Altair Glide Computer and Navigation System User Manual download pdf (Page 92)

XCSoar User Manual

if you leave a thermal at the top and later return at low altitude, the

position on the map shows the predicted location of the thermal at

that low altitude (which is further upwind than the top).

If the wind changes and the thermal source is still active, its po-

sition on the map reﬂects the wind change; that is, the thermal at

altitude will be projected downwind at the new wind estimate.

6.8 Convection forecast

If the glider is equipped with an outside temperature and humidity

probe, a simple convection forecast system estimates the convec-

tion ceiling and the cloud base. The humidity probe is optional

and is mainly required for estimating cloud base.

Prior to takeoff or during ﬂight the pilot can modify the maximum

forecast temperature on the ground, by adjusting the value in the

“Forecast Temperature” InfoBox.

The forecast convection ceiling is determined by the altitude at

which the atmospheric temperature equals the maximum forecast

temperature on the ground, cooled adiabatically as it rises accord-

ing to the dry adiabatic lapse rate. Typically the glider will not

climb as far as the convection ceiling and so the measured values

are extrapolated to ﬁnd the ceiling. If the atmosphere is stable, the

convection ceiling is reported as zero altitude.

The maximum forecast temperature on the ground is entered using

the basic settings dialog described in Section 5.4.

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Altair Glide Computer and Navigation System User Manual Page 92