Reading Your Vario: Climb and Sink as Numbers

Climb and sink are the same measurement with opposite signs, and both are just altitude change over time. Turning them into numbers you can compare between flights is straightforward, and the comparison is where the value is.

Sink

The sink rate calculator takes altitude lost and the time it took:

• 100 m in 90 s → 1.11 m/s (218.72 ft/min) — moderate
• 100 m in 60 s → 1.67 m/s (328.08 ft/min) — moderate
• 300 m in 60 s → 5.00 m/s (984.25 ft/min) — very strong
• 500 m in 60 s → 8.33 m/s (1,640.42 ft/min) — very strong

The first figure is roughly what an ordinary glider does in still air at trim. The last two are not still-air numbers at all — they are what happens in strong sink, or during a deliberate rapid-descent manoeuvre.

Climb

Same arithmetic, other direction. From the thermal lift calculator:

• +200 m in 120 s → 1.67 m/s (328.08 ft/min) — moderate
• +300 m in 120 s → 2.50 m/s (492.13 ft/min) — moderate
• +150 m in 180 s → 0.83 m/s (164.04 ft/min) — weak

The tool will also project a further climb at the same average rate: at 1.67 m/s, another 500 m would take 300 seconds — five minutes of continuous circling at that average. Seeing that written down is a useful corrective, because five minutes in a thermal is a long time and real thermals rarely hold a steady average for it.

Averages hide the thing you care about

Both numbers are averages over the period you measured, and thermals are not uniform. A climb averaging 1.67 m/s might be 4 m/s in the core and near zero at the edges, and the average tells you nothing about which part you spent your time in.

That is precisely why the projection above should be read as arithmetic rather than as a forecast. It answers "if this rate continued" — and the rate is the thing least likely to continue.

What the categories are for

The weak/moderate/strong labels are rough general context, and they are most useful as a shared vocabulary for comparing your own flights over time rather than as any kind of standard. Your "strong" and a pilot's in a different range on a different day are not the same thing.

The genuinely useful practice is logging: rate, time of day, site, month. Over a season that builds a picture of what your site actually does — when it switches on, when it peaks, how long the good band lasts — which is worth considerably more than any single number and cannot be got any other way.

The limits, which are the important part

Reading air is not a calculation. Thermal strength changes with time of day, terrain, ground cover and the state of the atmosphere; sink is often strongest right beside the lift; and a strong climb near cloudbase is a situation with its own hazards that no vario reading describes.

These tools turn logged numbers into comparable figures after the fact. They do not tell you where the lift is, whether to take it, or when to leave it — and the judgement involved in all three is taught through certified instruction and supervised practice rather than through arithmetic. Treat them as a way of understanding your own flying afterwards, which is a real and modest use.

Measuring cleanly

Both figures are only as good as the interval you measure over, and short intervals are noisy. A ten-second sample of a thermal tells you about ten seconds of air, not about the thermal.

For climbs, a full turn or several is a more meaningful unit than an arbitrary slice, because it averages across the core and the edges you actually flew through. For sink, a straight glide of a minute or more gives a figure worth comparing; anything shorter is dominated by whatever the air happened to be doing.

Altitude sources matter too. Barometric altitude drifts with pressure changes through the day, and GPS altitude is noisier vertically than horizontally. Neither is wrong; both are worth knowing about when two logs of the same flight disagree.

What a season of logging is worth

The single-flight numbers are mildly interesting. The accumulated record is genuinely useful, and it is the reason to bother.

Logged over months, climb rates by time of day and month describe when your site works. Sink rates on glides between thermals describe the air you routinely cross. Together they tell you what a realistic task looks like from your local hill — something no general advice can provide, because it is specific to that piece of ground.

The part the numbers do not touch

Knowing a thermal averaged 2.5 m/s tells you nothing about whether it was a sensible thermal to be in. Proximity to terrain, to other pilots, to cloudbase, and the state of the air around the lift are the questions that matter in the moment, and all of them are read visually and by feel.

Those are taught through certified instruction and supervised practice. The arithmetic here is for the debrief, and for building a picture of your site over time — both of which make you a better-informed pilot on the ground, which is where this kind of understanding belongs.