Thermals should not be confused with ridge lift. We want
sim thermals to have appropriate strength, have a life-cycle, have cumulus clouds, relate to ground features.
Thermals are relatively simple, but often badly explained. The Sun heats the ground, some areas warm better then others (e.g.
those areas are less damp, or facing into the sun more), the warmer air tends to collect near the surface and then rise in a steady column. The
air cools steadily as it rises (because it is EXPANDING due to lower pressure, not because it's otherwise losing heat, so this is
easily predictable). Give the prevailing HUMIDITY, at some altitude (i.e. temperature) the water contained in the air-mass will condense
out and form a cloud. Given all this, Cumulus clouds would have perfectly flat bottoms (because that condensing temperature occurs at
a particular height). In fact the center of the rising column of air is rising faster than than the air around it, and this is enough
to push the center of the bottom of the Cumulus cloud UP a little, so the underside of active Cumulus clouds is typically concave. You
can easily see this when thermalling under a cloud because as you reach cloud base the edges of the cloud will be lower than you
are (which is slightly unsettling as you can't see the horizon).
Some key points:
- Cumulus clouds are FAR AND AWAY the most important indicators of thermals to soaring pilots. The absence of Cu's on
a sunny day (because a layer of warmer air stops the thermal reaching the condensating altitude/temperature) is referred
to as a 'blue' day and thermalling on those days is awful, with racing competitors forming huge gaggles because no-one
has any idea where the thermals are so the best technique is to find another glider circling and join them.
- In the UK (and probably elsewhere), cloud-flying is permitted for gliders. This is a wonderful experience, with
the lift of the thermal continuing (the condensing water vapour actually ADDS energy to the thermal) but in my
experience the lift becomes smoother and the sounds inside the cloud seem slightly dampened which is somehow
relaxing as you sit in the gray watching the Attitude Indicator. The best bit is
emerging from the cloud with all the juicy extra height you've gained, into a castellated landscape of dense Cu's with
a long straight glide on track until you're back below cloudbase.
- People that draw Cumulus clouds as symmetical fluffy blobs of cotton wool have obviously never actually looked at one,
the bottoms of Cu's are FLAT. People are mysteriously blind to this or struggle to believe it, so even in flight sims the
Cu's are always fluffier on the bottom than they should be.
- At some point the thermal will stop rising, and the Cu will 'die' at that point with the condensed vapour dissipating
and the cloud gradually disappearing. Soaring pilots aim to reach clouds while they are still 'active', but we have all
had the frustrating experience of circling under a great looking Cu and finding nothing, having timed our arrival perfectly for the
death of the thermal.
- The weight of the air rising in a thermal can be 100,000 to 1,000,000 TONNES.
- In Northern Europe, typical thermals are often 2..4 knots (1..2m/s). In Southern Europe or the USA you might typically see
3..5knots (1.5..2.5m/s). Places like Texas have much stronger thermals. HOWEVER, the entire art of competition soaring is to
find the STRONGEST thermals and only circle in those. E.g. in Germany a glider pilot will 'dolphin fly' through the 3..5knot
thermals (i.e. pull up in lift but keep flying on track) and stop and circle in the relatively rare 6+ knot (3m/s) thermals.
- As you lose height, you naturally get less picky about how strong the thermal has to be for you to stop and climb in
it. This has a big impact on your cross-country speed because the fact you're low means you have further to climb, but you may
have to accept a crap thermal, so your chances of winning the race are seriously diminished.
- Of course all days are different. A little bit of 'high cover' (e.g. some high Cirrus clouds) radically reduces the strength
of the Sun hitting the ground, and the thermal will be weaker. This is often only for part of a task and the technique is to
take as high a climb as you can while you're still in the good sunshine before you head off into the murk. Also the Cu's
may start the day looking fantastic (this is actually a warning sign) but over-convect so they join together and cut off the
life-giving sunshine on the ground that created the thermals in the first place. Eventually holes in the cloud layer will
appear (because the thermals have died) and glider pilots will look for those spots on sunshine on the ground as a likely
source of lift. A similar phenomenon is 'spread-out': great looking active Cu's grow rapidly, but the tops of the clouds
reach a warmer layer and spread out rather than continuing to rise. This higher 'spread-out' layer can join up, and
similarly cut off the sunshine to the ground.
- Within a given area on a given day, the stronger the thermals, the further apart they are likely to be. NE USA has great
soaring weather with stronger thermals the the UK, going much higher, but in the UK the weedy thermals are closer together.
- Sim soaring often has thermals considerably stronger than typical in RL, with the effect that every day is soaring heaven. IMHO
this is partly ignorance on the part of developers, partly giving sim pilots a more enjoyable experience. In my competition
soaring career it was the weak days that were the most interesting, and separated the pilots, so long as it was still possible to
make it around the task. Sim's have an advantage here in that you can test and confirm a task is possible, so a task can be a
challenge and still enjoyable - this opportunity seems underrated in sim soaring currently.
Current sim thermal status Aug 2021
MSFS essentially has no thermals, but the support for them seems to exist within the sim as modest lift can be detected in the
proximity of thunderstorms. The Kinetic Assistant add-on for MSFS
generates 'thermal lift' with a life-cycle that can be shared by multiple players but has no way of linking that lift to the MSFS
simulated Cumulus clouds.
X-Plane has built-in thermals but not currently related to cumulus clouds.
Condor 2 is the only flight sim with thermals with a life-cycle, linked to Cumulus clouds, shareable in multi-player.