XC Therm visualizes the exact same data as Toptask, flugwetter.de, and Alptherm. This data is produced by Deutscher Wetterdienst (DWD) using the Regtherm model.
The various offers differ in the number of available regions and their visualization. Toptask and XC Therm offer forecasts for the next 5 days and for all the 289 regions. While Toptask is designed to be used only on a PC and is aimed at glider pilots, XC Therm offers a smartphone-compatible visualization that addresses mainly paragliding, hang gliding, and rigid wing pilots.
The smaller the grid size of a numerical model, the more computing power is necessary for its calculation. Therefore, the forecast range is shorter for models with a small grid size.
For a 5-day forecast as is offered by XC Therm, ICON-EU with a grid size of 7km is the model with the smallest grid size that still covers Europe for the next 5 days. Models with a 2.2km grid (COSMO-D2, ICON-D2) are either only computed up to 47h in advance, or they only cover small parts, such as COMSO-2E from MeteoSwiss, which covers 5 days in advance, but only for the Alps.
Comparisons have shown however that there are no significant differences in the quality of the thermal forecasts computed by Regtherm between using ICON-EU or COSMO-D2 as input model.
The following table shows the update times for the thermal forecasts. Because the computation of Regtherm takes a few minutes, the forecasts for the following days are updated with a few minutes delay.
Usually dew forms at night on the ground because excessive humidity condensates due to the cooling-down. This humidity cannot be detected by monitoring stations that are located at 2m above ground and hence is not included in the midnight run of ICON-EU. Therefore, the Regtherm forecasts at 3:30 UTC tend to be too dry and hence the predicted cloud base is often too high and the cloud size too small.
The dew on the ground starts to evaporate with the insolation after sunrise. Once evaporated, this humidity can be detected by monitoring stations again. This is the reason why Regtherm computes two additional runs at 5:30 and 6:30 UTC using the values from the midnight run of ICON-EU that have been corrected by the live measurements.
Numerical weather forecast models are rather generic models that are designed to cover a broad range of applications. Many of the parameters that are essential for a high-quality thermal forecast are not considered by such models, mainly for capacity reasons.
The Regtherm model on the other hand is a specialized thermal forecast. However, Regtherm is not a standalone weather model, but builds on top of numerical models. By adding thermal-specific parameters to these models, Regtherm computes a thermal forecast optimized for gliders, paragliders, hang gliders and rigid wings. This kind of model is called a post processing model.
More technical details about Regtherm are available here.
The primary focus of XC Therm and the underlying Regtherm model is to provide a forecast for thermal quality. The output of the Regtherm model contains some wind values, but since only wind values from one grid point of the ICON-Model are available per region , this is by far not sufficient for flight planning in mountainous regions.
Example: The region named Berner Oberland stretches from Rochers-de-Naye at Lake Geneva to Pilatus in Central Switzerland, which corresponds to a distance of 120 km. The grid point, from which the wind values are taken, lies approximately in the middle, at the southern Lake of Thun. While the regions were carefully selected such that the thermal conditions are homogenous, the wind values within this region can differ significantly.
For that reason, we strongly recommend numerical models with a small grid size (e.g. COSMO-2 or even COSMO-1 where available) for detailed wind forecasts in mountainous regions.