General glossary of mountain weather

30/30 rule: At 30 seconds or less between flash and thunder, take shelter; wait 30 minutes after the last thunder before setting off again. (ch. 9)
Adiabatic: Describes a transformation without any exchange of heat with the outside. A rising bubble cools adiabatically, simply by expanding. (ch. 1)
Air mass: A very large volume of air with almost uniform characteristics (temperature, humidity), acquired through a long stay over a source region. Named after its origin (Arctic, polar, tropical) and its track (maritime, continental). (ch. 4)
Air-mass thunderstorm: Thunderstorm produced by daytime heating alone in moist, unstable air, with no front; it follows the sun’s clock. (ch. 9)
Albedo: Share of the solar radiation reflected by a surface. A high albedo (snow) means ground that heats little; a low albedo (dark rock, forest) means ground that absorbs a lot. (ch. 8)
ANASYG, PRESYG: Météo-France’s graphical synoptic analysis and forecast: enhanced frontal charts (activity, jets, areas of significant weather). (ch. 5)
Anvil: Spread-out, fibrous top of a cumulonimbus, made of ice, flattened beneath the tropopause. (prelude, ch. 9)
Arcus: Horizontal roll of cloud at the front of a cumulonimbus, beneath its base; marks the gust front. (prelude, ch. 9)
Barrier effect: Enhancement of cloud and rain on the windward side of a massif, where the air is forced to rise. In the Alpine region it is often called “Stau”. (ch. 4)
Bise: Cold, dry north-easterly wind, between an anticyclone to the north and a low to the south; a low-level wind, accelerated in valleys and through passes. (ch. 11)
Blue (dry) thermal: Thermal that reaches its ceiling without condensing, in dry air: no cloud marks it. (ch. 8)
Blue day: Day on which the air is too dry for the bubble to saturate before reaching its ceiling: real lift, with no cumulus to mark it. (ch. 2)
Boundary layer (friction layer): The lowest layers, about 500 to 1,500 m deep, where friction with the ground slows and deflects the wind; this is where most of our flying takes place. (ch. 3, ch. 6, ch. 10)
Breeze front: Boundary between the cool air of a lake or sea breeze and the warmer air inland; a convergence line. (ch. 7)
Bubble method: Following an ideal bubble along its rails and comparing it with the environment curve to forecast the base, the top, the ceiling and the strength. (ch. 2)
Buys Ballot’s law: In the northern hemisphere, with your back to the wind, low pressure is on the left, a little ahead. (ch. 3)
CAPE: Convective available potential energy (J/kg): the area where the bubble is warmer than the air; the fuel for lift, and for thunderstorms above about 1,000 J/kg. (ch. 2)
CAPE and CIN: Energy available for convection (the area where the bubble is warmer than the surrounding air) and energy needed to break through a stable layer (the area where it is colder), in J/kg. (ch. 1)
Castellanus: Cloud species (mainly altocumulus) bearing turrets; a sign of instability aloft. (prelude)
Ceiling: Maximum usable height in thermals: the cumulus base or, on a blue day, the height at which the dry bubble meets the environment curve. (ch. 2)
Chimney: Continuous thermal, permanently fed by a strong source and anchored to one point of the terrain. (ch. 8)
CIN (convective inhibition): Energy that must be supplied to the bubble for it to reach the LFC (J/kg): the toll. (ch. 2)
Cloud street: Line of cumulus parallel to the wind, beneath which the lift is almost continuous. (prelude, ch. 8)
Cloud suck: Lift that strengthens under the base of a fast-growing cumulus, to the point of exceeding the descent capability of a wing. (ch. 9)
Cold-air pool: Accumulation of cold air on the floor of a valley or in a basin, capped by an inversion. Fog and frost form in it. (ch. 7)
Compression: Squeezing together of the streams of air above a ridge, which makes the wind accelerate at the summit. (ch. 6)
Condensation level: Altitude at which rising air reaches saturation; it is the flat base of cumulus (chapter 1). (prelude, ch. 1, ch. 2)
Congestus: Cumulus clearly taller than it is wide, tower-shaped, with a cauliflower top. (prelude, ch. 9)
Convergence: Meeting of two airflows which, unable to accumulate, rise. A line of lift and of cumulus. (ch. 7, ch. 8)
Convergence / divergence: Horizontal accumulation (or removal) of air, balanced vertically: convergence at the surface forces ascent, divergence at the surface draws down subsidence. (ch. 3)
Coriolis force: Apparent force due to the rotation of the Earth. Deflects every movement to the right in the northern hemisphere, in proportion to its speed; zero at the equator. Never creates or accelerates motion. (ch. 3)
Corona effect: Crackling, buzzing metal, hair standing on end: immediate danger of lightning. (ch. 9)
Cumulonimbus (Cb): Thunderstorm cloud that rises to the tropopause; violent updraughts and downdraughts, hail, lightning. (ch. 9)
Cut-off low: Pocket of cold air aloft detached from the main flow, spinning on itself, moving slowly and erratically. Scattered thunderstorms, unreliable forecasts, little signature at the surface. (ch. 5, ch. 11)
Cut-off time: Time set before setting off, at which you turn back (or land) whatever happens. (ch. 9)
Dew point: Temperature to which the air would have to be cooled to become saturated; it measures the amount of vapour. (ch. 1, ch. 2)
Dew-point spread: The difference T − Td: how much the air must be cooled for it to become saturated. (ch. 1)
Downslope breeze (katabatic): Downward flow of air cooled against a slope. In the evening and at night, or permanently over a glacier. (ch. 7)
Dry adiabat: Cooling rail of a rising unsaturated bubble: −1 °C / 100 m, the effect of expansion alone. (ch. 2)
Ensemble (ensemble forecast): Series of forecasts from the same model, starting from slightly different initial states. The spread between the members measures the uncertainty. (ch. 10)
Entrainment: Mixing of surrounding air into a thermal through its edges; it dilutes the excess heat and slows the lift. (ch. 1)
Environment curve: The real profile of temperature against altitude, at a given place and time. (ch. 1, ch. 2)
Foehn: Warm, dry wind that blows down the lee side of a massif, after leaving its moisture on the other side. Detailed in chapter 6. (ch. 4, ch. 6, ch. 11)
Fractus: Ragged shred of cloud, often beneath a rain cloud or in a damp valley. (prelude)
Front: Narrow transition zone between two air masses of different densities. Its passage brings abrupt changes in temperature, humidity and wind. (ch. 4)
Frontal system: A wave on the polar front, wound up around a low, with a warm front and a cold front, moving from west to east. (ch. 4)
Frontolysis, frontogenesis: Weakening or strengthening of a front. A front in frontolysis often arrives “empty”: a veil with no rain. (ch. 5)
Geopotential height: Altitude, in metres or decametres, at which a given pressure is found (for example 500 hPa). High: warm air; low: cold air. (ch. 5)
Geostrophic wind: Wind in balance between pressure and Coriolis: parallel to the isobars, with low pressure on the left in the northern hemisphere. A good approximation above the friction layer. (ch. 3)
Glacier wind: Katabatic wind that blows down a glacier all the time, even in sunshine, and emerges cold at the snout of the glacier tongue. (ch. 7)
Grid point: The model box that contains the requested place, at the altitude of the smoothed terrain — not necessarily that of the launch or the summit. (ch. 10)
Grid spacing: Side of the boxes into which a model divides the atmosphere. A model correctly describes only phenomena larger than five to eight grid spacings. (ch. 10)
Gust front: Leading edge of the cold air from a thunderstorm downpour, which spreads along the ground far ahead of the storm, with a strong, sudden wind that changes direction. (ch. 9, ch. 11)
Hadley cell: Circulation loop between the equator, where the air rises, and 30° of latitude, where it sinks again; responsible for the subtropical anticyclones and the deserts. (ch. 3)
Halo: Ring of light of about 22° around the sun or moon, caused by the crystals of a cirrostratus. (prelude)
Height contour: Line of equal geopotential height on an upper-air chart. Read like an isobar: the upper wind follows it, low values on the left. (ch. 5)
Hydraulic jump: Abrupt, turbulent rise of the air at the foot of a lee-side downdraught, comparable to the hydraulic jump at the foot of a dam. (ch. 6)
Hypothermia: Fall in the core temperature of the body; serious when confusion sets in and the shivering stops. (ch. 9)
Inversion: Layer in which temperature increases with height; a lid on convection. (ch. 1)
Isobar: Line of equal pressure (reduced to sea level) on a chart. The equivalent of a contour line: close together means a steep “slope” and a strong wind. (ch. 3, ch. 5)
Jet stream: Ribbon of very strong winds (150 to 350 km/h) at high altitude. The polar jet stream, at around 9 to 12 km, on the boundary between polar and tropical air, is the one that steers our frontal systems: its waves (troughs, ridges) create and guide the lows. The subtropical jet, near 30° of latitude, blows higher. (ch. 3)
Lake breeze (sea breeze): Wind that blows from the cooler water towards the shore in the afternoon. Stable air over the water. (ch. 7)
Latent heat: Heat absorbed by water as it evaporates and given back when the vapour condenses, about 2,500 J per gram. (ch. 1)
Lead time: Time between the start of the calculation and the forecast time. Reliability falls as the lead time increases, more or less quickly depending on the parameter. (ch. 10)
Lee-side downdraught: Violent wind rushing down the lee slope when a stable layer presses the air against the terrain. (ch. 6)
Lenticular: Smooth lens-shaped cloud, stationary despite the wind, formed at the crests of a mountain wave. (prelude, ch. 6)
Level of free convection (LFC): Height above which the bubble becomes warmer than the air and rises unaided. (ch. 2)
MacCready theory: Rule for choosing the speed on glide: the stronger the expected thermal, the faster you fly; you speed up in sink and slow down in lift. (ch. 8)
Mamma: Rounded pouches hanging beneath an anvil or a cloud base; associated with turbulence. (prelude)
Mixing ratio: Mass of water vapour per kilogram of dry air, in g/kg. (ch. 1, ch. 2)
Model run: One complete calculation by a model, named after its start time in Universal Time (“the 06:00 UTC run”). Published several hours after that time. (ch. 10)
Mountain breeze (down-valley breeze): Wind that blows down the valley at night. (ch. 7)
Mountain wave: Vertical oscillation of stable air in the lee of a range, stationary relative to the terrain. (ch. 6)
Nocturnal inversion: Layer resting on the ground in which temperature increases with height, created by the ground cooling at night; destroyed from below during the morning. (ch. 2)
Occlusion: The stage at which the cold front has caught up with the warm front, the warm air being pushed aloft. A sign of an ageing frontal system. (ch. 4)
Orographic: Describes a cloud produced by terrain that forces the air to rise. (prelude)
Overdevelopment: Excessive vertical growth of cumulus, leading to congestus and then to cumulonimbus. (ch. 9)
Pannus: Ragged shreds of low cloud beneath the rain of a nimbostratus, clinging to the slopes: the hiker’s frontal “fog”. (ch. 4)
Parcel (or air parcel): Imaginary bubble of air that is followed as it rises, assuming it is isolated from its environment. (ch. 1)
Pileus: Smooth cap above a building tower; a sign of very rapid lift. (prelude)
Plan B: Fallback route, site or plan decided in advance, usable if the day turns bad. (ch. 10)
Polar front: Semi-permanent boundary between polar air and tropical air in the mid-latitudes, with the jet stream above it. The cradle of frontal systems. (ch. 4)
Post-frontal air: Polar maritime air behind the cold front: unstable, crystal clear, cumulus and isolated showers. Active (frequent showers, wind) or dry (the unstable fine weather after the front). (ch. 4, ch. 11)
Pressure force (pressure gradient force): The engine of the wind: it pushes air at right angles to the isobars, from high to low pressure, all the more strongly the closer together they are. (ch. 3)
Refreeze: Overnight freezing of snow or meltwater; it makes snowfields hard and slippery in the morning. (ch. 9)
Relative humidity: Ratio of the vapour present to the maximum possible at the air temperature, as a percentage. (ch. 1)
Restitution lift: Gentle, wide evening lift, caused by surfaces giving back the heat stored during the day (forests, rocks, villages, west faces). (ch. 8)
Retour d’est (easterly return flow): Moist easterly flow pushed against the French–Italian border by a Mediterranean low (Gulf of Genoa): cloud and precipitation held on the border ridges, fine weather further west. The corresponding easterly wind is also called the “lombarde”. (ch. 5)
Reversal: Change in the direction of a breeze, in the morning or the evening, often preceded by a calm and accompanied by shifting winds. (ch. 7)
Ridge (of high pressure): At the surface: a U-shaped crest of high pressure. Aloft: a northward wave, warm air, subsidence; fine weather, and a heatwave if it becomes blocked. (ch. 5)
Ridge bubble: Bubble launched from ridge height with the temperature of the air over the sunny slopes, to estimate the base over a massif. (ch. 2)
Ridge lift (dynamic lift): Lift created by wind forced to rise over a slope facing it. Used in ridge soaring. (ch. 6)
Rotor: Eddy with a horizontal axis in the lee of terrain, beneath a mountain wave; strong turbulence. (prelude, ch. 6)
Saturated adiabat: Rail of the bubble inside its cloud: ≈ −0.55 °C / 100 m (0.4 in warm air to 0.8 in cold air). Condensation releases heat, which slows the cooling. (ch. 2)
Sea of clouds: Flat-topped sheet of low cloud, trapped beneath an inversion, with the summits rising out of it. (prelude, ch. 11)
Sink: Air coming back down to compensate for lift: around thermals, between cloud streets, under dissipating clouds. (ch. 8)
Slack pressure gradient: Extensive area of almost uniform pressure: no synoptic wind, breezes in control, heat thunderstorms possible in summer. (ch. 5, ch. 11)
Solar time: Time set by the position of the sun (solar noon = sun at its highest). In summer in the southern French Alps, clock time ≈ solar time + 1 h 30 min. (ch. 7)
Spread-out: Cumulus whose top runs into an inversion and spreads out into a sheet; it covers the sky and cuts off the thermals. (ch. 11)
Squall line: Organised line of thunderstorm cells, ahead of or along an active cold front: gusts, driving rain, hail. (ch. 4)
Step voltage: Difference in voltage between two points on the ground around a lightning strike; it drives the current from one foot to the other, hence the instruction to keep the feet together. (ch. 9)
Subsidence: Slow, large-scale sinking of air, typical of anticyclones; the air warms and dries as it sinks. (ch. 1, ch. 3, ch. 11)
T850, T500: Temperature at 850 hPa (around 1,500 m) and at 500 hPa (around 5,500 m). The first describes the air mass and gives the freezing level; the difference between them indicates the instability. (ch. 5)
Trigger: Terrain feature or contrast that makes the layer of warm air come unstuck: break of slope, ridge, summit, forest edge, edge of a village. (ch. 8)
Trigger temperature: Temperature the ground must reach for a bubble to break through the nocturnal inversion: the top of the inversion brought back down to the ground along the dry adiabat. (ch. 2)
Troposphere, tropopause: Lowest layer of the atmosphere, in which temperature falls with height and clouds form; the tropopause is its ceiling, at around 11 km. (ch. 1)
Trough: At the surface: a V-shaped valley of low pressure. Aloft: a southward wave in the flow, cold air. Its forward (eastern) side deepens surface lows. (ch. 5)
Trough / ridge: Dip (towards the south) and bulge (towards the north) of a wave in the upper flow. Lows deepen ahead of troughs; ridges bring fine weather. (ch. 3)
Upslope breeze (anabatic): Light wind that blows up a slope heated by the sun. It feeds the thermals and the cumulus above the ridges. (ch. 7)
Valley breeze: Wind that blows up the valley by day, because the air in the valley warms more than the air over the plain. (ch. 7)
Venturi effect: Acceleration of the wind through a constriction: pass, gap, gorge. (ch. 6)
Virga: Trails of precipitation that evaporate before reaching the ground; sink and gusts beneath. (prelude)
W* (convective velocity): Characteristic speed of the rising air in the convective layer, calculated by the models from the heat flux at the ground and the depth of the layer. (ch. 8)
Warm sector: Area of tropical maritime air between the warm front and the cold front: grey, mild, fairly stable weather, low bases. (ch. 4)
Whiteout: Total loss of contrast on snow-covered ground under an overcast sky or in fog: no relief, no horizon. (ch. 9)
Wind barb: Wind symbol: the shaft points to where the wind comes from; barb 10 kt, half barb 5 kt, pennant 50 kt. (ch. 2)
Wind chill: Increased heat loss from a body exposed to the wind; expressed as a feels-like temperature. (ch. 6, ch. 9)
Wind gradient, general wind: Increase in wind speed with height above the ground, due to friction. The general wind (or free-air wind) is the wind that blows above the boundary layer, little slowed by the ground. (ch. 6)
Wind shear: Rapid change in the strength or direction of the wind over a short distance, vertical or horizontal. Zone of choppy air. (ch. 6, ch. 8)
Windgram: Time × altitude graph for a site: wind, ceiling, cloud at each level, rain. The central tool of the pilot the day before. (ch. 10)