If you fly over mountainous or rough terrain you'll see cumulus clouds and thunderstorms popping up in the same place day after day, on top of the ridgeline. Why is this?

Inbound solar rays are absorbed by the Earth's surface, which heats the atmosphere during the day and cools it at night. Generally, winds move upslope starting in the late morning hours, until the flow reverses in the late evening, when sunset stops inbound solar radiation.

Increased Surface Area Over Mountains

Mountain slopes have a greater surface area than a flat plain. With this increased surface area, stronger thermals are created on mountain slopes. These thermals move upslope. When this upslope flow meets at the ridgeline or peak, it converges and moves straight up, causing the mid-day cumulus buildups pilots often see.

To form a thunderstorm you need moisture, instability, and lifting action. This upslope flow convergence creates a 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) to that above a plain in Omaha, Nebraska (OAX). You'd expect to encounter rising thermals in both locations, but we only see convective formations over the ridgeline in ABQ.

Looking at radar imagery the pop-up thunderstorms follow the topography of the Sacramento mountain range, this is not a coincidence, it's upslope flow convergence in action.

Now let's confirm that what you are seeing is actually taking place in real time using PIREPs.

The first PIREP shows turbulence in the flight levels, reported by an Embraer 175, a regional jet. It also indicates that cloud tops are reaching FL350, supporting the convective activity depicted on the radar. The second PIREP relays information about low level windshear.

With this supporting information you might decide to alter your routing around the problematic area.

Flight Planning

By understanding the patterns of mountain weather and the mechanisms behind them, you can implement your knowledge during flight planning.

Identifying the ingredients of convective formation, and avoiding the area altogether will result in less turbulence, vertical shear, and overall workload on you.

Consider alternative routing around a mountain range if you see a combination of these 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 an option, consider launching for your flight early in the morning or later in the evening, when convective activity has subsided. This decision should also consider the risk of an engine failure, and forced off-field landing at night in mountainous terrain.

Advanced Scenarios

While the focus of this case study is upslope flow and convergence, our Mountain Weather Course dives into deeper detail with over 14 hours of video content including scenario based case studies, advanced weather topics, and genuinely useful insights for your next flight.

Building on the basics of ridgeline convergence and diurnal wind systems, there are other factors that you can look out for, that affect the strength of thermal systems.

• Oceans and Deserts
• Strong Winds Aloft

Oceans and deserts with close proximity to mountain ranges can provide steeper humidity differences in the collision of the rising upwind and downwind mountain slope air masses, resulting in a more violent ride. Click here to learn about what happens when moist and dry air masses collide.

Just like rough terrain, oceans and deserts can develop distinct diurnal (daily) wind patterns, which create on-shore and off-shore winds as air rises and sinks.

During the day the winds moving towards the top of the mountain ridge help not only generate lifting action and instability, they also prevent the storm from moving significantly. This can be disrupted by frontal systems, strengthening winds aloft, or evening subsidence. Subsidence is the technical term for the outward settling of air as the atmosphere cools after sunset.

Make better decisions flying around terrain this winter.

It's easy to think that mountain weather only happens in places like the Rockies. But the hills of Eastern Ohio can produce the same types of weather year-round. If you've ever flown near the Appalachians, you probably experienced mountain weather, even if you didn't realize it was happening.

Whether you're flying on the East Coast, the Coastal Ranges of California, or any of the rough terrain in between, Boldmethod's Mountain Weather course makes you confident and comfortable flying around the mountains.

You'll learn how to evaluate mountain weather during your planning and while you're in flight. You'll also learn how terrain generates updrafts, downdrafts, turbulence, and storms, and changes the direction of the wind throughout the day.

Plus, for less than the cost of a cross-country flight, you get lifetime access to tools that increase your confidence and make your flights more fun.

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