Weather systems (the big picture) · Canadian Paragliding Exam Study Guide

Pressure, wind and the northern hemisphere

What causes wind? Horizontal differences in pressure. Air moves from high pressure toward low pressure. The Earth's rotation (the Coriolis effect) bends the flow so that, well above the ground, the wind blows parallel to the isobars, not straight across them. Friction with the ground does not cause wind; it slows it.

Isobars are lines of equal pressure on a weather map. Tightly packed isobars = a steep pressure change = strong wind. Widely spaced isobars = light wind.

	High pressure (anticyclone)	Low pressure (cyclone, depression)
Rotation in the northern hemisphere	Clockwise	Counter-clockwise (anticlockwise)
Vertical motion	Air descends (subsides)	Air rises
Weather	Stable, dry, clear or hazy, light winds, inversions	Unstable, cloud, rain, stronger winds
For us	Smooth mornings, weak or capped thermals, inversion haze	Strong thermals, over-development, gusts, fronts

Wind and height. Coming down from about 3,000 ft AGL to the ground in the northern hemisphere, friction makes the wind back (turn counter-clockwise, for example from west to south-west) and decrease in strength. Going up, the wind veers (turns clockwise) and increases. Veering = clockwise change; backing = counter-clockwise change.

Stability, lapse rates and inversions

Lapse rate = how fast the temperature falls as you go up.

Dry adiabatic lapse rate (DALR): a parcel of unsaturated (dry) air cools about 3 °C per 1,000 ft (≈1 °C per 100 m) as it rises.
Saturated adiabatic lapse rate (SALR): once cloud forms, condensation releases heat, so the parcel cools more slowly, about 1.5 °C per 1,000 ft. So the lapse rate is greater in unsaturated air than in saturated air.
Environmental (actual) lapse rate: what the real atmosphere does that day.

Unstable air: the real atmosphere cools faster with height than a rising parcel does (environmental lapse rate greater than the DALR). A parcel that starts rising stays warmer than its surroundings and keeps rising: thermals, cumulus, turbulence, good visibility.

Stable air: the atmosphere cools slowly with height, or warms (inversion). A rising parcel soon becomes cooler than its surroundings and sinks back: smooth air, stratus, haze, fog, poor visibility.

Worked example (a favourite P4 question)

The weather office reports 0 °C at 6,000 ft ASL and 20 °C on the ground at 1,000 ft ASL. That is a 20 °C drop over 5,000 ft = 4 °C per 1,000 ft, which is more than the dry adiabatic rate of 3 °C. The air is unstable: expect strong thermals and probably cumulus. If the drop had been 1 °C per 1,000 ft, the day would be stable.

Air masses: a cold air mass moving over a warm surface is heated from below → unstable → bumpy, moderately turbulent flying with good visibility. A warm air mass moving over a cold surface is cooled from below → stable → smooth, poor visibility, stratus or fog.

Inversion = temperature increases with height (see Module 03 for what it does to thermals).

Convergence area = rising air where different air masses or winds meet (Module 03).

Fronts

A front is the boundary between two air masses.

Warm front (warm air sliding up over cold air): approaches slowly with high cirrus, then cirrostratus, altostratus, nimbostratus, lowering cloud, steady rain, stable air. For paragliding it is unfavourable: conditions deteriorate (lowering base, rain, wind shift) for many hours.

Cold front (cold air pushing under warm air): arrives faster, with cumulonimbus, squall lines, heavy showers, gusty winds and a sharp wind shift, then clearing, cooler, unstable air with excellent visibility behind it. The day after a cold front is often a classic strong Canadian thermal day, sometimes too strong.

Occluded front: a cold front catches a warm front; mixed weather.

Clouds you must recognise

Cloud	What it tells you
Cumulus (puffy, flat base, cauliflower top)	Thermals. Favourable for soaring. Small and well-spaced is best.
Towering cumulus / cumulus congestus (tall, growing fast)	Strong lift and turbulence, over-development coming. Be ready to leave.
Cumulonimbus (CB) (anvil top, dark base, rain)	Thunderstorm. Land, or get far away, before it matures.
Stratus (flat grey layer)	Stable air, no thermals. Unfavourable.
Nimbostratus, altostratus	Frontal cloud, steady rain. Unfavourable.
Cirrus, cirrocumulus (very high, thin, icy)	Often the first sign of an approaching warm front. Cirrus also cuts solar heating and weakens thermals.
Lenticular (smooth, well-defined, lens or almond shaped, stationary)	Strong winds at that altitude and mountain wave. Stable air with strong wind. Rotor below. Stay on the ground.
Altocumulus castellanus (turrets growing out of a mid-level layer)	Instability aloft: thunderstorms may develop later, possibly embedded (hidden) in the cloud layer.
Stratocumulus	Spreading, stable-ish layer; weak or dying convection.

Which clouds are favourable for performance (soaring) flying? Cumulus. Not stratus, not nimbostratus, not altostratus, and cumulonimbus is dangerous.

What decides the height of the cumulus base? The humidity of the rising air: the base is where the rising parcel cools to its dew point. Dry air (a big temperature–dew-point spread) gives a high base; humid air gives a low base. (Rule of thumb: base in feet ≈ spread in °C × 400.)

Wave (mountain wave) forms when a stable air mass flows over a ridge in strong wind that increases with height. The air oscillates downwind in smooth waves marked by lenticular clouds, with violent rotor underneath. Wave is a hazard for paragliders: strong wind, huge sink, rotor.

Thunderstorms and over-development

A thunderstorm needs unstable air, moisture, and a trigger (strong heating, a front, or terrain). It grows from a towering cumulus (cumulus stage) to a mature cumulonimbus with rain, lightning and downdrafts, then dissipates. Over-development (OD) is when the cumulus clouds grow and spread until they cover the sky and shut off the sun, often leading to storms.

Hazards for a paraglider:

Cloud suck: widespread strong lift under a growing cloud that can pull you into it. Widespread lift beneath clouds and rapid cloud build-up is the signal to leave.
Gust front: the cold downdraft from a storm spreads out along the ground and races ahead of the storm. An average gust front can reach about 15 km ahead (large storms much further, 25 km and more). It arrives as a sudden strong wind shift with dust, long before the rain.
Downbursts, hail, lightning, heavy rain, poor visibility.

Escape path. When you see widespread lift under clouds or rapid build-up, fly upwind (into the wind), because storms and their gust fronts move downwind and lift is weakest upwind of the developing cloud. Do not fly toward shaded ground under the cloud (that is where the storm is), and do not run downwind ahead of a gust front you cannot outrun. If you are low, land early, well before the gust front, and secure the wing.

Upper instability inside a large cloud area or haze layer

You may not see a thunderstorm growing if it is embedded in an existing cloud layer or haze. Upper instability (for example altocumulus castellanus) creates the danger of embedded thunderstorms. This is different from subsidence (sinking air in a high) and from a squall line (a visible line of storms along a cold front).

Nepal bridge: monsoon storms and Canadian storms

In Nepal you know pre-monsoon afternoon storms and you stop flying early. Canadian storms often come with a cold front and can arrive on a day that started smooth and blue; the gust front can hit a landing field 10–20 km from the storm. The Canadian pilot's rule: when cumulus tops grow faster than they spread, and bases darken, you land or leave now, not after the next thermal.

Reading Canadian weather reports (HAGAR and P4)

METAR (hourly aerodrome observation) and TAF (aerodrome forecast) report wind direction in degrees true and speed in knots. Example: 27015G25KT = from 270° true, 15 knots, gusting 25.
The GFA (Graphic Area Forecast) shows clouds, weather, icing and turbulence for a region.
Wind aloft forecasts (FD) give wind and temperature at 3,000, 6,000, 9,000 ft and higher.
All aviation weather uses UTC (Zulu) time.
Sources: NAV CANADA's aviation weather website and Flight Information Centres (FICs) by phone.

Key numbers from this module

DALR ≈ 3 °C / 1,000 ft (1 °C / 100 m). SALR ≈ 1.5 °C / 1,000 ft. Lapse rate is greater in unsaturated air.
Actual lapse rate > 3 °C/1,000 ft → unstable; < 1.5 → stable; temperature rising with height → inversion.
Northern hemisphere: high = clockwise & descending & stable; low = counter-clockwise & rising & unstable.
Descending to the ground the wind backs and slows.
Average gust front reaches ~15 km ahead of a storm; big storms 25 km+.
Escape from over-development: upwind.
METAR wind: true degrees, knots.
HPAC operating limits for P3: base wind 25 km/h, gusts 30 km/h, gust change 8 km/h in 5 s, no thermals over 800 fpm (4 m/s), no significant vertical cloud development.