Thanks to the Darling Downs Soaring Club for permission to reproduce this item
from its newsletter CHAOTIC.
Being a cross country pilot is not just being able to fly.
You should also:
- Be in good physical condition
- Understand what's around you and how to read the conditions
- Know your glider (you need about 100 hrs hours on type to be comfortable)
- Have good instrument
- And finally…..Practice, Practice. Practice
Cumulus Days
Good lift is usually found under the best cu's where the tops are smaller than the
bases. Look for the best part of the cu and remember that the darkest part is usually the
thickest part of the cloud, and this is where the lift is strongest. Check where the sun
is shining as it will give more heating to that part of the cloud. Eg in the afternoon,
the Western side of the cloud will be better.
To estimate the distance to next cu from cloud base, look at the shadows of the streets
on the ground.
Winds
Winds can displace the lift. When you are at a low height, you will need to look for
trigger sources and the cu's above will not be a good indicator.
Trigger sources can be fixed and are likely to "pulse". If you have ever seen
time-lapse photography on cloud formation, it is easy to see how thermals and hence clouds
often develop from a fixed source.
When analysing the ground sources think about wind direction, how strong will the lift
be, and look for areas of potential stored heat such as dark fields, high crops,
vegetation, and forests.
These sources will be more or less active depending on the time of day. Take note of the
wind strength and direction in working out where the lift will be.
On windy days there is more mixing of cool air at ground level, and thermals are broken
down low.
When lift is very strong, the thermals will be further apart.
Trigger Points
To give you an idea of the potential of some ground sources, the following list shows
radiant heat energy of different types of ground features:
- Cropped paddocks and dark ground <15%
- Bare ground/light soils/rocks <38%
- Desert <28%
- Snow <80%
When looking for trigger sources, look for protected areas where the sunlight has had
time to heat the surface. Eg ravines and bowls in mountain areas.
Use your imagination and think - look for hot spots
Trigger points in calm conditions - look for an area of temperature difference.
- Mountain ridges - either side differential heating temperature
- Edges of forests or wooded areas
- Lake/river banks
- Trucks and tractors are triggers
- Crops and dams - watch for disturbance coming across the surface
When looking for new thermals, don't always go for a nicely-formed cu. By the time you
get there, it will probably already be dissipating. Instead look for the haze domes, as
shown below.
Cloud Streets
Thermal under a cloud street if you are below cloud base, or have reached the end of
the cloud street and need to leave as high as possible, or if there are no other streets
to go under along course.
You should continue to fly under worthwhile cloudstreets when it is close to desired
course, or you are flying into strong wind. Your cruising speed under cloudstreets is
higher than elsewhere. Use the cloudstreets and cut across (like tacking in yachts) if
they are not aligned on track.
Leaving a Thermal
Don't leave the thermal at thermalling speed as you will lose height crossing the sink
on the side of the thermal. Always increase speed about ½ a turn before leaving, so you
cross the sink with more speed
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