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Cloud Streets


Introduction to Glider Flying > Soaring Weather > The Atmosphere > Cloud Streets

Cumulus clouds are often randomly distributed across the sky, especially over relatively flat terrain. Under the right conditions, however, cumulus can become aligned in long bands, called cloud streets. These are more or less regularly spaced bands of cumulus clouds. Individual streets can extend 50 miles or more while an entire field of cumulus streets can extend hundreds of miles. The spacing between streets is typically three times the height of the clouds. Cloud streets are aligned parallel to the wind direction, thus they are ideal for a downwind cross-country flight. Glider pilots can often fly many miles with little or no circling, sometimes achieving glide ratios far exceeding the still-air value.

Cloud streets usually occur over land with cold-air out-breaks, for instance, following a cold front. Brisk sur-face winds and a wind direction remaining nearly constant up to cloud base are favorable cloud street conditions. Wind speed should increase by 10 to 20 knots between the surface and cloud base, with a max-imum somewhere in the middle of or near the top of the convective layer. Thermals should be capped by a notable inversion or stable layer.

A vertical slice through an idealized cloud street illus-trates a distinct circulation, with updrafts under the clouds and downdrafts in between. Due to the circula-tion, sink between streets may be stronger than typically found away from cumulus. [Figure 9-13]

Figure 9-13. Circulation across a cloud street.

Thermal streets, with a circulation like Figure 9-13, may exist without cumulus clouds. Without clouds as markers, use of such streets is more difficult. A glider pilot flying upwind or downwind in consistent sink should alter course crosswind to avoid inadvertently flying along a line of sink between thermal streets.

Air Masses Conducive to Thermal Soaring
Thermal Waves
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