If you soar over mountainous or rough terrains, you’ll frequently notice cumulus clouds and thunderstorms popping up in the same exact spots day after day, right atop the ridgeline. Now, why is this happening?
Inbound solar rays are absorbed by the Earth’s surface, which heats the atmosphere during the day and cools it at night. Generally speaking, winds start to ascend the slopes in the late morning hours and reverse their flow in the late evening when sunset ceases the incoming solar radiation.
Increased Surface Area of Mountains
The slopes of mountains have a greater surface area compared to a flat plain. With this enhanced surface area, stronger thermals are created on mountain slopes. These thermals ascend the slopes. When this upslope flow meets at the ridgeline or peak, it converges and shoots straight up, giving rise to the midday cumulus buildups that pilots often observe.
To form a thunderstorm, you need moisture, instability, and lifting action. This upslope flow convergence provides the necessary lifting action that fuels the development of a thunderstorm.
Case Study: August 17th, 2024
Let’s compare the strength of thermal instability above a mountain ridgeline in Albuquerque (ABQ) with that above a plain in Omaha, Nebraska (OAX). You would expect to encounter rising thermals in both locations, but we only see convective formations above the ridgeline in ABQ.
Examining the radar imagery reveals that the pop-up thunderstorms follow the topography of the Sacramento mountain range, and this is no coincidence; it’s the action of upslope flow convergence at work.
ForeFlight
Now let’s confirm that what you are witnessing is indeed taking place in real time using PIREPs.
The first PIREP reports turbulence at flight levels, as experienced by an Embraer 175, a regional jet. It also indicates that the cloud tops reach FL350, supporting the convective activity depicted on the radar. The second PIREP relays information about low-level wind shear.
With this supporting data, you might decide to modify your routing around the problematic area.
Flight Planning
By understanding the patterns of mountain weather and the mechanisms behind them, you can incorporate this knowledge into your flight planning.
Identifying the elements that contribute to convective formation and avoiding the affected area altogether will result in less turbulence, vertical shear, and a reduced workload for you.
Consider alternative routing around a mountain range if you notice the following combination of factors:
- Clear (or mostly clear) daytime conditions.
- Weak winds aloft, up to 10,000 feet above the ridge line.
- Infrared imagery of stationary clouds over ridgelines.
- Stationary and intensifying radar returns over mountain ridgelines.
If alternative routing is not feasible, think about taking off for your flight early in the morning or later in the evening when the convective activity has subsided. This decision should also consider the risk of an engine failure and the potential for a forced off-field landing at night in mountainous terrain.
Advanced Scenarios
While the focus of this case study is on upslope flow and convergence, our Mountain Weather Course delves deeper with over 14 hours of video content, including scenario-based case studies, advanced weather topics, and truly valuable insights for your next flight.
Building on the fundamentals of ridgeline convergence and diurnal wind systems, there are other factors to look out for that can influence the strength of thermal systems.
- Oceans and Deserts
- Strong Winds Aloft
Oceans and deserts that are close to mountain ranges can create steeper humidity differences when the rising air masses from the upwind and downwind mountain slopes collide, resulting in a more intense experience. Click here to learn about what happens when moist and dry air masses meet.
Just like rough terrains, oceans and deserts can develop distinct diurnal (daily) wind patterns, which give rise to onshore and offshore winds as the air rises and sinks.
During the day, the winds moving towards the top of the mountain ridge not only help generate lifting action and instability but also prevent the storm from moving significantly. However, this can be disrupted by frontal systems, strengthening winds aloft, or evening subsidence. Subsidence refers to the outward settling of air as the atmosphere cools after sunset.
Fly with More Confidence Around Terrain This Summer
It’s easy to assume that mountain weather only occurs in places like the Rockies. But the hills of Eastern Ohio can also produce similar weather throughout the year. If you’ve ever flown near the Appalachians, you likely experienced mountain weather, even if you didn’t realize it at the time.
Whether you’re flying along the East Coast, the Coastal Ranges of California, or any of the rough terrains in between, our Mountain Weather course will make you feel confident and at ease when flying around the mountains.
You’ll learn how to assess mountain weather during your planning and while in flight. You’ll also discover how terrain generates updrafts, downdrafts, turbulence, and storms, and how the wind direction changes throughout the day.
Plus, for less than the cost of a cross-country flight, you gain lifetime access to tools that boost your confidence and make your flights more enjoyable.
Are you ready to get started? Click here to purchase Mountain Weather now.
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