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Unit 13


Unit Overview

This unit discusses some of the mechanisms responsible for cloud formation and ultimately precipitation; the unit also examines the related concept of air masses. The main sections are:

  • Lifting mechanisms that produce precipitation
  • Convergent-lifting precipitation
  • Convectional precipitation
  • Orographic precipitation
  • Frontal (cyclonic) precipitation
  • Air masses in the atmosphere

The production of precipitation involves multiple processes, including the general mechanisms of air parcel lifting. The four major lifting mechanisms are: convergent lifting, convection, orographic lifting, and frontal lifting. These mechanisms initiate the upward movement of air parcels. Adiabatic cooling will eventually yield a cloud if both the dew point temperature is achieved and if sufficient cloud condensation nuclei are present.

The stability of the atmosphere affects the type of cloud (e.g., cumulonimbus vs. stratus). The four lifting mechanisms are associated with air masses, which are large regions of the atmosphere having relatively homogeneous temperature and moisture characteristics. Convergent, convectional, and orographic lifting can all occur within an air mass, whereas frontal lifting occurs when two different air masses interact. A "front" is the term for that zone of interaction, and the two major types of fronts that yield air parcel lifting are cold fronts and warm fronts.

Unit Objectives

  • To discuss the four basic mechanisms for producing precipitation
  • To develop the concept of air masses - their character, origin, movement patterns, and influence on precipitation
  • To distinguish between cold fronts and warm fronts, and to describe their structure and behaviour as they advance

Glossary of Key Terms

Air mass A very large parcel of air (more than 1600 km [1000 mi] across) in the boundary layer of the troposphere that possesses relatively uniform qualities of density, temperature, and humidity in the horizontal dimension; it is also bound together as an organized whole, a vital cohesion because air masses routinely migrate as distinct entities for hundreds of kilometers.
Cold front Produced when an advancing cold-air mass hugs the surface and displaces all other air as it wedges itself beneath the preexisting warmer air mass; cold fronts have much steeper slopes than warm fronts and thus produce more abrupt cooling and condensation (and more intense precipitation).
Convection Spontaneous vertical air movement in the atmosphere.
Convectional-precipitation Convection is the spontaneous vertical movement of air in the atmosphere; convectional precipitation occurs after condensation of this rising air.
Convergent-lifting precipitation Precipitation produced by the forced lifting of warm, moist air where low-level windflows converge; most pronounced in the equatorial latitudes where the Northeast and Southeast Trades come together in the InterTropical Convergence Zone (ITCZ), especially over the oceans.
Front The surface that bounds an air mass, along which contact occurs with a neighboring air mass possessing different qualities; this narrow boundary zone usually marks an abrupt transition in air density, temperature, and humidity. A moving front is the leading edge of the air mass built up behind it.
Frontal precipitation Precipitation that results from the movement of fronts whereby warm air is lifted, cooled, and condensed; also frequently called cyclonic precipitation.
Life cycle of a thunderstorm The stages marking the formation, progression, and termination of a thunderstorm: the developing stage, mature stage, and dissipating stage.
Orographic precipitation The rainfall (and sometimes snowfall) produced by moist air parcels that are forced to rise over a mountain range or other highland zone; such air parcels move in this manner because they are propelled both by steering winds and the push of other air parcels piling up behind them.
Rain shadow effect The dry conditions - often at a regional scale as in the U.S. interior West - that occur on the leeward side of a mountain barrier that experiences orographic precipitation; the passage of moist air across that barrier wrests most of the moisture from the air, whose adiabatic warming as it plunges downslope sharply lowers the dew point and precipitation possibilities.
Source region An extensive geographic area, possessing relatively uniform characteristics of temperature and moisture, where large air masses can form.
Tornado A small vortex of air, averaging 100 to 500 m (330 to 1650 ft) in diameter, that descends to the ground from rotating clouds at the base of a severe thunderstorm, accompanied by winds whose speeds range from 50 to 130 m per second (110 to 300 mph); as tornadoes move across the land surface, they evince nature┬┐s most violent weather and can produce truly awesome destruction in the natural and cultural landscapes.
Warm front Produced when an advancing warm air mass infringes on a preexisting cooler one; when they meet, the lighter, warmer air overrides the cooler air mass, forming the gently sloping, upward-sloping warm front (producing far more moderate precipitation than that associated with steeply sloped cold fronts).

Unit Outline

  • Lifting mechanisms that produce precipitation
    • All precipitation is from moist air that has been cooled adiabatically to below its condensation (dew) point
  • Convergent┬┐Lifting Precipitation
    • Convergent-lifting precipitation occurs when warm, moist air converges, rises, expands, and condenses into precipitation
      • Inter-Tropical Convergence Zone (ITCZ)
  • Convectional precipitation
    • Convectional precipitation occurs when a column of air (convection cell) rises spontaneously, expands, cools, and condenses into precipitation
    • Thunderstorms are produced by convectional precipitation, common in low and mid-latitudes where there is a large amount of latent heat
      • developing stage
      • mature stage (anvil top)
      • dissipating stage
    • Thunderstorm-related phenomena
      • hail in squall-line storms
      • lightning
      • thunder
      • supercells
      • mesoscale convective complexes
  • Orographic Precipitation
    • Orographic precipitation occurs when air masses are forced to rise over areas of higher altitude
    • The rain shadow effect causes very dry areas on the leeward side of mountains
  • Frontal (Cyclonic) Precipitation
    • Frontal precipitation occurs when warm air is lifted, cooled and its water vapour is condensed as a result of frontal movement.
    • There are two types of fronts, warm and cold
  • Air masses in the atmosphere
    • Maritime (m) or continental (c)
    • Tropical (T)
    • Polar (P)
    • Maritime Tropical (mT)
    • Continental Polar (cP)
    • Maritime Polar (mP)
    • Continental Arctic (cA) or Continental Antarctic (cAA)
    • Maritime Equatorial (mE)
    • Colder (k)
    • Warmer (w)
    • Stable (s)
    • Unstable (u)
    • Movements of air masses
      • Air masses are somewhat affected by their contacts with the surface, but retain many of their original characteristics far from their origin

Review Questions

  1. Describe the life cycle of a thunderstorm, using Fig. 13.3 as a reference.
  2. Compare and contrast orographic and frontal precipitation.
  3. What conditions characterize the windward slope of a mountain barrier?