06 Dec 2023

Weather forecast in gliding: interview with legendary met man Hermann Trimmel

Austrian meteorologist Hermann Trimmel is a renowned figure in the gliding community. With over 50 years of experience, he has made an exceptional contribution to the promotion of the sport, which earned him the prestigious FAI Gold Air Medal in 2018.

Trimmel has held various positions in championships at international and national level, serving as a Met Director and task setter. He represented Austria in the International Scientific and Technical Soaring Organisation (OSTIV) and served on the board for over three decades. Additionally, he faithfully represented the Austrian Aeroclub within the FAI for around ten years. In addition to his achievements as a pilot, which includes winning titles at multiple national championships, Trimmel has excelled as a trainer and mentor.

Although he stepped down from his official positions a few years ago, Trimmel continues to be highly respected for his extensive expertise and knowledge. We had the privilege of interviewing him to gain insights into the evolution of weather forecasting and the role of met men over the years, as well as what the future holds for the gliding community in this field.

When did you begin working as a meteorologist, and what kind of weather forecasting was available then for gliding? What type of information did you receive?

I started to work as an aviation meteorologist at the airport of Vienna in 1973. At this time, we had neither computer models nor satellite pictures. Fortunately, a good network of weather observations and radiosondes was already in place. So, our forecast was based on surface and upper air charts in different levels, which we had to first draw by hand. Leveraging this information, we could start analysing and developing a comprehensive synoptic picture of the current weather situation and the expected development. Due to the lack of computers, we had to shift frontal systems kinematical and for forecasting thermal conditions we used tables of solar energy. Although very simple, sometimes it worked surprisingly well and forced us to think rather than to interpret ready-made calculations.

How has weather forecasting for gliding competitions evolved over time?

I remember the World Gliding Championships in 1978 in Chateauroux, there were 8-10 meteorologists from different countries to support their teams. We exchanged our knowledge very openly, and many friendships were formed. This also led to the birth of the OSTIV MET-Panel. In this pioneering group the “Handbook of Meteorological Forecasting for Soaring Flight” was developed, drawing on the expertise of 20 experts from all over the world. This compendium was finally published by the World Meteorological Organization (WMO)  in 1993 as Technical Note No 158, and updated in 2009 as Technical Note No 203. During this period the MET-Panel held annual meetings. Based on this knowledge the forecasting tools continuously improved, evolving the original subjective and manual interpretation of the weather to sophisticated computer models running on the world’s most powerful supercomputers.

In today's world, with a multitude of sources providing weather information to both the public and professionals, how has the role of a gliding meteorologist changed?

The role of gliding meteorologists has changed significantly. Nowadays a meteorologist on site of a competition cannot beat the model-output in time and accuracy. Nevertheless, given the range of models available, putting the findings into the right context is still of value. However, much can be done glider pilots themselves and in today’s competitions tactical decisions often play a bigger role than meteorological decisions.

In today’s world, I note that the main task for a gliding meteorologist is in educating and training pilots, to understand the meteorological processes on a macro and meso scale to comprehend the limits of model forecasts.

Currently, pilots can access forecasts on their onboard flight computers. With so many weather forecast sites and services available to everyone, what is the current and future role of a competition met man in gliding?

Supporting the task setter and the competition director not only in optimising the daily tasks but also assisting in identifying and avoiding hazardous situations. This still plays an important role in today’s competitions.

On a team level however, I believe that the days of the met man is over. Pilots must take decisions at a scale of minutes which can best be supported by satellite and real-time radar pictures which are nowadays accessible on board.

Looking ahead, what advancements can we expect in weather forecasting? We can already predict mountain waves using flight computers and laptops. Will we be able to do the same for thermals in the future?

The main question is what resolution in time and space do we expect? We must keep the accuracy of the model outputs in mind. Sometimes they suggest a level of precision and detail, which looks compelling but in some cases is not valid.  By nature, you cannot get out more than you put in. While the speed and capacity of computers will evolve further, it is everything but certain that the granularity of inputs evolves at the same speed. A reduction of factor two in time and space requires a computing power of factor 16 (exponent 4). This seems to be possible, but will the measuring network also keep up with the change? This doesn’t seem very realistic, but what we have learnt in the past is that you can never be sure. Lidar systems installed in a glider could for instance allow to “see” thermals ahead.

While general thermal forecast on a larger scale will evolve in terms of reliability, I cannot imagine forecasts of precise positions, time, and strengths of thermal lifts. My belief is that as nature isso complex, gliding and in particular actual vs. expected conditions will continue to surprise pilots for the generations ahead…

In days gone by when wooden gliders were prevalent, pilots would often ask if there would be enough thermals. Nowadays, they are more concerned about whether thermals will occur frequently enough. With modern gliders designed for high speeds and glide ratios, has the focus on forecasts shifted?

To efficiently use “energy-lines” the identification of e.g. convergence lines which are already embedded in some models will increasingly be of great help. Also, products highlighting favourable conditions for thermal lines, whether in blue conditions or when marked with clouds are available today and will continue to evolve.

How do different competition sites vary when it comes to weather forecasting?

While the basic processes governed by the laws of physics are the same everywhere, each site and its surrounding areas come with particularities. Different topographies e.g. mountains vs. flat lands, available radiation energy depending on latitude and time of year, continental vs. maritime airmasses as well as surface conditions ranging from dry to wet and different vegetation all influence the local weather conditions. While at some sites the forecasting can be quite reliable and straightforward, in other locations in particular when mountainous terrain and different airmasses are involved, even the most powerful models of today can reach their limits. This however does apply more to the reliability of multiday outlooks and less to actual competition days which typically only span a couple of hours.

As an experienced weather forecaster for gliding expeditions worldwide, you had the task of predicting weather conditions in remote locations. How do you go about forecasting weather in distant areas?

I do not like forecasting weather from remote without actual knowledge of the respective area. In the time when I assisted the Austrian team in World or European Gliding Championships, I always tried to explore the competition area by glider ahead of the actual competition to familiarise myself with the actual local characteristics and particularities. While computer models are incredibly helpful, for me, forecasting is still a combination of art and science. Building a mental model of how the weather in a particular area will develop over the course of a day and experiencing it as if sitting in the cockpit of an actual glider is super important for me to develop my forecasts. Despite all the advances in technology, this part hasn’t changed over all the years.

Hermann Trimmel with his wife Traude