NQ

Meteorology – Core Vocabulary

Origin of Weather

  • Planetary Motions drive fundamental energy imbalances
    • Rotation of Earth: west ➜ east, 24\,h per turn → diurnal heating / cooling.
    • Moon’s orbit: anticlockwise \approx27.5\,d → tidal bulges modify coastal micro-climates (e.g.\, sea-fog onset).
    • Earth’s revolution: anticlockwise, \approx365\,d on an elliptical path → annual variation in solar distance (perihelion/aphelion) modulates received insolation.
    • Axial tilt 23.5^{\circ} → seasons; solstices (Dec 22, Jun 22) & equinoxes (Mar 21, Sep 23).
    • Uneven solar heating (latitude, season, day/night) = prime cause of atmospheric circulation, pressure cells & weather systems.

The Atmosphere

  • Vertical structure
    • Troposphere: all “active” weather, depth \sim16\,km at equator, \sim6\,km at poles.
  • Heat transfer mechanisms
    • Conduction: molecule-to-molecule; greatest at surface (soil ⇆ air).
    • Convection: mass transport; warm air/water rises (density ↓), cold sinks → thermals, cumulus, sea/land breezes.
    • Radiation: short-wave from Sun, long-wave re-radiated by Earth (greenhouse interaction).
  • Atmospheric physics linkage
    P \leftrightarrow T \leftrightarrow \rho + water-vapour content → governs stability, cloud, wind.

Density

  • Molecules per unit volume.
  • Decreases with height → lighter, less dense air aloft.
  • Implication: aircraft & mountain meteorology (density altitude) and vertical convection potential.

Atmospheric Pressure

  • Defined as weight of air column per unit area.
  • Mean sea-level pressure (MSLP) 1013.25\,\text{mb}.
  • Standard lapse: \Delta P \approx -1\,\text{mb}/10\,\text{m} ascent.
  • Isobars: equal-pressure lines on synoptic charts; tight spacing ⇒ strong winds.
  • Pressure Gradient (PG): \frac{\Delta P}{\Delta d}, drives wind.

Pressure Instruments

  • Mercury / aneroid barometers measure P.
  • QNH: MSLP setting for aviation.
  • Corrections:
    • Altitude: +1\,\text{mb} per 10\,\text{m} instrument is above sea level.
    • Index error vs trusted reference.
  • Barograph: continuous record (barogram) = tendency predictor.

Temperature & Lapse Rates

  • Pressure ↓ ⇒ density ↓ ⇒ expansion cooling.
  • Dry Adiabatic Lapse Rate (DALR): 1^{\circ}\text{C}/100\,\text{m} ascent (unsaturated parcels).
  • Temperature differences drive global & local wind systems.

Winds

  • Air moves from HP ➜ LP (PGF).
  • Coriolis deflection: right in NH, left in SH; zero at equator, ↑ with latitude & wind speed.
  • Circulation around cells:
    • NH: LP counter-clockwise, HP clockwise.
    • SH opposite.
  • Geostrophic wind: balance PGF = Coriolis, flows parallel to isobars above friction layer; speed read from geostrophic scale.
  • Boundary-layer wind: surface friction slows & cross-isobar flow; typically \approx \tfrac23 geostrophic speed.
  • Land/Sea Breezes: diurnal reversal due to differential heating of land vs water.
  • Buys Ballot’s Law: stand with wind at back, NH LP on left (≈90–130^{\circ}).
  • Handheld / fixed anemometers measure wind speed.

Global Circulation

  • Non-rotating aqua-planet ⇒ single Hadley cell; real Earth ⇒ three-cell model (Hadley, Ferrel, Polar) with ITCZ, trade winds, westerlies, polar easterlies.
  • Seasonal ITCZ migration: S in Jan/Feb, N in Jul/Aug (monsoon driver).
  • Continentality distorts pressure belts (January vs July charts).

Air Masses

  • Large homogeneous bodies separated by fronts.
  • Two-letter code: first moisture (m/c), second temperature (E, T, P, A).
    Examples: mT (warm, moist), cP (cold, dry).
  • Modify downstream weather via advection & frontal interactions.

Humidity Concepts

  • Humidity = water-vapour mass per air mass.
  • Saturation: max vapour at given T.
  • Relative Humidity (RH) =\frac{\text{actual}}{\text{sat}}\times100\%.
  • Dew Point: T at which parcel reaches saturation at constant P; when T drops below dew point → condensation (dew, fog, cloud).

Hygrometer & Dew-Point Calculation

  • Stevenson screen with wet & dry bulb thermometers.
  • Wet-bulb depression + table → dew point (examples provided).
  • Principle: evaporation cools wet bulb; greater depression indicates drier air.

Clouds

  • Require condensation nuclei (dust, salt, smoke).
  • Formation: lifting (orographic, frontal, convergence, convection, turbulence) → expansion, cooling below dew point.
  • Latin taxonomy:
    • Cumulus (heap)
    • Stratus (layer)
    • Cirrus (hair)
    • Nimbus (rain).
  • Height groups:
    • Low (St, Sc, Ns)
  • Cloud chart: Ci → halo; Cb → anvil with heavy showers.

Visibility

  • Greatest distance object can be identified unaided.
  • Good: warm air over cold surface, strong winds (dispersion).
  • Poor: aerosols (dust, smoke, salt) or hydrometeors (mist, drizzle).
  • Code figures correspond to distance bands.

Fog

  • Cloud at surface reducing vis <1000\,\text{m}.
  • Cooling to dew point at/near ground (no ascent).
  • Types & mechanics:
    • Advection/Sea fog: warm moist air over cold surface (conduction).
    • Radiation fog: nocturnal radiative cooling; clears with sun/wind.
    • Precipitation (frontal) fog: evaporation of rain into cool air.
    • Valley fog: cold air drainage.
    • Upslope fog: ascent cooling along slope.
    • Arctic sea smoke: very cold air over warmer water.

Precipitation

  • Hydrometeors falling to ground (cloud/fog ≠ precipitation).
  • Droplet growth:
    • Warm clouds: collision-coalescence.
    • Cold clouds: Bergeron (vapour pressure) ice-crystal growth.
  • Types: rain, drizzle, snow, sleet, hail, glaze, spray, dew, hoar frost, rime.

Thunderstorms

  • Hazards: torrential rain, squalls, lightning (static & magnetic effects).
  • Requirements: moisture, instability, lift.
  • Life cycle:
    1 Towering Cu (updraft) \le6\,km.
    2 Mature Cb 12–18\,km, up- & downdrafts, hail, gusts.
    3 Dissipating (downdraft dominates).
  • Charge separation: + top, – base → lightning; thunder = rapid thermal expansion.

Monsoons

  • Seasonal reversal of wind & rainfall; Asia & W Africa classic examples.
  • Winter: land cools ➜ HP, offshore dry flow.
  • Summer: land heats ➜ LP, moist onshore flow, heavy orographic/convection rain.

Fronts

  • Boundary between contrasting air masses; warm air overruns cold.
  • Stationary: airflows parallel; symbol alternate red semicircle/blue spike.
  • Cold: blue spikes toward warm air; steeper slope, rapid weather change, veering wind (NH).
  • Warm: red semicircles toward cold; gentler slope, widespread stratiform rain.
  • Occluded: faster cold front overtakes warm; purple alternating symbols; mixed properties.

Isobaric Patterns & Weather

  • Low (cyclone): closed isobars, LP centre, bad weather, windy.
  • High (anticyclone): closed HP, settled dry.
  • Ridge: HP elongation.
  • Trough: LP elongation, unsettled.
  • Col: slack area between two HP & two LP; calm/fog (winter) or thundery (summer).

Tropical Storms (Cyclones/Hurricanes/Typhoons)

  • Warm-core LP systems deriving energy from latent heat.
  • Development requirements:
    • SST \ge26.5^{\circ}\text{C} to 50\,\text{m} depth.
    • High humidity, rapid cooling aloft, low vertical wind shear.
    • Latitude \ge5^{\circ} for Coriolis spin.
    • Pre-existing disturbance.
  • Formation zones: Eastern ocean basins 10–30^{\circ}\text{N/S}; seldom within 5^{\circ} equator.
  • Structure: eye (calm, clear, lowest P), eyewall (max wind/rain), spiral rainbands.
  • Motion: westward by trade steering, poleward deflection, recurvature by westerlies.
  • Dissipation: landfall, colder water, strong shear, upwelling.
  • Shipboard warning signs: fast falling P, wind shift/strengthen, long swell, high cloud bands on horizon, radar echoes ≤100\,nm.
  • Dangerous vs navigable semicircles (based on motion & hemisphere) dictate avoidance manoeuvres (plots supplied).
  • Synoptic symbols differentiate disturbance, depression, storm, hurricane.

South African Local Weather Highlights (METAREA VII)

  • Sub-tropical anticyclones: South Atlantic (SAH), South Indian (SIH), continental HP; dominate winter interior.
  • SAH ridging eastward → SE coast strong winds, inland moist inflow & rain; “budding” HP splits & drifts.
  • Mid-latitude cyclones (westerly wave lows/cold fronts): winter rainfall, coastal changes.
  • Coastal lows: coast-trapped shallow LP; berg winds offshore (hot/dry) followed by cool moist onshore flow & fog.
  • Cut-off lows: deep closed upper troughs, heavy rain/floods (spring/autumn); often blocked by quasi-stationary anticyclones.

Synoptic Charts & Station Model Interpretation

  • Snapshot of met elements over area.
  • Station model encodes: wind (direction by arrow, speed in knots via barbs), cloud cover (octas), temperature, dew point, MSLP (last 3 digits), present/past weather symbols.
  • Additional symbols: isobars, fronts, highs/lows, tropical systems.
  • Reading geostrophic wind scale (2 mb spacing) ⇒ speed @ 1000\,\text{mb} & 10^{\circ}\text{C}; adjust for latitude if printed; surface wind \approx \tfrac23 geo.
  • Practical examples: cold-front sequences, warm-sector conditions, occlusion passages.

Weather Reporting & Logs

  • METAREA VII forecasts via SANHO 1 schedules: deep-sea, coastal, radio-fax charts, Port Meteorological Services.
  • Ship’s logbook: Beaufort scale, weather coded letters, visibility & sea state codes; entries each watch & hourly in storms.
  • Standard coded weather report TV 4/102 at 0000Z, 0600Z, 1200Z, 1800Z; ‘priority’ routine, ‘immediate’ for special (e.g.\, tropical cyclone).
  • Examples: MV VOORBEELD & MV SKAAPSTAD messages showing synoptic code groups.

Quick-Reference Formulae & Numbers

  • \text{MSLP}=1013.25\,\text{mb}.
  • \Delta P/\Delta z \approx -1\,\text{mb}/10\,\text{m}.
  • \text{DALR}=1^{\circ}\text{C}/100\,\text{m}.
  • Surface wind = \tfrac23 geostrophic.
  • Dew-point & wet-bulb tables supplied (see Appendix).
  • Tropical storm threshold SST \ge 26.5^{\circ}\text{C}.

Ethical / Practical Implications

  • Accurate barometer calibration and log keeping are IMO & SOLAS safety obligations.
  • Fog & thunderstorm avoidance critical for aviation & maritime collision/grounding risk.
  • Tropical cyclone track forecasting saves lives; understanding semicircular hazards guides ship routing.

Connections & Applications

  • Global circulation sets stage; fronts & local circulations modulate daily weather.
  • Instruments (barometer, hygrometer, anemometer) quantify state variables feeding synoptic charts.
  • Synoptic interpretation links theoretical dynamics to operational decisions (route planning, flight levels, port closures).
  • Seasonal ITCZ migration underpins monsoon rains—vital for agriculture yet hazardous for navigation.
  • Cut-off lows & blocking highs illustrate upper-air coupling with surface systems, reminding forecasters to integrate 3-D atmosphere.