Weather knowledge part I: Weather maps & weather models

"Sometimes when I see the weather',
I get violent fantasies, and the weather fairy
would' be the first victim of my aggression,
although I know: What's the point,
if you take her at her word and force her
to jump into the North Sea in a bikini."
- Wise Guys, Jetzt ist Sommer Everyone has probably experienced for themselves that you can't always believe the weather forecast. You slide around on icy slopes with reverse camber skis because the forecast for fresh snow doesn't materialize, or you sink with slalom skis when it suddenly snows a lot more than predicted. In order to explain incorrect forecasts, or even to get an idea of the weather for your next trip to the snow, it is useful to know at least the basics of how forecasts are made.weather forecasts are based on data spit out by numerical models. The TV weather fairy presents this data to us in easily digestible morsels, as small sun or cloud icons in front of a blue box. Between the weather report and the raw data from the models are the weather maps: the results of the model calculations are displayed graphically, broken down into various meteorological parameters. A large number of such maps are freely available online and allow even frustrated freeriders to put together their own weather report.

Overview of the general weather situation

To get an initial overview of the general weather situation, it is best to consult geopotential maps for high levels. Here, the height of a certain pressure surface, for example 500hPa, is shown with isolines. In areas with low geopotential, the pressure is also low - low-pressure areas and high-pressure centers are clearly visible, large-scale flow patterns can be seen and the weather pattern for the next few days can be estimated. The map below shows the European section of the American GFS model. The heights of the 500hPa pressure surface are plotted in black - the geopotential. The ground pressure is also shown in white. The color scale provides further rough indications: Blue/purple - low pressure, yellow/orange - high pressure.

Troughs and wedges

Once we have identified troughs and wedges (areas of low and high pressure), humidity maps can tell us something about how important they are for our weather patterns. Roughly speaking, more moisture in a system also means more precipitation. The position of individual fronts can be determined in the humidity, but this takes some practice. Ground maps, such as those from the German Weather Service or the UKMO, in which the fronts are already marked, can help with orientation.

Equivalent potential temperature / theta-e values

Another useful parameter is the equivalent potential temperature, or the so-called theta-e values. These describe the temperature and humidity in an air mass and the corresponding maps can usually be interpreted quite intuitively depending on the color scheme. Low equivalent potential temperatures (blue, purple) indicate cold air and give an indication of its moisture content. Air mass boundaries and associated weather changes and fronts are also easy to recognize. Perhaps most interesting for snow enthusiasts is that the snow line can often be better estimated on the basis of the theta-e values than with pure temperature data, in which the humidity is not taken into account. At a theta-e temperature of 12 degrees it snows at sea level, at 24 degrees at a thousand meters, at 36 degrees at 2000 meters and so on. As an example, here is the GFS Europe section again, the color scale shows us the equivalent potential temperature in degrees Celsius, the white lines the ground pressure.

Precipitation maps

Finally, of course, we have precipitation maps. These always refer to a previous period, usually as a total over 6 hours. Precipitation is generally difficult to predict as it is caused by non-linear processes and can only be poorly parameterized. Large uncertainties arise, especially in the mountains, due to the poor topographic resolution of most models. The Alps are usually approximated as a uniform mountain range that reaches just under 2000 meters and has no valleys, passes or foothills. Only the best, smallest-scale models take rudimentary terrain structures such as the Brenner Pass into account. Precipitation patterns in particular are strongly influenced by the local topography and spatially very limited processes, which is why automatically generated forecasts are often significantly off the mark in terms of precipitation amounts and duration. A good forecaster is familiar with the local conditions and knows how the various models work in detail. They can assess when, for example, unresolved congestion effects or slope wind systems influence the weather and how, and apply appropriate corrections to the model data. To do this quantitatively and accurately requires experience and meteorological knowledge that goes far beyond the areas discussed here. But if you look carefully out of the window and occasionally take a look at a map or two, you will quickly notice when it tends to snow a lot in your area and when it tends to stay dry. This way, you are no longer at the mercy of the weather forecast and can adjust your ski choice accordingly.