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Spotting tornadoes
Dr Harold Brooks
 Tornadoes
and severe thunderstorms producing large hail and strong wind gusts have
fascinated people throughout the ages. Aristotle and Lucretius wrote about them
and they also feature in the mythology of aboriginal North Americans.
Machiavelli described a tornado in his diary and many of the great scientists of
the 18th and 19th centuries tried to explain them.
Our understanding of their distribution, causes, and effects did not begin to
develop significantly though, until the 1880s when John Park Finley of the
United States Signal Service (the forerunner of the US National Weather Service)
systematically began to record their occurrence and their relationship to
observed surface conditions in the environment.
It wasn't until 1925 that meteorologists first collected information on the
conditions above the ground near a tornado, when a tornado struck Borculo, in
the Netherlands.
Gathering data
It would be another 30 years, however, before their would be enough data
collected for people to first try to describe the general conditions associated
with severe thunderstorms and tornadoes and that would only occur because the US
Weather Bureau started trying to forecast the occurrence of storms in 1953. One
of the collateral effects of the forecasting effort was that the collection and
archival of reports of severe weather began. What followed was little short of
an explosion in our estimates of what was actually occurring and in scientific
research. Suddenly, the number of reported tornadoes in the US increased from
150 to over 600 and meteorologists began serious research to attempt to
understand why they occur.
 The
systemisation of data collection in the US, and the corresponding rise in the
number of reports has led many people to think that tornadoes are something that
happens only in the US. In fact, we know that tornadoes have occurred on all
continents except Antarctica. Unfortunately, only a few countries have official
mechanisms for recording reports. As a result, meteorologists interested in the
world-wide occurrence of severe thunderstorms depend on interested individuals,
typically going through the reports in their own countries, to collect reports.
Based on these efforts, we've been able to learn that it appears that the kinds
of severe thunderstorms we see are the same all over the world; it's just that
some places have more than others.
We've also learned that the lack of systematic reporting leads to vast
underestimates of what goes on. In France, for instance, where the entire record
has been collected in much the same way as the pre-1953 US record was collected,
we've found that the distribution of tornadoes by intensity is the same as in
the pre-1953 US record. As a result, it seems likely that if a systematic
forecasting and data collection effort in France took place, the number of
reports would increase seven-fold, as it has in the US over the last 50 years.
Ingredients for a tornado
 Through
the collection of observations of events and the environments in which they
form, as well numerical simulation and statistical analysis of the data,
meteorologists have pieced together a picture of the basic ingredients needed to
make severe thunderstorms, and, to a lesser extent, tornadoes. Relationships
between the environmental conditions and observed severe weather have been
developed, primarily based on American observations. In order for the atmosphere
to make a severe thunderstorm, a few basic ingredients are needed:
1) warm, moist air near the ground
2) relatively dry, cold air aloft,
3) something (such as a front or other boundary between air masses with
different properties) to lift the warm, moist air up into the relatively dry,
cold air aloft
4) horizontal winds in the environment that increase in speed from the surface
to several kilometres above the ground and change direction, typically from the
equator at the surface to out of the west aloft at perhaps 5-6 km above the
ground (vertical wind shear)
The change of phase of water vapour to liquid or solid water, in the form of
clouds, provides the energy that drives the storm, and the change of winds with
height organises the storm, making it much more likely to produce severe
weather. In general, the easiest way to get the dry air aloft is to take air and
heat it over elevated terrain, and then blow it eastward over the moist surface.
This means we typically expect to see severe thunderstorms east of mountain
ranges and poleward of moisture sources, such as warm bodies of water or
tropical rainforests. Tornadic environments are characterised by the conditions
that lead to severe thunderstorms, with the additional need for strong vertical
wind shear in the lowest kilometre and high relative humidity in the lowest kilometre
or so, leading to low cloud bases.
Recently, a new tool for looking at the atmosphere allows meteorologists the
opportunity to take the relationships developed from the American observations,
and apply them to the rest of the world. In effect, we assume that the same
conditions that lead to severe thunderstorms and tornadoes in the US should lead
to severe thunderstorms and tornadoes in the rest of the world. We find how
often those conditions occur in an analysis of the atmosphere around the world
and use that as a proxy for the pattern of occurrence of severe thunderstorms
and tornadoes (Figure below). There are a number of things that are left out of
figures such as these: the presence of the source of lift for the warm, moist
air, but they're reasonably plausible. They're also only based on three years
worth of data (1997-9), so if those years were unrepresentative of typical
atmospheric conditions, the picture may not be accurate.
Another important thing that is missing is good reporting data to check the
quality of the relationships outside of North America. It is possible that there
may be environmental conditions that are conducive to severe thunderstorms that
occur rarely in the United States. If so, then the relationships that have been
developed between events and environments will not be accurate. This issue
becomes more important when one considers that possible climate change could
lead to changes in the distributions of the favourable environments. Extreme
weather events are where climate change will meet people's lives on a daily
basis. As such, it is important for us to develop better understandings of
where, when, and why severe thunderstorms occur. There is no substitute for
better observations of the events!
More Information:
Harold Brooks is the Head, Mesoscale Applications Group, NOAA/National Severe
Storms Laboratory, Norman OKlahoma.
www.nssl.noaa.gov/~brooks/papers/ECSS2002.pdf
www.tordach.org - Includes links to
tornado information around the world, focusing primarily on Central Europe
www.spc.noaa.gov
www.nssl.noaa.gov - The US National
Severe Storms Laboratory, the premier research organisation for severe
thunderstorms in the world
Photos copyright of the NOAA Photo Library, NOAA Central Library; OAR/ERL/National
Severe Storms Laboratory (NSSL)
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