Mid-latitude synoptic and mesoscale systems - Atmosphere, Weather and Climate - page 226

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CHAPTER NINE

Mid-latitude synoptic
and mesoscale systems

LEARNING OBJECTIVES

When you have read this chapter you will:

n understand the air mass concept, the characteristics of the major air masses and their
geographical occurrence
n know the mechanisms of frontogenesis and the various frontal types
n understand the relationships between upper-air and surface processes in forming frontal cyclones
n know the major types of non-frontal cyclone and how they form
n be familiar with the role of mesoscale convective systems in severe weather

This chapter examines the classical ideas about
air masses 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 severe
weather phenomena.

any vertical cross-section, as shown in Figure 9.1 ,
isobars and isotherms are parallel.
Three factors determine the nature and degree
of uniformity of air-mass characteristics: (1) the
nature of the source area where the air mass
obtains its original qualities; (2) the direction of
movement and changes that occur as an air mass
moves over long distances; and (3) the age of the
air mass. Air masses 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 AIR-MASS CONCEPT
An air mass 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 kilometers. The
theoretical ideal is a barotropic atmosphere where
surfaces of constant pressure are not intersected
by isosteric (constant-density) surfaces, so that in

B NATURE OF THE SOURCE
AREA
The basic idea of air-mass formation is that
radiative and turbulent transfers of energy and