistic” and “Forecast” are vastly different things. A forecast is based on current and expected atmospheric conditions at a location; a “probabilistic forecast” is based on what several meteorologists at SPC, after long experience, have observed typically happens in a certain location when a particular set of atmospheric conditions exist there. Big difference.

Note that the spc Outlook doesn’t indicate hail will occur in the location, only that it typically does. Check it out day to day and your confidence in its accuracy will grow. That doesn’t mean, however, hail is not possible elsewhere, given certain atmospheric conditions. What are they? Just five principal ones of interest to pilots; storm height above the freezing level, wind direction and velocity at higher levels, speed of storm movement, radar reflectivity and storm shapes.

Many NEXRAD uplink vendors provide storm height, movement and speed of movement. Otherwise, go to www.aviation
and you’ll be covered with tons of WX information. For a quickie, Flight Service should also be able to give you storm heights and speed of movement. There are numerous websites for aviation weather in other nations. Or the world GOES images will give you an indication of how high storms may exist. If you don’t know how to read temperatures in a GOES image, the formulars are listed below it.

For height information on your airborne radar, simply select +10º on tilt and apply the rule of 60 – at 1 nautical mile a degree is 100 feet in linear measure. Therefore, at 10 miles each degree is 1,000 feet. With +10º on tilt, an echo at 10 miles reaches to at least 10,000 feet above your current altitude; at 20 miles, 20,000 feet; at 30 miles 30,000 feet. Again, note, that’s above your current altitude.

Obviously, if the storm’s max height is below the freezing level, no hail will form. How much above the freezing level must one grow before it likely will? Many years ago thunderstorm research pilot, James M. Cook, the original “Project Rough Rider” pilot, after penetrating hundreds of them, found that almost every one with a height exceeding the freezing level by 10,000 feet or more had hail of some size in it. Hail damage on his aircraft made the point. Over the USA that means any storm reaching to or above about 30,000 feet may be spitting hail. In areas towards the Poles, the freezing levels tend to be much lower. In equatorial regions, much higher.

One must always consider
the geographic area.

Once upon a time, radar intensity was available from any Controller or Flight Watch Briefer (the latter are no longer available). It was given to us as a VIP number. VIP 5 signified hail possible; VIP 6 hail certain. But several years ago NWS and FAA bureaucrats, in their anti-litigious minds evidently, decided pilots don’t need that much detail. They dropped those precise VIP numbers and switched to meaningless abstractions. Now they tell us only that a weather echo is “Light, Moderate, Heavy or extreme”, all of no help to a pilot wanting to avoid hail. For example, “Heavy” could mean a simple heavy rain shower or could mean a true thunderstorm with hail. “Extreme” can mean it may contain hail or it definitely does contain hail. Depends on whether it’s low “Extreme” or high “Extreme”. So for most pilots, NEXRAD – as gross as it is – has to do.

But, you must also consider that NEXRAD only tells you where it was hailing 5 or 10 minutes ago, not where it is hailing at this instant, or where it will be hailing 10 minutes from now when you get there. On NEXRAD, deepest red, magenta or deep blue at the top of the color scale all indicate hail or possible hail.

For radar-equipped pilots, on most airborne radars there’s a method for sorting it out. When red weather is displayed, turn the CAL control down (misnamed “GAIN” on most systems) to the approximate mid position. If red is still displayed, think hail in that echo. Many radars display magenta to indicate storm intensity; magenta signifies hail, little question. (Once more, several years ago the know-nothing bureaucrats decreed that magenta may no longer be used on airborne radars to signify a hail-containing echo; however it can on NEXRAD. Go figure. Today, only turbulence is indicated with magenta, on radars that have the TURB function.)

Next, shapes. Begin with round symmetrical echoes. They are most likely just showers or baby thunderstorms. Hail not likely. The more the shape varies from simple round, the greater the possibility of hail. Shapes are only grossly discernable on NEXRAD. The standouts are “pendant” shaped ones and “fingers” of echo. “Pendant Shaped” meaning it looks sort of like a lady’s pendant with a notch in the large end. Sometimes they look like a snake with its mouth wide open. The mouth will be on the downwind end of the formation. (Yes, even for flights that never get above 12,000 or so, it’s critical that pilots know both direction and velocity of winds in the flight levels.) With airborne radar, hail-indicating shapes can be seen clearly, even down inside the red part of the echo, by careful twiddling of that “CAL” control.

So, you suspect hail. How far away should you fly to avoid it? The usual advice is 20 nm. But that’s just a partial answer. It’ll help in making the avoidance decision to know that hail strikes rarely occurs in flight above 30,000 feet. Few occur during departures. Most are during arrivals. Reasons are obvious; on departures, it’s usually possible to make wide deviations. Not so on arrivals. In making an avoidance decision, you must also consider turbulence potential.

If the storm is wicked enough to be generating hail it’s also generating heavy to extreme turbulence far out from the echo on the downwind side. That’s in reference to winds above 24,000, not low-level ones. To avoid all possible downwind turbulence from a potential hail producer, circumnavigate to a distance equal to the winds at 24,000 feet and above. That’s even if you’re at only 8,000 feet or thereabouts.

So, Hail? No! You just have to know the signs and be observant. Cunning also helps.

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