A
Cloud Street is an arrangement of
cumulus clouds in a line, which allows
soaring
aircraft, such as
hang gliders,
paragliders, and
sailplanes
to easily fly for long distances.
Cumulus clouds are formed when
thermals reach an altitude where the
moisture in them
condenses, thus cumulus clouds are good markers for
lift. The layer of air between the
surface and the altitude where the
dew point (condensation) is reached is known as the
convective layer, and is the volume of air within which soaring takes place. Thermals are born at the surface
from points more or less evenly distributed (on the very large scale) and tend to be produced at regular
intervals of time. If there is a steady breeze then there will be a line of thermals at various stages of life
downwind of each thermal producing spot. If the breeze
is not so strong that it breaks up the thermals at altitude then there will be a line of cumulus clouds
marking the line of thermals. The best conditions for cloud streets are when the maximum wind speed is
within the convective layer (near the top).
A pilot can climb to near
cloudbase and then glide to the area under the next cloud downwind and
stand a very good chance of finding lift, climbing back to cloudbase, and repeating as desired. The
combination of flying directly downwind and not having to "hunt" for lift allows pilots to focus
on efficiency and speed, leading to long flight distances.
As the line of thermals and clouds forms the convection along the line is enhanced by lines of
sink along each
side. The line of shadows the clouds cast on the ground increases the temperature
differential
on the surface along corresponding lines. This leads to large areas of
parallel cloud streets,
especially where the surface disruptions are minor; the streets usually have some curvature which
illustrates the large scale flow of the
airmass. The areas between cloud streets are areas of
strong sink, and often only
gliders with exceptional
performance can fly from one street to
another. "Streets" can be present where thermals are produced but that are too dry for clouds to form -
"blue thermals" - but without the visual cues such streets are extrememly difficult to detect and exploit.