Weather to Fly

Weather forecasting 101

By Honza Rejmanek
Originally published in USHPA Pilot, July/August 2019

To become proficient at soaring flight requires advancement on two fronts. The first and most obvious is the gradual yet constant improvement of piloting skills. The second is a relentless quest to better understand the dynamics of the invisible medium into which we choose to cast ourselves. This article is aimed to focus on the latter.

Having a basic understanding of meteorology is fundamental to the safe and enjoyable pursuit of soaring flight. As pilots, we start to develop a basic mental model of how the air behaves. We continually refine this mental model throughout our flying career. In practice, we usually arrive at launch with some knowledge of the weather forecast for the given area. To this forecast, we have to add our prior experience of what a similar forecast in the past actually meant for flying conditions. We then take a moment, before choosing to fly, to assess whether the conditions are similar to or different from what we had anticipated. Based on this assessment, and an honest evaluation of our pilot skills, we make a fly/no-fly decision. Finally, if we choose to launch, we quickly find out if our assessment of current conditions from the ground matches up with what we feel when we are in the air.

If conditions in the air are very different from what we inferred them to be while sitting on launch, then we need to refine our mental model of how the air behaves. The reason why we have to continually update and improve this mental model is that the relatively small, micrometeorological features that we encounter are not resolved on the relatively coarse grids of operational weather models. Much of what we fly in is considered sub-grid scale. Examples of these sub-grid, micrometeorological features are: turbulence, thermals, ridgetop compression of wind, valley winds, and small-scale venturi effects caused by acceleration through gaps. The takeaway message is that weather models see a much-smoothed-out version of the topography and the surface type over which we choose to fly.

However, weather models are increasingly accurate at predicting the arrival of large weather systems over our region. They are also good at forecasting the general, synoptic wind associ-ated with these systems. Generally speaking, high-pressure systems will have light synoptic winds. This is because the pressure gradient, which leads to wind, is weak in a high. Pressure gradients are depicted on a surface-pressure chart by isobars, contours of equal pressure values. A surface-pressure chart resembles a topographical map. Regions of high pressure have clear skies because air is sinking slowly overhead. Sinking air warms due to compression and clouds tend to dry out and dissipate. Conversely, a low-pressure system, with its associated fronts, will be windier. The pressure gradient is much stronger in a low. The isobars often resemble a bullseye. Clouds and rain are associated with a low-pressure system due to widespread lifting of air over the region. Rising air cools due to expansion, eventually cooling enough to reach its dew point. The level at which this occurs is the cloudbase. It should be noted that the widespread sinking rates in a high and lifting rates in a low are generally very slow compared to the sink rate of a soaring aircraft.

The synoptic wind that is forecast is an important piece of knowledge that a soaring pilot needs to keep in mind. If your flying site is strictly a ridge-soaring site then you want to know the forecast strength and direction of wind for that elevation and maybe one level above. If you plan on soaring up in thermals, it is imperative to be aware of the strength and direction of the synoptic wind at multiple levels. These levels range from the surface all the way up to several thousand feet above the highest level where you anticipate thermals to reach. This is because if strong wind exists overhead it can mix down on a day with good thermals.

As a novice pilot, it is particularly important to your safe progression to avoid flying in very windy situations. Strength of turbulence increases as a square of the wind speed. Turbulence produced by a 20mph wind will be four times as strong as it would be in a 10mph wind. This is why being aware of the forecast synoptic wind is very important. However, this is still not enough. As discussed, the models do not resolve the true topography. Wind can compress and accelerate over a ridge or through a saddle or gap. It can be blocked by topography or it can even stagnate on the upwind face of certain mountains. The wind does not see the terrain the same way we see it. It tries hard to smooth it out. It does so with a combination of stagnation zones and eddies that we often call rotors. Observing how water interacts with large boulders in a stream is a very useful exercise in starting to build a mental model of how wind interacts with terrain.

Visualizing wind already starts to get complicated if we just think of the forecast wind interacting with complex topography, but it actually gets far more intricate. This is because the wind that we feel at any particular spot is a superposition or summation of winds at different scales. At least three scales should be considered: local, regional, and synoptic. Local winds are upslope and up-valley winds that set up as a result of the sun heating the surface, which in turn heats the overlying air. The air above the surface heats up, decreases in density, and naturally begins to flow upslope, eventually peeling off skyward as a thermal. A thermal can best be thought of as an invisible smokestack that we can circle around in and gain elevation. This is the magic of soaring flight! If a thermal reaches high enough and cools enough for its air to reach dew point, it will suddenly become visible and manifest itself as a cumulus cloud.

Over higher terrain, once it is mostly snow-free, it becomes easier to heat up the air than out over the adjacent plains. A daily pressure drop of a few millibars will be common in the summer months over the mountains. This is an example of a regional heat-low. It sets up and dissipates on a daily cycle. In response to this pressure drop, a low-level flow develops from the plains to the mountains. This flow strengthens valley winds, especially in the large principal valleys. These valley winds can become strong enough in summer months that landing in a valley mid-day can be overwhelming. This is especially true if the valley narrows and this regional wind accelerates to speeds faster than the forward speed of our gliders. Weather forecasts will not resolve this wind! The models do not resolve mountain valleys.

Understanding of the daily cycle of local and regional winds can help a soaring pilot anticipate strong valley wind. In some mountainous areas in late spring and summer, a strong valley wind can start to set up as early as 10 or 11 a.m. and can blow past sunset. However, the launch site that might be several thousand feet above the valley will probably be above the depth of the valley wind. Under synoptically benign conditions, when a high pressure is centered over your area, there might be only nice thermal cycles flowing up the face of the mountain. This can lure the unaware pilot into the sky. There can be a strong, fast-moving river of air flowing through the landing area in the valley below.

As a novice pilot, it's important to understand that just because someone else is flying does not necessarily mean that conditions are appropriate for your skill level. For the first few years in your flying career, it is important to realize that it is very likely that you do not know what you do not know. Approaching soaring flight with this humble realization coupled with motivation to constantly better your piloting skills and continually refine your mental model of how the air behaves will gradually reward you with amazing cross-country flights.

For a soaring forecast, it is worth subscribing to XC Skies, whose goal is “to provide timely and highly useful soaring forecasts to allow pilots to make better decisions on when and where to fly.” Additionally, windy.com is a free service that allows access to two global models for comparing forecasts. Lastly, it is very useful to learn to read a balloon sounding called a Skew-T. This is especially true if there is a weather balloon launched near your flying site. There are many online tutorials on how to read such a sounding and on the very relevant topic of atmospheric stability.

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