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UPPER-AIR FEATURES SEEN IN IMAGERY

The cloud patterns discussed in this section are associated with upper-level ridges, troughs, and jet streams.

Upper-Level Ridges

The distribution of middle and high clouds is the key in locating and positioning upper-level ridges. A sharp upper ridge has a narrow cloud band associated with it, and the clouds end abruptly at the ridgeline. See figure 10-3-15. The cloud pattern associated with medium amplitude ridgelines is broader, and the ridgeline ranges in position from the leading edge of the cloud deck to a few degrees back into it. When the cirrus deck extends beyond the ridge, the ridgeline closely approximates the leading edge of the middle cloud deck. If there are no cirrus clouds present, the ridgeline is positioned at or slightly behind the leading edge of middle clouds. Flat, low amplitude ridges have their axes well back into the cloud pattern and are often related to anticyclonically curved striations. The axis lies very close to the axis of the curved striations seen in the pattern.

Upper-Level Troughs

Changes in the appearance of frontal bands denote the position of upper-level troughs. A 500-mb trough is positioned along a line extending from the center of the upper-level vortex southward to the point on the frontal band where the clouds become ragged and discontinuous in

Figure 10-3-15.ó500-mb analysis and satellite photograph showing narrow cloud band associated with a sharp ridge.

appearance. See figure 10-3-16. If a comma-shaped cloud area (PVA MAX) is present equatorward of the vortex, the trough is drawn to the west of the comma cloud.

Jet Streams

The distribution and appearance of cirrus clouds help us in locating jet streams. Jet-stream cirrus are usually much higher than other widespread cloud patterns. In visual imagery, they occasionally cast shadows on lower cloud decks or surface features. These shadows appear as long, narrow dark lines with a slight anticyclonic turning. A jet is located parallel to and within 1 degree of latitude poleward of such shadows. Figure 10-3-17 shows a shadow line cast by a higher cirrus deck. A jetís position can also be determined by the texture and brightness of clouds in its vicinity. Poleward of a jet, the clouds have a bumpy appearance, while on the equatorward side the clouds are smooth. Another cirrus pat-tern associated with jet streams is transverse cloud bands. See figure 10-3-18. This pattern is not ordinarily seen in the IR mode. The jet axis is located poleward and runs perpendicular to the bands. The cloud pattern most frequently associated with a polar jet is a large, anti-cyclonically curved cirrus shield with a sharp poleward edge. Look for this pattern between the upper-level trough and the downstream upper-level ridge. Jet streams are also found in conjunc-tion with long narrow streaks of cirrus; however, accurately pinpointing a jetís location is more difficult with this pattern, because the streaks lack continuity. The streaks nearly parallel the windflow and are equatorward of the jet axis.

Learning Objective: Identify land and water features seen in imagery.

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