SUMMARY

Ideal airmasses are defined in terms of barotropic conditions, where isobars and isotherms are assumed to be parallel to each other and to the surface. The character of an airmass is determined by the nature of the source area, changes due to airmass movement, and its age. On a regional scale, energy exchanges and vertical mixing lead to a measure of equilibrium between surface conditions and those of the overlying air, particularly in quasi-stationary high-pressure systems. Airmasses are conventionally identified in terms of temperature characteristics (Arctic, polar, tropical) and source region (maritime, continental). Primary airmasses originate in regions of semi-permanent anticyclonic subsidence over extensive surfaces of like properties. Cold airmasses originate either in winter continental anticyclones (Siberia and Canada), where snow cover promotes low temperatures and stable stratification, or over high-latitude sea ice. Some sources are seasonal, such as Siberia; others are permanent, such as Antarctica. Warm airmasses originate either in shallow tropical continental sources in summer or as deep, moist layers over tropical oceans. Airmass movement causes stability changes by thermodynamic processes (heating/cooling from below and moisture exchanges) and by dynamic processes (mixing, lifting/subsidence), producing secondary airmasses (e.g. mP air). The age of an airmass determines the degree to which it has lost its identity as the result of mixing with other airmasses and vertical exchanges with the underlying surface.

Airmass boundaries give rise to baroclinic frontal zones a few hundred kilometres wide. The classical (Norwegian) theory of mid-latitude cyclones considers that fronts are a key feature of their formation and life cycle. Newer models show that instead of the frontal occlusion process, the warm front may become bent back with warm air seclusion within the polar airstream. Cyclones tend to form along major frontal zones - the polar fronts of the North Atlantic and North Pacific regions and of the southern oceans. An Arctic front lies poleward and there is a winter frontal zone over the Mediterranean. Airmasses and frontal zones move poleward (equatorward) in summer (winter).

a cold front. They are most common in autumn, when cold air moves over relatively warm seas.

Newer cyclone theories regard fronts as rather incidental. Cloud bands and precipitation areas are associated primarily with conveyor belts of warm air. Divergence of air in the upper troposphere is essential for large-scale uplift and low-level convergence. Surface cyclogenesis is therefore favoured on the eastern limb of an upper wave trough. 'Explosive' cyclogenesis appears to be associated with strong wintertime gradients of sea-surface temperature. Cyclones are basically steered by the quasi-stationary long (Rossby) waves in the hemispheric westerlies, the positions of which are strongly influenced by surface features (major mountain barriers and land/sea-surface temperature contrasts). Upper baroclinic zones are associated with jet streams at 300 to 200 mb, which also follow the long-wave pattern.

The idealized weather sequence in an eastward-moving frontal depression involves increasing cloudiness and precipitation with an approaching warm front; the degree of activity depends on whether or not the warm-sector air is rising (ana- or kata-fronts, respectively). The following cold front is often marked by a narrow band of convective precipitation, but rain both ahead of the warm front and in the warm sector may also be organized into locally intense mesoscale cells and bands due to the 'conveyor belt' of air in the warm sector.

Some low-pressure systems form through non-frontal mechanisms. These include the lee cyclones formed in the lee of mountain ranges; thermal lows due to summer heating; polar air depressions commonly formed in an outbreak of maritime Arctic air over oceans; and the upper cold low, which is often a cut-off system in upper wave development or an occluded mid-latitude cyclone in the Arctic.

Mesoscale convective systems (MCSs) have a spatial scale of tens of kilometres and a timescale of a few hours. They may give rise to severe weather, including thunderstorms and tornadoes. Thunderstorms are generated by convective uplift, which may result from daytime heating, orographic ascent or squall lines. Several cells may be organized in a mesoscale convective complex and move with the large-scale flow. Thunderstorms associated with a moving convective system provide an environment for hailstone growth and for the generation of tornadoes.

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