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Monday, 21 December 2015

How a small blue bird could help protect us from hail

Julian Brimelow (Edmonton, AB)

In recent years, hailstorms causing billions of dollars’ worth of damage have occurred over Europe, North America and Australia. It is therefore important to continue to improve the lead time of warnings and to include information in warnings that will encourage people to take preventative action. Reporting hail characteristics in real time is a critical step towards achieving this because this information a) is invaluable in helping forecasters issue the best possible forecast, and b) assists researchers by providing them with reliable data to develop even better forecast tools.

Reporting the size and amount of hail is important because the damage potential (or kinetic energy) is primarily a function of both the size and the amount of hail. In copious amounts, even pea-sized or grape-sized hail can be devastating for crops and clog drains in urban areas.  Reporting and measuring hail is, however, a tricky business: hailstones melt quickly, and are slippery and cold. This is where a small blue bird could help us report hail in a way that makes the information useful to forecasters and researchers.

More on that later.

Despite advances in the skill of weather forecasts in recent years (today’s four-day forecast is a good as the one-day forecast 30 years ago), forecasting the occurrence and size of hail remains a challenge. Major limiting factors are that not all thunderstorms produce hail and that when they do, the hail swaths (narrow bands of hail deposited by hailstorms) often cover a small area. This makes it difficult to identify where hail actually fell and what the maximum hail size was. Because conventional weather radar does not directly identify the presence or size of hail, meteorologists are reliant on hail reports from the public.

 Public hail reports have issues though. Consider a day when nasty storms are visible on radar but no reports of hail are received. Was this because there was no hail, or did the storm affect a sparsely populated area, or did no one report the hail for some reason? Even if a hailstorm affects a densely populated area when it is producing the largest hail, there is no guarantee that people reporting the hail will be able to a) identify the largest hail, or b) make accurate measurements of the hail. Someone may, for example, estimate the size of hail on their lawn by looking out their kitchen window, or measure the hail after it has been on the ground for a while, or, after showing it to others.

A recent study in the U.S. found that the mean hail diameter of reports in the U.S. Storm Database was over 1 cm smaller (about 30%) than that measured by teams of students who measured hail along several traverses across hail swaths. This negative bias represents a significant barrier for developing reliable forecast tools, especially empirically derived ones. The lack of skillful guidance undermines the forecasters’ confidence, makes them wary of issuing warnings and may prevent them from including important information in warnings about hail.

The best tool for objectively measuring hail remains the hail pad. A hail pad is typically a 30-cm square pad of painted styrofoam that records the indentations made by hailstones. These data are then used to estimate the size and number of hailstones and the kinetic energy. Unfortunately, hail pads are of no use to forecasters because they do not provide real-time information. Additionally, maintaining a sufficiently dense network of hail pads (i.e., spaced about 3 km apart) is time-consuming and labour-intensive. Some might argue, in jest, that cars are plentiful and make convenient hail pads, but using a car as a hail pad would be exactly what we are hoping to avoid.

This all seems rather hopeless doesn’t it? Fortunately, this is where a small blue bird enters the equation. I am referring to Twitter! In recent years, Twitter has exploded onto the severe weather scene, with eager citizens tweeting about all sorts of severe weather phenomena, real or perceived.

Notifying forecasters of hail has never been easier or quicker, and “quicker” is critical for increasing the lead time of warnings. Also, information from the hail swath (e.g., the maximum hail diameter, typical hail size, depth of hail and damage) allows forecasters to include important information in the warnings. People are more likely to be proactive in heeding a warning and taking preventative action if they hear that a storm producing baseball-sized hail is on its way.

There is a downside to legions of people tweeting reports of hail. First and foremost, they will likely not be trained storm-spotters. There is a danger of there being a deluge of well-intentioned but unhelpful tweets that researchers and, more importantly, forecasters have to sift through. Tweeting “Huge hailstorm west of Moose Jaw!” is not particularly helpful. Why? There could be more than one storm in the vicinity. It is also unclear if “huge” refers to hail size or storm size. Also, no details on the hail size or time of the event (the time of the tweet does not always correspond to the time the hail was falling) are provided. That is why I carefully worded the title of this post “How a small blue bird could help protect us from hail”. Make no mistake, the potential for Twitter to improve warnings and to modernize hail research is real, but like any tool, we have to use it properly in order to maximize its usefulness.

Part of my job at Environment and Climate Change Canada is to develop (and test) hail forecast products, including products from Canada’s radar network. A critical component of this research is to verify the hail products against observations. During the summer of 2014, I collected hundreds of tweets with embedded photos reporting hail over the Canadian prairie provinces. After carefully examination, however, I could use only about a quarter of them for research.

The most common problem was that the photos did not include an object that could be used to estimate the size of the hail. A photo of someone holding a large stone in their hand looks neat and can serve as confirmation that hail fell, but that is pretty much where its use for research ends. Estimating the size of hail by sight alone is unreliable because people tend to be biased towards reporting certain size classes of hail.

A colleague and I dealt with this bias when using the U.S. Storm Database for a journal paper—specifically, we found that there were a significantly disproportionate number of reports of golf ball-sized hail compared to the adjoining size categories. Apparently people are predisposed to “seeing” golf ball-sized hail; perhaps this phenomenon has something to do with the popularity of golf.

What we need is a standardized tool that allows people to objectively and accurately report hail, while not introducing disincentives or delays. If people had access to a universal application (app), this would go a long way in avoiding some of the pitfalls currently associated with tweeting hail reports. All the required features for such an app have been developed—they just need to be combined into a single app by someone with the required skills.

Rather than estimating the diameter, the most accurate way to determine the size and kinetic energy of a hailstone is to measure its mass. Given that hail (at least hard hail that we are interested in) has a fairly consistent density, if we know the mass then we can get a really good estimate of its equivalent diameter, without having to worry about which of the hailstone’s axes to measure and all that fiddly stuff. So, ideally, the app would convert the smart phone into a scale that can be used to measure the mass of hailstones. Yes, there really is an app for that, more than one in fact!  

Additionally, the app could automatically include a scale on the photo or have a feature that changes your screen into a ruler to permit the user to accurately measure the hailstone’s dimensions. The app would geo tag the phone’s current location. There would be no need to manually enter the latitude and longitude, and there is no danger of typos. The app could also include a time stamp of when the app was activated (this could be overridden to reflect the time the hail started). Lastly, the user could have the option of entering more details about the event. Hit send and voilĂ , the report with an image and all the critical information is tweeted to Environment and Climate Change Canada. Such an easy to use, nifty tool would improve the usefulness of hail reports and increase the number of citizen reports.

A final observation—I am often surprised during the summer when someone casually notes that it is hailing and doesn’t think about reporting it. First off, “Hello, it is hailing! Where is my scale?” Seriously though, I asked someone why this was. They said that there had been watches for severe thunderstorms, so of course the meteorologists were aware of the hail. I’m flattered by their confidence in us, but no! Another reason why people don’t think of reporting hail is that it’s “only small”. That may be, but knowing where there is small hail is still very useful for developing and verifying forecast tools, not to mention that sufficient amounts of small hail can also be very damaging.

So, only if it is safe, please do send along your hail information! Your participation would be greatly appreciated.

Acknowledgements: I’m very grateful to Gabrielle Gascon (ECCC), Neil Taylor (ECCC) and Nyree Sharp for their valuable feedback on earlier versions of this post.

Julian Brimelow, PhD., Physical Sciences Specialist, Applied Environmental Predictions Science Meteorological Service of Canada (MSC), Prairie and Northern Region, Environment & Climate Change Canada (ECCC).

This blog post has been written by Julian Brimelow, Environment Canada, who will be presenting on February 1, 2016 at CatIQ's Canadian Catastrophe Conference during the "Dealing with Hail Risk" session at 3:00 pm.

Environment Canada Twitter: @environmentca

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