Unit 14


Unit Overview

This unit examines weather systems and the transfers of energy and moisture associated with the systems. The main sections are:

  • Low-latitude weather systems
  • Weather systems of the middle and higher latitudes
  • Energy and moisture within weather systems

A weather system is a set of interrelated atmospheric features that is present from minutes to weeks. Weather systems include storms, which vary in latitudinal position, geographic scale, duration, and intensity.

A hurricane is an example of a weather system that forms at low latitudes. Hurricanes have wind speeds greater than 119 km per hour and a central surface pressure below 900 mb, although wind speeds may be much greater. The origin of hurricanes appears to be related to easterly waves and the equatorial trough of low pressure. Other low-latitude systems include easterly waves, tropical depressions, and tropical storms.

The middle and high latitudes experience the polar front jet stream and mid-latitude cyclones. This unit examines, in detail, the life cycle of mid-latitude cyclones. Each weather system, although distinctive in detail, has a characteristic internal organization in which warm air is transformed into faster-moving air, and moisture become precipitation.

Unit Objectives

  • To demonstrate the importance of migrating weather systems in the global weather picture
  • To discuss the significant tropical weather systems, particularly hurricanes
  • To explain how midlatitude cyclones are formed, and to describe the weather patterns associated with them

Glossary of Key Terms

Cyclogenesis The formation, evolution, and movement of midlatitude cyclones.
Easterly wave A wave-like perturbation in the constant easterly flow of the Northeast and Southeast Trade winds that produces this type of distinctive weather system; westward-moving air is forced to rise on the upwind side (producing often heavy rainfall) and descend on the fair-weather down-wind side of the low-pressure wave trough.
Eye The open vertical tube that marks the center of a hurricane, often reaching an altitude of 16 km (10 mi).
Eye wall The rim of the eye or open vertical tube that marks the center of a welldeveloped hurricane; the tropical cyclone┬┐s strongest winds and heaviest rainfall occur here.
Hurricane A tropical cyclone capable of inflicting great damage. A tightly organized, moving low-pressure system, normally originating at sea in the warm moist air of the tropical atmosphere, exhibiting wind speeds in excess of 33m per second (74 mph); as with all cyclonic storms, it has a distinctly circular and pressure field.
Occluded front The surface boundary between cold and cool air in a mature midlatitude cyclone; caused by the cold front undercutting and lifting the warm air entirely off the ground as in Fig. 14.10c.
Open wave The early-maturity stage in the development of a mid1atitude cyclone; surface cyclonic air motion transforms the original kink on the stationary front into an open wave, around which cold and warm air interact in the distinct ways shown in Fig 14.10b.
Polar Front jet stream The upper atmosphere jet stream located above the subpolar latitudes, specifically the Polar Front; at its strongest during the half-year centered on winter.
Stationary front The boundary between two stationary air masses.
Storm An organized, moving atmospheric disturbance.
Storm surge The wind-driven wall of water hurled ashore by the approaching center of a hurricane, which can surpass normal high tide levels by more than 5 m (16 ft); often associated with a hurricane┬┐s greatest destruction.
Weather system Organized phenomena of the atmosphere, with inputs, outputs, and changes of energy and moisture.

Unit Outline

  • Low-latitude weather systems
    • Weather systems are organized phenomena with inputs, outputs, and changes in energy and moisture
      • storms are moving disturbances of the atmosphere
    • Easterly (tropical) waves
      • result of the trade winds in tropical latitudes
    • Tropical depressions
      • low-pressure troughs with cyclonic organization
    • Tropical storms
      • sustained winds exceed 63 km/hr (39 mi/hr)
    • Hurricanes (tropical cyclones)
      • sustained winds exceed 119 km/hr (74 mi/hr)
      • called hurricanes in western Atlantic and eastern Pacific
      • called typhoons in western north Pacific
      • called (tropical) cyclones in Indian Ocean
      • open eye structure in center
      • strongest winds and rain in eye wall
    • Hurricane development
      • develop between 5 and 25 degrees latitude
    • Hurricane destruction
      • storm surge
      • tornadoes
  • Weather systems of the middle and higher latitudes
    • The polar front jet stream
      • important implications for surface pressure patterns and weather
    • Cyclogenesis
      • formation, evolution, and movement of midlatitude cyclones
      • called depressions or extratropical cyclones
      • stages of development
        • stationary
        • open-wave
        • occluded front
        • dissipation
      • atmospheric events
        • steady rain and falling pressure as cyclone approaches
        • warm front passes, winds shift, pressure steady, occasional showers
        • cold front arrives, intense rain, wind shifts, abrupt cold temperatures
        • Energy and moisture within weather systems
    • Inputs into weather systems
      • warm air carrying latent heat
      • water vapour

Review Questions

  1. Describe the process of mid-latitude cyclogenesis.
  2. List the differences between a tropical depression, tropical storm, and hurricane.