Micrometeorology (the weather at the site)

Air flows like water

Air goes over and around objects, always along the path of least resistance. Behind every object the flow breaks into eddies: turbulence.

How much turbulence you get depends on three things together: the size of the object, its shape, and the speed of the wind. Exam answer: all of the above.

• Shape: a smooth, rounded, streamlined shape (a gentle rounded hill) disturbs the air least. A sharp edge, a cliff, a building or a tree line disturbs it most.
• Size: bigger objects make bigger eddies that reach further downwind.
• Speed: turbulence (like drag) grows with the square of the wind speed. Double the wind → four times the turbulence.

Wind gradient

Near the ground, friction slows the wind. The wind speed increases with height in the first few tens of metres. This change of speed with height is the wind gradient.

• Stronger gradient in stronger wind.
• Stronger gradient over rough ground (trees, buildings, crops) than over smooth ground (water, short grass).
• Effect on you: descending into slower wind reduces your airspeed. Keep speed on final (Module 02). Climbing through it after launch increases airspeed and can make the wing surge.

Rotor and wind shadow

Rotor is rolling, turbulent air that forms where the wind separates from the terrain. Expect rotor:

1. Downwind (lee side) of a hill, ridge or cliff. The wind cannot follow the sudden drop, so it tumbles.
2. Upwind of a cliff, at its base. Wind hitting a vertical wall rolls back on itself in front of the wall.
3. Downwind of the top of a hill if the slope changes too quickly (a rounded top followed by a steep back side).
4. Behind any obstacle: trees, buildings, a row of vehicles on launch.

As the wind increases, the rotor becomes stronger and reaches further back from the obstacle. A rotor that was 100 m behind a ridge in light wind can be 500 m behind it in strong wind.

Wind shadow is the region directly behind an obstacle where the wind is blocked. The air there is calm, or gusty and unreliable. A landing field behind a line of tall trees is in wind shadow: you can lose your headwind (and your airspeed) suddenly.

Venturi

When wind is forced through a gap (a pass, a saddle between two hills, the space between two buildings) or over a ridge top, the air is squeezed and speeds up. This is the venturi effect.

What it does to you: the extra wind speed can stop your forward progress, blow you off course, trap you in or behind the gap, and make it impossible to fly back to clean air. A venturi over a ridge top is why the wind at launch can feel fine while the wind 50 m above is too strong to penetrate.

Reading the wind before launch

Natural wind indicators that show both strength and direction: smoke, water surface (ripples, wave lines), trees (leaves, branches, whole trunk), flags and streamers, grass and crops, dust, clouds (movement and shape), birds, and other gliders.

Signs of unstable or turbulent air before launch (name two for the P2): gusty wind; wind direction that keeps shifting; cumulus clouds growing; other gliders or birds flying unevenly, bouncing; thermals cycling through launch; dust devils.

Signs that launch is in lee-side (rotor) conditions: wind that rapidly and irregularly changes direction and strength, sometimes from the back. Compare: regular cycles of headwind then calm are thermal cycles, which are normal on a thermic day. A steady light wind from behind is simply a tailwind. Irregular, chaotic wind is the rotor signature.

Where the rotor is, the lift is not. If you drift downwind while thermalling above a mountain launch and the thermal drifts you behind the mountain, you are entering the lee side: expect major sink and rotor turbulence. Do not follow a thermal over the back unless you have the height and the plan to fly out the other side.

Thermals

How a thermal forms. The sun heats the ground. The ground heats the air touching it. The warm air becomes lighter than the air around it, breaks free and rises. Cooler air flows in below to replace it.

Where thermals come from (the best ground):

• Dry ground beats wet ground (water takes energy to evaporate instead of heating the air).
• Dark ground beats light ground (absorbs more sun). Rock, ploughed earth, tarmac, dark roofs.
• Sun-facing slopes: south-facing in Canada (and Nepal); east-facing in the morning, west-facing in the late afternoon because the sun moves.
• High altitude, dry ground heats fastest and has the best lift (thin air, strong sun).
• Rocky clearings on a sunny slope, sun-baked rock formations in the mountains.
• Poor thermal ground: forests, green crops, swamps, lakes, moist valleys, and the shaded slopes.

Blue thermals (thermals on a cloudless day): look for areas of marked temperature difference on the ground: the edge between a dark field and a light one, between dry rock and forest, between town and country. Contrast makes thermals break away.

Thermal structure. The core (centre) rises faster than the thermal as a whole. Near the edges the lift is weaker and there may be sink around the outside. In a constant lapse rate, a rising thermal mixes with the air around it: it grows in volume and weakens in strength as it goes up.

How thermals change the weather at launch: variable gusts, shifts in wind direction, changes in air temperature, and building cumulus clouds that can over-develop.

Coring (P4). Your vario climbs to a peak, then falls to weaker lift: you flew through the best part. To move toward the core, momentarily flatten (decrease) your bank just before the peak, then resume your bank. Flatten toward the good air, tighten in the weak air.

Thermals with clouds. A cumulus that is still making lift has a clean-cut, flat base, is growing in volume, and often has a wider base than top (triangular, like a cauliflower on a flat plate). A cloud that is dissolving has a ragged base and wispy edges. Under a cumulus the best lift is under the windward (upwind) side, because the thermal feeding the cloud leans downwind as it rises, so it enters the cloud from the upwind side.

When a cloud forms, it releases heat. Condensation gives back the energy used to evaporate the water. The cloud generates heat, so the air inside it becomes even more buoyant. This is why lift under a cloud can increase ("cloud suck") and why you must leave before you are pulled in.

Inversions

An inversion is a layer where temperature increases with height (normally it decreases). Cold, heavy air sits under warm air. An inversion is stable: rising air cannot punch through it.

• Nocturnal inversion: the ground cools at night and the air near it becomes cold. Next morning the air is smooth (good for a sled ride, no thermals). Convection begins only when the sun has heated the ground enough to break the inversion.
• A thick overnight inversion delays the start of thermals: triggering may happen only in the late afternoon, or not at all.
• A well-established inversion the night before → next morning's flights will be in smooth air.
• An inversion during the day caps the thermals: cloud base or the thermal tops sit at the inversion. A haze layer often marks it.

Valley winds and slope winds

• Anabatic wind: upslope, during the day, as the sun heats the slopes and the warm air climbs them.
• Katabatic wind: downslope, at night and in the evening, as the slopes cool and heavy cool air slides down.
• Valley wind: up-valley during the day (air flows from the plains into the warming mountains), down-valley at night. In a big valley the afternoon valley wind can be strong and is the main wind you fly in.
• Landing at the valley mouth at day's end: expect the wind to be flowing down the valley; land facing up-valley.
• Late afternoon, calm wind, in the mountains: hunt thermals on the west-facing slopes (the sun is in the west).

Sea breeze, lake breeze and convergence

Sea breeze: during the daytime, land heats faster than water, air rises over the land, and cooler air flows in from the sea onto the land. At night the flow reverses (land breeze). A lake breeze works the same way beside a large lake. Sea breezes are often smooth and steady but can bring a wind shift, a low haze layer and a sea-breeze front.

Convergence: where two air masses or two winds meet, the air has nowhere to go but up. A line of lift forms along the meeting line, often marked by a line of cumulus. Examples: sea breeze meeting the valley wind, two valley winds meeting, wind splitting around a mountain and rejoining behind it.

Ridge (dynamic) lift

Wind hitting a slope is turned upward. The lift is strongest when the wind hits the slope squarely (perpendicular) and when the slope is steep: a cliff turns almost all the wind upward and gives the most lift for a given wind speed; a long shallow slope gives the least.

Holding a course parallel to the ridge in a crosswind means flying at a crab angle (nose pointed partly into the wind). If the wind increases, you can keep the same track either by flying faster at the same crab angle or by increasing the crab angle at the same airspeed. Exam answer: either.

Where the wind is too strong for ridge soaring you get pushed back over the ridge into the rotor. Where it is too light, only the steepest part of the slope works.