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Synoptic scale is defined as the scale of the migratory high- and low-pressure systems of the lower atmosphere, with wavelengths of 1,000 to 2,500 km. Cloud patterns that relate to the synop-tic and subsynoptic scale are associated with fronts, high- and low-pressure systems, jet streams, etc.


The cloud patterns discussed in this section pertain to those features seen in surface chart analysis.


Distinctive, long cloud bands identify fronts on satellite imagery. Some extend several thousand miles and exceed 300 miles in width. Frontal zones contain both stable and unstable cloud forms, and through the use of infrared imagery, we can determine frontal stability and whether all or part of a front is active or inactive. The active or inactive classification pertains to the clouds and precipitation produced by a front or portion of a front. Active fronts, or por-tions thereof, produce appreciable cloudiness and usually precipitation, while inactive fronts, or portions thereof, have few clouds and no precipi-tation. As a rule, frontal cloud bands are more con-tinuous over oceans than overland. This rule is related to the availability of moisture. Overland, frontal cloud bands can be discontinuous and fragmented, while the same front over water may appear well-developed and continuous.

ACTIVE COLD FRONTS.— These fronts appear as continuous, well-developed cloud bands, especially over oceans. Overland, they are much more fragmented and discontinuous because of topography. The bands are made up of stratiform, cumuli form, and cirriform cloud layers. In IR imagery, the frontal band appears off-white with lines of bright white (convective activity) within the band.

INACTIVE COLD FRONTS.— Over oceans, these fronts appear most often as narrow, fragmented, discontinuous cloud bands. They are similar in appearance to active cold fronts overland. The clouds are mainly low-level cumuli form and stratiform, but some cirriform clouds may be present. Overland, inactive cold fronts, or portions thereof, have few or no clouds.

ACTIVE QUASI-STATIONARY FRONTS.— The cloud bands associated with these fronts are wide, and frontal waves are fre-quently seen within the band. The frontal wave creates a bulge in the cloud band that is visible in both visual and IR pictures. 

INACTIVE QUASI-STATIONARY FRONTS.— These cloud bands are fragmented and usually devoid of low and mid clouds. They are most often found in lower latitudes and generally are oriented east-west. 

WARM FRONTS.— Active warm fronts are, at best, difficult to locate on satellite pictures, while inactive warm fronts cannot be located at all, because they are, for the most part, cloud free.

OCCLUDED FRONTS.— The cloud bands of occluded fronts spiral toward their associated vortices. The cloud band bulges at the occlusion’s apex, and the apex is sometimes located by a change in the clouds. Poleward of the apex, the clouds take on a cobblestone appearance, while equatorward, they appear smooth. Occasionally, cloud bands resemble fronts that have undergone the occlusion process, but in reality this is not the case. Figure 10-3-1 shows a comma-shaped cloud mass (PVA MAX) associated with a short-wave trough moving across an east-west oriented, quasi-stationary front. The clouds of the trough merge with those of the PVA MAX and result in a cloud band resembling an occluded front. Such east-west-oriented fronts appear in imagery to jump from the wave stage to a mature occlusion without going through the occlusion process. These occluded-like frontal bands are termed instant occlusions.

FRONTAL WAVES.— Frontal waves appear similar in visual and IR imagery with regard to the bulge in the frontal cloud band. In visual pictures, frontal waves usually have a uniform white appearance, while the IR presen-tation exhibits several shades of gray, indi-cating clouds of varying height and thickness. The larger convective clouds stand out as globs of intense white. Frontal waves that appear gray in imagery are classified as stable, while those that are extensively white (cold temperatures) are unstable. An illustration of stable and unstable wave appearance is seen in figure 10-3-2.

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