Geoscience Reference
In-Depth Information
Northern Hemisphere), where the translational
and rotational speeds are combined. Destruction
results not only from the high winds, because
buildings near the path of the vortex may explode
outward owing to the pressure reduction outside.
Intense tornadoes present problems as to their
energy supply, and it has recently been suggested
that the release of heat energy by lightning and
other electrical discharges may be an additional
energy source.
Tornadoes commonly occur in families and
move along rather straight paths (typically between
10 and 100km long and 100m to 2km wide) at
velocities dictated by the low-level jet. Thirty-year
averages indicate some 750 tornadoes per year in
the United States, with 60 percent of these during
April to June (see
Figure 9.32B
). The largest out-
break in the United States occurred on 3-4 April
1974, extending from Alabama and Georgia in the
south to Michigan in the north and from Illinois
in the west to Virginia in the east. This 'Super
Outbreak' spawned 148 tornadoes in 20 hours with
a total path length of over 3200km (
Plate 9.5
).
Tornadoes in the United States caused an
average of 59 fatalities per year during 1975-2006
and about 1800 injuries annually, although most
of the deaths and destruction result from a few
long-lived tornadoes, making up only 1.5 percent
of the total reported. For example, the most severe
recorded tornado traveled 200km in three hours
across Missouri, Illinois and Indiana on 18 March
1925, killing 689 people. Tornado fatalities were
almost 19,000 during the period 1880-2005 but
the annual rate has declined considerably from the
1920s-1930s. The highest proportion (44 percent)
of fatalities occur in mobile homes.
Tornadoes also occur in Canada, Europe,
Australia, South Africa, India and East Asia. They
are not unknown in the British Isles. During
1960-1982 there were 14 days per year with
tornado occurrences. Most are minor outbreaks,
but on 23 November 1981, 102 were reported
during southwesterly flow ahead of a cold front.
They are most common in autumn, when cold air
moves over relatively warm seas.
Ideal air masses are defined in terms of barotropic conditions, where isobars and isotherms are
assumed to be parallel to each other and to the surface. The character of an air mass is determined
by the nature of the source area, changes due to air-mass movement and its age. On a regional
scale, energy exchanges and vertical mixing lead to a measure of equilibrium between surface
conditions and those of the overlying air, particularly in quasi-stationary high pressure systems.
Air masses are conventionally identified in terms of temperature characteristics (arctic, polar,
tropical) and source region (maritime, continental). Primary air masses originate in regions of semi-
permanent anticyclonic subsidence over extensive surfaces of similiar properties. Cold air masses
originate either in winter continental anticyclones (Siberia and Canada), where snow cover
promotes low temperatures and stable stratification, or over high-latitude sea ice. Some sources
are seasonal, like Siberia; others are permanent, such as Antarctica. Warm air masses originate
either in shallow tropical continental sources in summer or as deep, moist layers over tropical
oceans. Air-mass movement causes stability changes through thermodynamic processes
(heating/cooling from below and moisture exchanges) and by dynamic processes (mixing,
lifting/subsidence), producing secondary air masses (e.g., mP air). The age of an air mass determines
the degree to which it has lost its identity as the result of mixing with other air masses and vertical
exchanges with the underlying surface.
