Local winds

The forecast promised light winds all day. Yet on the training hill the windsock hangs limp at breakfast, stands up the slope by late morning, and in the evening flops round to point downhill. Nothing is wrong with the forecast. You are watching local winds: breezes the landscape makes for itself as the sun warms it and the night cools it.

Local winds decide a lot of training days. They can make a calm slope soarable, bring a tailwind to launch in the evening, or push a gusty sea breeze across your landing field (LZ). Learn the patterns and you will see the changes coming.

Why the landscape makes its own wind

Slope, valley and sea breezes all start with uneven heating. On a sunny day rock, dry ground and bare slopes warm quickly, while water and forest warm slowly¹. Air touching a warm surface warms too, becomes lighter and rises. At night the ground cools, the air against it becomes denser, and it sinks and drains downhill. The landscape breathes in by day and out at night.

These breezes are usually weaker than the large-scale wind, so a strong gradient wind swamps them; on their own, though, they can still exceed your limits. You see them most clearly on settled days with a weak gradient wind, typically under high pressure and clear skies². Foehn, at the end of this lesson, is different: it is strong wind forced over a mountain range.

Slope winds: up by day, down at night

On a sunny, calm day the air over sun-warmed slopes rises along them as an upslope or anabatic wind. It typically blows at about 11–18 km/h7–11 mph, it is hundreds of metres deep, and it sometimes builds cumulus just above the ridge tops⁵.

At night the slopes cool and the air against them drains downhill as a downslope or katabatic wind. It can be just as fast, typically 11–29 km/h7–18 mph, but it is only 10–100 m35–330 ft deep and smoother than the daytime upslope flow⁵.

The switch follows the sun, not the clock. When the sun is low, the slope facing it can still have upslope wind while the slope across the valley, already in shade, has turned to downslope⁵. Watch the shadow line creep across the hillside in the evening: it shows you where the wind is about to change.

Two things follow for you. A sunny slope can feel soarable on an otherwise calm morning, but that breeze is thermal and arrives in cycles. And when the sun leaves the slope, the upslope breeze fades and the drainage flow can arrive from behind you on launch: a tailwind for any launch down that slope.

Valley winds: the daily cycle

A mountain valley adds a second, larger circulation. By day the slopes draw air upwards and the valley replaces it with an up-valley wind, the valley wind, blowing in from the plains. At night the cold air that drains off the slopes pools on the valley floor and flows out down the valley as the mountain wind⁶.

The changeovers are where the surprises happen. In the evening the cool drainage flow meeting the warmer valley air can create a weak convergence that sets off the last thermal of the day⁶. Where the air squeezes through a gap or a narrow pass, it speeds up, and the edges of the jet can be turbulent⁷.

By the afternoon the valley wind can blow hard across the landing field, so ask local pilots how strong it gets and when. When each wind starts and stops is the part no book can give you. It depends on the season, the size and direction of the valley and the day’s weather, and there is no simple clock rule. Your site briefing and local pilots are the best guide.

Figure: The daily wind cycle in a mountain valley, looking up the valley. Choose a time of day and watch the slope winds and the wind along the valley; notice how the sunny and shady slopes disagree when the sun is low, and how a strong gradient wind wipes the pattern out. Real timings vary from valley to valley.

Sea breezes

Near the coast, the same uneven heating works on a bigger scale. On a fairly calm, sunny day the land warms faster than the sea, and once the land is about five degrees Celsius (nine Fahrenheit) or more warmer, cool air from over the sea starts to flow onshore: a sea breeze⁸.

A fully developed sea breeze blows at anything from 4–36 km/h2–22 mph at the coast, typically about 22 km/h14 mph, already above most student limits, in a layer 500–1,000 m1,650–3,300 ft deep⁸. It starts near the coast in the morning and pushes inland through the day, typically 20–60 km12–37 mi by evening, with extremes anywhere from 10 km6.2 mi to 200 km124 mi. Towards the end of the day it dies away, and at night a weaker offshore land breeze often takes its place⁸.

The leading edge of the sea air, the sea-breeze front, behaves like a small cold front. It brings a wind shift, often marked by a line of building cumulus or a curtain of cloud over strong lift. The thermals along it are turbulent, in moist unstable air it can set off showers and even thunderstorms, and once it has passed, the cool sea air can end thermal flying for the day⁹.

Figure: A sea-breeze cell on a sunny afternoon. Cool sea air flows onshore near the surface and lifts the warmer land air at the front, where a line of cumulus often forms. Notice that the thermals stop behind the front. Not to scale.

The gradient wind changes the picture. A weak onshore gradient wind helps the front push inland faster, a strong onshore wind can stop a separate sea breeze forming at all, and a moderate offshore wind usually holds it near the coast. Because the sea breeze and the gradient wind add together, the wind at a coastal site can strengthen or swing round through the day¹⁰.

If you train near the coast, ask how the sea breeze behaves at your site: a smooth morning breeze can build beyond your limits by the afternoon.

Foehn: warm, dry and dangerous

When a strong wind blows across a mountain range, the air that pours down the far side can arrive warm, dry and often at gale force. This is foehn, and North America’s chinook is the same effect¹¹.

On the lee side, foehn has a recognisable face¹¹:

• a bank of cloud sitting on the crest, the foehn wall;
• an unusually clear, cloud-free patch of sky above you, the foehn window, with excellent visibility;
• lens-shaped lenticular clouds, sometimes called foehn fish;
• lee waves with rotor beneath them.

In the Alps a south foehn typically arrives with a slow-moving low-pressure system over the Bay of Biscay or the British Isles, and at Altdorf, a classic Swiss foehn valley, the strongest winds of the year reach 100–140 km/h62–87 mph¹¹. USHPA’s Novice (P2) syllabus lists lenticular clouds among its weather danger signs¹².

For a student the rule is simple: any sign of foehn means no flying. It brings exactly the strong, gusty, turbulent air that student limits exist to keep you out of¹³. A foehn forecast is a reason not to fly that day. A forecast of no foehn is no guarantee either: on site, any sign of foehn still means no flying, however calm launch feels¹⁴⁴.

When the gradient wind takes over

The breezes above need a fairly calm day. Once the gradient wind is strong, it overrides them²; foehn is the exception, because it is the strong wind. The gentle upslope breeze vanishes under a wind that blows wherever the weather system sends it, and the textbook valley cycle may not show at all.

Two habits keep you out of trouble. First, check the forecast wind at launch height and above, not just at the surface (forecasts and the go/no-go decision). Second, keep re-checking through the day: local winds change with the sun, and the wind that suited you in the morning may not suit you after lunch.