Weather: Better get used to more of those intense hail years

Thursday, April 2, 2009

Last year, crops and trees across the province took a pounding from hail. What's behind the increase in severe hailstorms?

by HENRY HENGEVELD

On June 9, 2008, a severe hailstorm passed through the Chatham-Kent area of Ontario, pounding crops and trees, and seriously damaging cars and buildings with hail stones the size of golf balls. Two weeks later, another storm dropped more large hail stones on the same area.

The Chatham-Kent hailstorms were just the harbinger of much more to come. Areas within the Niagara-St. Catharines region, for example, were hit hard on July 22. On Aug. 2, farms in the Grand Bend area on Lake Huron and south of London also suffered. In fact, by the time summer was over, significant hail damage had occurred over wide areas of Ontario. Most orchards in the province were hit at last once, some several times. Reports on related damage to crops indicate extensive losses in 2008, both in terms of the severity and frequency of damage. The areal extent of damage, at about 110,000 acres, was more than four times that of the average for the previous five years.

The hail season was sufficiently unusual for David Phillips, Environment Canada's weather guru, to list it as one of the top 10 weather stories of 2008, and to dub it as "a hail of a summer." 

Yet the summer was not unusually hot or humid. Since humidity is generally associated with severe convective storms, it raises the question, "Why all that hail?"

A recent research article, published in the prestigious scientific journal Geophysical Research Letters, offers some clues as to the answer. In that article, Environment Canada researcher Zuohao Cao first took a thorough look at the variations and trends in severe hail storm behaviour in Ontario between 1979 and 2002 (see Figure 1). His results indicate that, over the 24 years of the study, Ontario experienced a yearly average of 20 or so severe hail storm events (defined as events with hail stones having diameters of at least two centimetres).

However, he also noted a significant increase in the number of these events over time, with the last decade experiencing about six more such events than the previous one. In fact, the last five years of the analysis, 1998 through 2002, were all among the
10 most intense hail seasons. 

Cao then compared the atmospheric conditions for the 10 individual hail seasons (defined as the period from May to October) having the greatest number of severe events with those for the 10 seasons with the least number of events. He initially looked at the differences in a temperature index between the bottom of the atmosphere and that at a height of 500 hPa air pressure (about five to six kilometres elevation). This temperature difference is one indicator of the stability of the air mass above any location.

Not surprisingly, he found that the atmosphere was less stable, and hence more prone to strong convective thunderstorms, during the years with higher number of severe hail events than for the low-event years. He also noted that the amount of water in the atmosphere available for precipitation was greater during the more intense hail years.

He then compared the stability of the atmosphere with surface temperatures. As expected, years with warmer surface temperatures generally coincided with a higher level of severe hail storm activity. That is because the higher temperatures at the bottom of the air mass increased the vertical temperature gradient and enhanced the air mass instability. More instability, more severe thunderstorms and thus a greater risk of a major hail storm event.   

The curious thing is that the summer of 2008 doesn't quite fit the pattern that Cao found in his earlier 24-year record. Last year, the lower atmosphere was neither unusually warm nor very humid.

In this case, the increased vertical temperature gradient and enhanced air mass instability occurred because temperatures at the 500 hPa level were cooler than normal. Different factors, but the same results.

The prognosis for future decades, as the world continues to warm, is for warmer surface temperatures and higher atmospheric moisture – both factors that suggest increased summer air mass instability!

That in itself is not proof that the recent trend is due to global warming. However, it does suggest, everything else being equal, that we may need to get used to the concept of increased frequency of intense hail years.

It would seem to be prudent to keep those insurance premiums up to date! BF

Henry Hengeveld is Emeritus Associate, Science Assessment and Integration Branch/ACSD/MSC, Environment Canada.