Geoscience Reference
In-Depth Information
Mid-latitude synoptic and mesoscale
systems
Learning objectives
When you have read this chapter you will:
Understand the airmass concept, the characteristics of the major airmasses and their geographical
occurrence,
Know the mechanisms of frontogenesis and the various frontal types,
Understand the relationships between upper air and surface processes in forming frontal cyclones,
Know the major types of non-frontal cyclone and how they form,
Be familiar with the role of mesoscale convective systems in severe weather.
This chapter examines the classical ideas about
airmasses and their role in the formation of frontal
boundaries and in the development of extratropical
cyclones. It also discusses the limitations of those ideas
and more recent models of mid-latitude weather
systems. Mesoscale systems in mid-latitudes are also
treated. The chapter concludes with a brief overview of
weather forecasting.
as shown in Figure 9.1, isobars and isotherms are
parallel.
Three factors determine the nature and degree of
uniformity of airmass characteristics: (1) the nature
of the source area where the airmass obtains its original
qualities; (2) the direction of movement and changes
that occur as an airmass moves over long distances; and
(3) the age of the airmass. Airmasses are classified on
the basis of two primary factors. The first is temperature,
giving arctic, polar and tropical air, and the second is the
surface type in their region of origin, giving maritime
and continental categories.
A THE AIRMASS CONCEPT
An airmass is defined as a large body of air whose
physical properties (temperature, moisture content
and lapse rate) are more or less uniform horizontally
for hundreds of kilometres. The theoretical ideal is
a
barotropic
atmosphere where surfaces of constant
pressure are not intersected by isosteric (constant-
density) surfaces, so that in any vertical cross-section,
B NATURE OF THE SOURCE AREA
The basic idea of airmass formation is that radiative and
turbulent transfers of energy and moisture, between the
land or ocean surface and the atmosphere, give rise to
