Class #9 Study Summary - Brampton Flight Centre

Winds

Hemispheric Prevailing Winds & Surface Friction

Unequal solar heating drives global circulation: warm air rises at the equator (low pressure), cold dense air sinks at the poles (high pressure)
Three circulation cells per hemisphere: Hadley (tropics), Ferrel (mid-latitudes), Polar
Surface friction slows wind speed, reducing Coriolis deflection, causing air to cross isobars at an angle toward low pressure
Over water deflection is ~10°; over rough terrain up to ~40°. Friction effect extends up to approximately 2,000 ft AGL

Buys Ballot's Law Coriolis Force Pressure Gradient Trade Winds Prevailing Westerlies

Upper Level & Surface Winds

Above the friction layer (~3,000 ft), wind flows parallel to isobars/contours: clockwise around highs, counterclockwise around lows (Northern Hemisphere)
Wind speed determined by pressure gradient: tightly spaced isobars = stronger wind
Surface winds are slower and cross isobars due to friction

Local Winds & Mountain Wave

Katabatic wind: Cold dense air flows downslope at night (mountain breeze)
Anabatic wind: Warm air flows upslope during the day (valley breeze)
Mountain wave: Air pours over a ridge and oscillates in powerful vertical waves downwind. Can produce downdrafts exceeding 2,000 fpm, severe turbulence in the rotor zone, extreme wind shear, altimeter errors, and severe icing
Cap cloud and lenticular clouds are visual indicators of mountain wave activity

Katabatic Anabatic Mountain Wave Rotor Cloud Lenticular Cloud

Diurnal Variations, Eddies & Dust Devils

Surface winds are typically stronger and gustier during the day than at night due to convective mixing with faster air aloft
Nocturnal inversion forms after sunset: surface winds decrease and back in direction
After sunrise, mixing resumes: surface wind increases, veers, and becomes gusty; peaks in the afternoon
Eddies form downwind of buildings, hangars, tree lines and hills — hazardous during takeoff and landing
Dust devils are short-lived whirlwinds caused by intense surface heating; most violent near the ground

Wind Shear

Sudden change in wind speed or direction creating turbulence; can occur at any altitude
Low-level wind shear near thunderstorms: gust front (up to 100 kt, 10 nm ahead) and downbursts (microbursts/macrobursts)
Nocturnal inversions produce shear between calm surface air and faster air above
Surface obstructions (mountains, large buildings, hangars) cause localized shear
Most hazardous during takeoff and landing when speed margins are thin and altitude is low

Exam focus: Wind shear can cause sudden airspeed loss leading to stall at low altitude. Direction changes of 180° and speed changes of 80 kt have been observed. Know the difference between gust front, microburst, and macroburst.

Jet Stream & Clear Air Turbulence (CAT)

Narrow bands of high-speed wind at 20,000–40,000+ ft; core speeds 100–250 kt; segments 1,000–3,000 nm long
Polar jet associated with the polar front; strongest in winter. Subtropical jet driven by equatorial heating
Wind speed decreases faster on the polar (north) side than the equatorial (south) side
CAT occurs near the jet stream boundaries (not in the core), especially where curvature is sharp
CAT is random and transient — almost impossible to forecast. Most frequent in winter
If encountering CAT with headwind/tailwind, turn right for smoother air

Jet Stream Isotachs CAT Tropopause

Humidity, Temperature & Stability

Humidity & Moisture

Relative humidity: Ratio of actual water vapour to the maximum the air can hold at that temperature. Rises as air cools; falls as air heats
Fog/low cloud likely when temperature is within 2°C of the dewpoint
Latent heat released during condensation/deposition fuels thunderstorms and hurricanes
Condensation requires nuclei (dust, salt, smoke) — most abundant near the surface
Dew forms when surface cools below dewpoint; frost forms when dewpoint is below 0°C (deposition of ice crystals)

Lapse Rates & Adiabatic Processes

Environmental lapse rate (ELR): Average ~1.98°C per 1,000 ft (ICAO Standard); actual rate varies widely
Dry adiabatic lapse rate (DALR): 3°C per 1,000 ft — unsaturated rising air
Saturated adiabatic lapse rate (SALR): ~1.5°C per 1,000 ft (average) — saturated rising air (slower cooling due to latent heat release)
Dewpoint decreases ~0.5°C per 1,000 ft in rising unsaturated air; temperature and dewpoint converge at ~2.5°C per 1,000 ft
Cloud base calculation: Spread (°C) ÷ 2.5 × 1,000 = cloud base AGL. Quick method: spread × 400

Exam focus: Given surface temp 15°C and dewpoint 5°C (spread = 10°C): cloud base = 10 ÷ 2.5 = 4,000 ft AGL. Freezing level in cloud: 3°C dewpoint at base ÷ 1.5°C SALR = 2,000 ft above base.

Inversions & Stability

Inversion: Temperature increases with height (opposite of normal). Traps fog, smoke and pollutants below
Common causes: radiation cooling at night (nocturnal inversion), warm air overriding cold air at a front, subsidence
Stable air: Displaced air returns to original level. Characteristics: smooth flight, poor low-level visibility, stratus cloud, steady precipitation, steady winds
Unstable air: Displaced air keeps moving. Characteristics: turbulence, good visibility (except in precipitation), cumulus/CB clouds, showery precipitation, gusty winds
Steep lapse rate = unstable; shallow lapse rate = stable

Stable Air Unstable Air Inversion Convection Orographic Lift

Air Masses

Classification & Modification

Continental Arctic (cA): Very cold, very dry — forms over Arctic ice/snow
Maritime Arctic (mA) / Maritime Polar (mP): Cold, moist — gains moisture over ocean
Maritime Tropical (mT): Warm, moist — Gulf of Mexico, Caribbean, tropical oceans
Air masses modify as they travel: gain moisture over water, lose moisture crossing mountains, warm from below over heated surfaces, cool from below over cold surfaces
Cold air mass (warmed from below): Unstable — turbulence, good visibility, cumuliform clouds, showers
Warm air mass (cooled from below): Stable — smooth air, poor visibility, stratiform clouds/fog, drizzle

Fronts

Warm Fronts

Warm air advances and overrides retreating cold air in a long gentle slope
Cloud sequence ahead of front: cirrus → cirrostratus → altostratus → nimbostratus
Frontal cloud/weather may extend 500+ nm ahead. Precipitation is steady, widespread
If warm air is unstable, CB/thunderstorms may be embedded in the stratiform layers
Passage marked by temperature rise, wind veering (gradual), visibility often poor with low stratus and fog
Freezing rain hazard: Rain falls from warm air aloft, supercools in the cold air below, freezes on contact

Cold Fronts

Dense cold air undercuts warm air violently; frontal slope is steep
Narrow band of weather (~50 nm) with cumuliform cloud, heavy showers, possible thunderstorms/hail
Fast-moving cold fronts (30+ kt) produce the most severe weather
Passage marked by sharp wind shift (veer), temperature drop, pressure rise, visibility improvement
Squall line: Line of thunderstorms 50–300 nm ahead of a fast-moving cold front; extremely hazardous

Occluded Fronts & Frontal Weather Summary

Cold front catches warm front, lifting warm sector off the ground
Warm occlusion: Cool air advances on colder air — warm front characteristics
Cold occlusion: Cold air advances on cool air — similar to warm front weather with possible embedded CB
Isobars bend into a V-shape (trough) at all fronts
Not all fronts bring bad weather; some produce only a windshift and slight temperature change

Exam focus: Know the weather sequence for warm fronts (cloud types in order), cold front characteristics (steep slope, narrow weather band, squall lines), and how to identify front passage (wind shift, temperature, pressure, visibility changes).

Precipitation & Fog

Precipitation Types

Drizzle: Fine drops from stratus (stable, little vertical motion)
Rain: Heavy rain indicates strong vertical motion; parent cloud extends well above freezing level
Hail: Formed in CB clouds; ice stones cycled up and down by updrafts, building alternating layers of hard and soft ice
Freezing rain (FZRA): Supercooled rain freezing on contact — rapid ice build-up, very hazardous
Ice pellets (PL): Rain freezes before reaching ground; indicates warm air aloft with freezing rain at higher altitude
Snow: Ice crystals formed by deposition (vapour directly to ice)

Fog Types

Radiation fog: Clear calm nights; ground cools by radiation, air cools to dewpoint. Dissipates with morning heating
Advection fog: Warm moist air moves over a cold surface. Can be widespread and persistent; wind change needed to clear it
Upslope fog: Air cools by expansion moving up terrain. Requires light upslope wind
Steam fog: Cold air over warm water; common over rivers/lakes in autumn
Precipitation-induced (frontal) fog: Evaporation of rain saturates cooler air below; associated with warm fronts
Ice fog: Extreme cold; ice crystals from fuel combustion moisture
Fog reported when visibility ≤ 1 km; mist when visibility 1–5 km

Radiation Fog Advection Fog Upslope Fog Steam Fog Frontal Fog Ice Fog

Thunderstorms (Introduction)

Life Cycle & Hazards

Three stages: Cumulus (updrafts only, building), Mature (updrafts + downdrafts, heaviest precipitation, up to 60,000 ft), Dissipating (downdrafts dominate)
Hazards include severe turbulence, lightning, hail, heavy precipitation, strong gusts, wind shear, and icing
Lake-effect snow squalls: cold dry air over warm Great Lakes water — convective instability produces bands of snow along lee shores

Note: Thunderstorms are covered in greater detail in Class #10 (Meteorology III). This introduction covers the basic cell lifecycle and lake-effect precipitation.

Meteorology II: Air Masses, Fronts & Hazards