What is a thermal? The three criteria that define one in a flight log

In the air a thermal is the thing pushing you up, the thing you circle to stay in. In a flight log there are only rows of latitude, longitude, altitude and time. For a program to say "there is a thermal here" it needs a measurable definition drawn from those rows. Variograph uses three criteria.

Criterion 1: Are you climbing?

A vertical speed is computed for every track point: the altitude difference between two points divided by the time between them. A point is a thermal candidate if its vertical speed is at least 0.5 m/s.

The threshold is deliberately low. Staying in a weak thermal at 0.7 m/s is a decision too; with a 1 m/s threshold that decision would vanish from the record.

Criterion 2: Are you turning?

Climbing alone is not enough. You also climb flying straight along a ridge, and that is not a thermal. The second criterion is turn rate: the change of heading between consecutive points, divided by the seconds between them. A point is a candidate if it is turning at at least 5 degrees per second.

A full circle is 360 degrees; 5 degrees per second means completing one in about 72 seconds. Real thermal turns take 15–25 seconds, so the threshold sits well below them. It is there to catch not turning, not to grade the turn.

Criterion 3: Did it last?

Points that meet both criteria at once are collected into a segment. The segment is recorded as a thermal only if it lasted at least 10 seconds and gained a net 10 metres from start to end.

Without this third criterion a single gust would count as a thermal. Duration and gain together are the measurable form of "something really happened".

How a segment is cut

Not every point inside a thermal is perfect. On the downwind side of a circle you sink for a few seconds; that does not mean the thermal ended. Variograph tolerates an interruption of about 3 seconds, scaled to the recording interval. A longer break closes the segment, and the next climb becomes a new thermal. A gap in the recording splits the segment as well.

If a flight yields no thermal under the three criteria, a second, relaxed pass runs: the vertical speed threshold drops to 0.3 m/s, the turning requirement is removed, duration becomes 5 seconds and gain 4 metres. Ridge flights with no circling then show something instead of an empty table.

What you see in the log

The thermals table on the advanced analysis page lists, for each segment, entry and exit time, entry and exit altitude, net gain, duration, average and peak climb rate, and average turn radius. The radius is derived from the total heading change in the segment and the average ground speed: tight circles give a small radius, wide ones a large one.

All four thresholds can be changed in that page's settings. Lowering one finds more thermals, raising it fewer; which is right depends on the question you are asking.

Limits

The measurement is only as good as the recording interval. At 5-second logging the turn rate gets coarse and short thermals disappear; see IGC data quality. The measured climb is yours, not the air's; for what the air actually did you need net vario. And the drift of the thermal carries information of its own: the wind profile.