Midlatitude Cyclones

The traveling low-pressure systems of the middle latitudes are the great weather makers of the temperate zones, sweeping clouds, rain, snow, and wind across continents in a recurring few-day life cycle.

Definition

A midlatitude cyclone is a synoptic-scale low-pressure system of the extratropics that develops along the boundary between warm and cold air, drawing its energy from horizontal temperature contrasts through baroclinic instability.

Scope

This topic covers the structure and life cycle of extratropical cyclones, the Norwegian frontal-wave and conveyor-belt conceptual models, the process of cyclogenesis, and the dynamical link between surface lows and the upper-level waves that drive their development.

Core questions

• How does a midlatitude cyclone form and progress through its life cycle?
• What is the role of the polar front and of upper-level waves in cyclone development?
• How do the conveyor belts organize a cyclone's clouds and precipitation?
• Why do some cyclones deepen explosively?

Key theories

Norwegian cyclone model

Bjerknes and Solberg described the cyclone as a wave that grows on the polar front, develops warm and cold fronts, and ultimately occludes, providing a life-cycle framework that organized weather analysis worldwide.

Baroclinic cyclogenesis

Surface cyclones intensify when an upper-level trough induces divergence aloft over a region of strong low-level temperature contrast, converting available potential energy into the kinetic energy of the storm.

Mechanisms

A midlatitude cyclone begins as a small wave on a frontal boundary where warm and cold air meet. When an upper-level trough moves overhead, the divergence ahead of it removes mass from above the surface low and allows the pressure to fall, while warm and cold conveyor belts transport air through the system to build its cloud and precipitation shields. As the cold front overtakes the warm front the cyclone occludes, cuts off from its energy source, and decays.

Clinical relevance

Midlatitude cyclones bring the bulk of cool-season precipitation, damaging winds, and snowstorms to the temperate continents, so forecasting their tracks and intensity, including rapidly deepening bomb cyclones, is central to protecting life, property, transportation, and commerce.

History

The life-cycle model of the midlatitude cyclone was set out by Jacob Bjerknes and Halvor Solberg of the Bergen School in 1922; later work added the upper-level dynamics of baroclinic instability and the conveyor-belt description of airflow, refining but not replacing this enduring conceptual model.

Key figures

• Jacob Bjerknes
• Halvor Solberg
• Tor Bergeron

Seminal works

• bjerknes1922
• bluestein1993

Frequently asked questions

What is the difference between a midlatitude cyclone and a hurricane?

A midlatitude cyclone draws energy from horizontal temperature contrasts and has fronts, forming in the temperate zones, whereas a hurricane is a warm-core tropical system powered by latent heat from warm ocean water and has no fronts.

What is a bomb cyclone?

A bomb cyclone, or bomb, is a midlatitude cyclone whose central pressure falls very rapidly, traditionally at least 24 hectopascals in 24 hours, producing an unusually intense storm with strong winds.