Geoscience Reference
In-Depth Information
concept of potential evapo-transpiration. The inter-war
period was particularly notable for the appearance of
a number of climatic ideas which were not brought to
fruition until the 1950s. These included the use of
frequencies of various weather types (Federov, 1921),
the concepts of variability of temperature and rainfall
(Gorczynski, 1942, 1945) and microclimatology
(Geiger, 1927).
Despite the problems of obtaining detailed measure-
ments over the large ocean areas, the later nineteenth
century saw much climatic research which was con-
cerned with pressure and wind distributions. In 1868
Buchan produced the first world maps of monthly mean
pressure; eight years later Coffin composed the first
world wind charts for land and sea areas, and in 1883
Teisserenc de Bort produced the first mean global
pressure maps showing the cyclonic and anticyclonic
'centres of action' on which the general circulation is
based. In 1887 de Bort began producing maps of upper-
air pressure distributions and in 1889 his world map
of January mean pressures in the lowest 4 km of the
atmosphere was particularly effective in depicting the
great belt of the westerlies between 30° and 50° north
latitudes.
typical associated weather types. By this time, although
the energetics were far from clear, a picture was
emerging of mid-latitude storms being generated by the
mixing of warm tropical and cool polar air as a funda-
mental result of the latitudinal gradients created by the
patterns of incoming solar radiation and of outgoing
terrestrial radiation. Towards the end of the nineteenth
century two important European research groups
were dealing with storm formation: the Vienna group
under Margules, including Exner and Schmidt; and
the Swedish group led by Vilhelm Bjerknes. The former
workers were concerned with the origins of cyclone
kinetic energy which was thought to be due to differ-
ences in the potential energy of opposing air masses of
different temperature. This was set forth in the work
of Margules (1901), who showed that the potential
energy of a typical depression is less than 10 per cent of
the kinetic energy of its constituent winds. In Stockholm
V. Bjerknes' group concentrated on frontal develop-
ment (Bjerknes, 1897, 1902) but its researches were
particularly important during the period 1917 to 1929
after J. Bjerknes moved to Bergen and worked with
Bergeron. In 1918 the warm front was identified,
the occlusion process was described in 1919, and the
full Polar Front Theory of cyclone development was
presented in 1922 (J. Bjerknes and Solberg). After about
1930, meteorological research concentrated increas-
ingly on the importance of mid- and upper-tropospheric
influences for global weather phenomena. This was
led by Sir Napier Shaw in Britain and by Rossby,
with Namias and others, in the USA. The airflow in the
3-10 km high layer of the polar vortex of the northern
hemisphere westerlies was shown to form large-scale
horizontal (Rossby) waves due to terrestrial rotation,
the influence of which was simulated by rotation 'dish
pan' experiments in the 1940s and 1950s. The number
and amplitude of these waves appears to depend on the
hemispheric energy gradient, or 'index'. At times of
high index, especially in winter, there may be as few as
three Rossby waves of small amplitude giving a strong
zonal (i.e. west to east) flow. A weaker hemispheric
energy gradient (i.e. low index) is characterized by four
to six Rossby waves of larger amplitude. As with most
broad fluid-like flows in nature, the upper westerlies
were shown by observations in the 1920s and 1930s,
and particularly by aircraft observations in the Second
World War, to possess narrow high-velocity threads,
termed 'jet streams' by Seilkopf in 1939. The higher
and more important jet streams approximately lie along
E MID-LATITUDE DISTURBANCES
Theoretical ideas about the atmosphere and its weather
systems evolved in part through the needs of nineteenth-
century mariners for information about winds and
storms, especially predictions of future behaviour. At
low levels in the westerly belt (approximately 40° to 70°
latitude) there is a complex pattern of moving high
and low pressure systems, while between 6000 m and
20,000 m there is a coherent westerly airflow. Dove
(1827 and 1828) and Fitz Roy (1863) supported the
'opposing current' theory of cyclone (i.e. depression)
formation, where the energy for the systems was
produced by converging airflow. Espy (1841) set out
more clearly a convection theory of energy production
in cyclones with the release of latent heat as the main
source. In 1861, Jinman held that storms develop where
opposing air currents form lines of confluence (later
termed 'fronts'). Ley (1878) gave a three-dimensial
picture of a low-pressure system with a cold air wedge
behind a sharp temperature discontinuity cutting into
warmer air, and Abercromby (1883) described storm
systems in terms of a pattern of closed isobars with
