EV1005 LECTURE 3 & 4

Atmospheric Pressure and Basic Concepts

• Atmospheric pressure is the weight of air above a unit area; it decreases with height because density falls off with altitude.
• Air near the ground is heated → warm air rises, expands, density decreases, and low pressure forms at the surface.
• As air rises, it cools; cooler air holds less moisture, so condensation and rain can result from rising air; descending air is typically dry and associated with high pressure.
• Record pressures mentioned:
  • Canadian record low: 940.2 mb (94.02 kPa) at Saint Anthony, NL, Jan 1977.
  • U.S. record low: 882 mb (26.02 in.) Hurricane Wilma, Oct 2005.
  • Earth’s record low: 870 mb (25.69 in.) Typhoon Tip, Oct 1979.
  • Strong high pressures: Canadian record high 1079.6 mb (107.96 kPa) at Dawson, NWT, Feb 1989; Earth’s record high 1084 mb (32.01 in.) Agata, Siberia, Dec 1968.
• Normal sea-level pressure: ~1013.2 mb (29.92 inHg).
• Unit conversions:
  • 1.0 inHg = 33.87 mb; 1.0 mb = 0.0295 inHg; 1.0 mb = 0.75 mm Hg; 1 lb/in² = 68.95 mb (approximate conversions vary with exact reference).
• Fraction of atmospheric mass in the troposphere: about 90% of the mass is within the troposphere.
• Density and pressure profiles with altitude:
  • Density is higher near the Earth’s surface and decreases with altitude (see density–altitude profile).
  • Temperature and density profiles vary through the atmospheric layers (troposphere, stratosphere, mesosphere, thermosphere, exosphere).

Atmospheric Circulation: Horizontal and Vertical Motion

• Air moves from regions of high pressure to low pressure; wind strength is determined by the pressure gradient (spacing of isobars):
  • Wide spacing = weak gradient = light winds.
  • Close spacing = steep gradient = strong winds.
• Isobars are lines of equal pressure; wind generally follows a path roughly parallel to isobars (geostrophic tendency) with a cross-isobar component due to friction.
• High pressure systems (anticyclones) in the Southern Hemisphere rotate anticlockwise; low pressure systems (cyclones) rotate clockwise.
• Low pressure systems are associated with uplift, stronger winds, and precipitation; high pressure systems are associated with sinking air and clearer skies.
• Vertical motion:
  • High pressure: sinking air, dispersing or compressing, leading to clearer conditions.
  • Low pressure: rising air, leading to condensation, clouds, and precipitation.

Coriolis Force and Global Deflection

• Coriolis force acts on moving air, deflecting its path:
  • Southern Hemisphere: apparent deflection to the left of the motion.
  • Northern Hemisphere: apparent deflection to the right of the motion.
• Relative magnitudes (illustrative values):
  • Equator: v ≈ 1675 km/h (≈ 465 m/s)
  • 60° latitude: ≈ 838–770 km/h (≈ 232–214 m/s)
  • Poles: deflection tends toward very small circles (negligible relative to equator).
• Consequences:
  • Air originating at the equator moves poleward into faster-moving atmosphere, causing it to have a west-to-east (westerly) relative motion as it moves poleward.
  • Combined with pressure gradients, this produces prevailing wind patterns and cyclonic/anticyclonic circulations.
• Important concept: Coriolis causes turning of geostrophic flow and interacts with pressure gradients and surface friction to produce three-dimensional wind fields.

Geostrophic Winds and Upper-Level Flow

• Geostrophic winds flow approximately parallel to isobars in the upper atmosphere where friction is small.
• Across-isobar flow is small and results from a balance among pressure gradient force, Coriolis force, and friction (more cross-isobar flow near the surface).
• In the upper troposphere, winds tend to be faster where isobars are closer together (stronger pressure gradient).
• World-wide patterns:
  • Trade winds near the equator.
  • Westerlies in mid-latitudes.
  • Subtropical highs and mid-latitude westerlies associated with jet streams.

Synoptic Features: Fronts, Ridges, Troughs, and Pressure Systems

• Synoptic charts depict MSLP (mean sea level pressure) with isobars and fronts.
• Ridge: elongated high-pressure region with isobars curving outward; generally associated with clear weather.
• Trough: elongated low-pressure region with a surge of lower pressure; associated with unsettled weather.
• Fronts:
  • Cold front: boundary where cold air advances into warmer air; often sharp wind shift and steep ascent, with colder air behind the front.
  • Warm front: boundary where warm air overrides cooler air; ascent is more gradual, widespread, long-lasting rainfall.
  • Stationary front: boundary between air masses that is not moving significantly.
  • Occluded front: when a cold front overtake a warm front; can produce complex weather patterns.
• Clouds commonly associated with fronts:
  • Nimbostratus, Cirrus, Altostratus, Cirrostratus, etc.
• Fronts in Australia often show wind shifts around NW ahead of the front to SW behind it.
• “Geostrophic winds” in synoptic context: winds flow along isobars with some cross-isobar flow caused by friction; strongest winds where isobars are closest.

Air Masses, Uplift, and Precipitation

• When air masses collide, they do not mix readily; fronts form (air mass boundaries).
• Uplift mechanisms:
  • Convergent lifting (airflows collide and forced upward).
  • Convectional lifting (buoyancy over warm surfaces).
  • Orographic lifting (air moves over mountains; rain on windward side, rain shadow on leeward side).
  • Front uplifting due to conflicting air masses.
• Condensation and cloud formation occur when rising air cools and moisture condenses; latent heat release influences cloud development and precipitation intensity.

Adiabatic Processes, Lapse Rates, and Moisture

• Adiabatic processes describe cooling/heating of air as it rises or descends without exchanging heat with surrounding air.
• Dry adiabatic lapse rate (unsaturated air): $ext{Γ}_{ ext{dry}} \,=\, 10\,^{\circ}\mathrm{C}/1000\mathrm{m}$
• Moist adiabatic lapse rate (saturated air): approximately $\Gamma_{ ext{moist}} \approx 5\,^{\circ}\mathrm{C}/1000\mathrm{m}$
• Dew point lapse rate: roughly $\Delta T_{\text{dew}} \approx 1.8\,^{\circ}\mathrm{C}/1000\mathrm{m}$
• Phase changes and latent heat:
  • Evaporation/vaporization requires energy; latent heat of vaporization: L_v \approx 540\ \text{cal g^{-1}}
  • Condensation releases latent heat: $\Delta Q = -L_v$ per gram condensed
  • Fusion and melting involve latent heats as well: latent heat of fusion ~ +80\ \text{cal g^{-1}} (absorbed) and freezing/solidification release ~ -80\ \text{cal g^{-1}}
• During uplift, air cools adiabatically; if it cools to its dew point, condensation occurs and latent heat release slows further cooling (wet adiabatic cooling).
• Through height, temperature and humidity profiles determine cloud types and precipitation potential.

Water, Humidity, and Clouds

• Water states and energy:
  • Sublimation: solid to gas (absorbs energy).
  • Deposition: gas to solid (releases energy).
  • Evaporation (vaporization): liquid to gas (absorbs energy).
  • Condensation: gas to liquid (releases energy).
  • Freezing: liquid to solid (releases energy).
  • Melting: solid to liquid (absorbs energy).
• Latent heat values (per gram):
  • Vaporization: L_v \approx 540\ \text{cal g^{-1}} (absorbed)
  • Condensation: -L_v \approx -540\ \text{cal g^{-1}} (released)
  • Melting: L_f \approx +80\ \text{cal g^{-1}} (absorbed)
  • Freezing: -L_f \approx -80\ \text{cal g^{-1}} (released)
• Water-holding capacity of air:
  • At 30°C, air can hold about 27.5\ \text{g kg^{-1}} of water vapor at saturation.
  • At 10°C, about 7.5\ \text{g kg^{-1}} at saturation.
• Humidity concepts:
  • Relative Humidity (RH) is the ratio of actual water vapor to the maximum water vapor the air can hold at that temperature.
  • Dew point is the temperature at which air becomes saturated and condensation begins.
  • Condensation leads to cloud formation; dew point decreases with rising altitude if moisture is unchanged.
• Cloud formation and types (illustrative):
  • Cirrus, Cirrostratus, Cirrocumulus (high clouds).
  • Altostratus, Altocumulus (middle clouds).
  • Stratus, Stratocumulus (low clouds).
  • Cumulus, Cumulonimbus (vertical development; anvil-headed for cumulonimbus).
• Uplift and cloud development: advantageous vertical growth leads to cumulonimbus and heavy rainfall; stable layers suppress cloud growth.

Clouds and Precipitation: Condensation Processes

• Condensation occurs when air rises and cools to its dew point; latent heat release can promote further uplift.
• Cloud types indicate the vertical development and moisture content; Cumulonimbus represents strong uplift and potential storms.
• The presence of condensing moisture and lifting mechanisms determine precipitation patterns across regions.

Regional Weather Drivers in Australia

• Monsoon and Madden–Julian Oscillation (MJO) influence rainfall and cyclone activity, especially in northern Australia.
• Indian Ocean Dipole (IOD) and El Niño/Southern Oscillation (ENSO) shape seasonal patterns.
• Australian climate features:
  • Monsoon trough and ITCZ location shift seasonally; NW winds in summer vs SE trades in winter.
  • Frontal systems and troughs contribute to mid-latitude weather in southern Australia.
  • Blocking highs can stall movement of weather systems, influencing droughts and heat waves.
• Tropical cyclones and depressions contribute to extreme rainfall and wind events in northern regions.
• West Coast dynamics include troughs and upper-level troughs; sea breezes and monsoonal influences play a role in coastal climates.
• Notable weather drivers listed: Monsoon, Tropical Cyclones, Madden–Julian Oscillation, Indian Ocean Dipole, Tropical Depressions, West Coast troughs, Trade Winds, Northwesterly Cloudbands, Easterly Trough, Subtropical Ridge (winter and summer), Frontal Systems, Cut-off Lows, Blocking Highs, Southern Annular Mode.

ITCZ, Hadley Circulation, and Global Wind Patterns

• ITCZ (Intertropical Convergence Zone) location shifts seasonally; the convergence of moist air at the equator results in heavy clouds and rainfall.
• Hadley circulation: rising moist air around the equator, moving poleward aloft, sinking in the subtropics, and returning near the surface as trade winds.
• Tropical to mid-latitude transitions produce the Hadley-Cell–Ferrel-Polar cell pattern; vertical motion and fronts accompany these cells.
• Tropical regions exhibit heavy rainfall due to convection and ITCZ activity; subtropics have descending dry air creating high pressure zones.
• Seasonal ITCZ shifts and associated rainfall patterns influence the Australian climate, particularly the northern monsoon region.

Monsoon System in Australia and Nearby Regions

• Monsoon: characterized by a seasonal reversal of winds due to differential heating of land and sea; offshore winds in winter and onshore winds in summer.
• ITCZ follows the sun’s position and the seasonal migration of heating; two major heat lows (Pilbara and Cloncurry) can attract the ITCZ.
• Summer monsoon features NW winds; winter features SE trades.
• MJO influence on cyclone activity in Australia: eastward-moving convective pulse that interacts with the monsoon trough; stronger influence to the north of the country.
• ENSO (El Niño / La Niña) modulates monsoon intensity and rainfall variability.

Severe Weather and Practical Implications in Australia

• Economic and ecological impacts from heat waves, bushfires, and monsoonal rains.
• Anticipated weather indicators include NW winds in dry seasons and SE winds in monsoon-active periods.
• Frontal systems, tropical cyclones, and monsoon troughs contribute to extreme weather events; blocking highs can lead to prolonged weather patterns.
• Understanding atmospheric processes helps predict rainfall distribution, drought risk, and temperature extremes.

Atmospheric Stability and Cooling/Heating Processes

• Atmospheric stability affects cloud formation and precipitation patterns; adiabatic processes govern temperature changes with altitude.
• Adiabatic temperature change with altitude:
  • Dry adiabatic lapse rate: $\Gamma_{dry} = 10\,\degree\mathrm{C}/1000\mathrm{m}$
  • Wet (moist) adiabatic lapse rate: approximately $\Gamma_{moist} \approx 5\,\degree\mathrm{C}/1000\mathrm{m}$ due to latent heat release slowing cooling.
• Dew point lapse rate: $\Delta T_{dew} \approx 1.8\,\degree\mathrm{C}/1000\mathrm{m}$
• When a parcel of air rises from sea level at 32°C to 1000 m, its temperature changes according to the appropriate lapse rate, affecting condensation potential and humidity.
• Relative humidity interacts with temperature to determine condensation likelihood; RH can exceed 100% if temperature falls while moisture content remains.

Atmospheric Layers, Profiles, and Measurements

• Temperature and pressure profiles across the atmosphere show:
  • Troposphere contains ~90% of atmospheric mass.
  • Temperature generally decreases with height in the troposphere; a lapse rate describes this gradient.
  • Stratosphere contains the ozone layer; temperature increases with height in parts of the stratosphere due to ozone absorption of UV radiation.
• Pressure at sea level is defined as 1013.2 mb (29.92 inHg); variations include deep lows and strong highs.
• Pressure scales and conversions:
  • 1 mb ≈ 0.0295 inHg; 1013.2 mb corresponds to 29.92 inHg.
  • Severe hurricane/low-pressure events: pressures can dip well below 1000 mb; high-pressure records rise above 1050–1080 mb.
• Typical cross-sectional views show the relationship between pressure cells, circulation, and tropopause heights (ITCZ and polar front interactions).

Synoptic Charts and Chart Reading

• Early synoptic charts illustrate pressure systems, fronts, troughs, ridges, and monsoon features; modern charts continue to show MSLP, fronts, troughs, monsoon troughs, fronts, and jet streams.
• Synoptic interpretation involves:
  • Identifying high and low pressure centers (H and L).
  • Reading isobar spacing to infer wind strength.
  • Recognizing fronts (warm, cold, stationary, occluded) and associated cloud types.
  • Noting troughs and ridges and their movements to predict weather changes.
• Example: a geostrophic wind diagram demonstrates air flowing around a low pressure center with isobars wrapping clockwise in the Southern Hemisphere (anticyclonic) and anticlockwise around highs.

Key Pointers for Exam Preparation

• Understand the cause-and-effect chain: differential heating → pressure differences → wind (pressure gradient) → Coriolis deflection → geostrophic balance in upper levels → fronts, troughs, ridges → uplift and precipitation.
• Be able to explain why ascending air cools and leads to cloud formation and rain, and why descending air leads to dry, stable conditions.
• Distinguish between dry and moist adiabatic lapse rates and explain how latent heat release modifies vertical temperature profiles.
• Recognize the main global wind systems and their relation to pressure belts: trade winds, westerlies, polar easterlies, Hadley, Ferrel, and Polar cells.
• Understand the ITCZ and its seasonal migration and how that affects monsoon systems and rainfall in Australia.
• Be able to describe how topography (orography) and surface friction modify wind patterns and rainfall distribution (e.g., rain shadows, sea breezes).
• Recall representative numerical values for quick reference: normal sea-level pressure, record pressures, lapse rates, latent heats, and Coriolis speed estimates.
• Connect synoptic cues (isobars, fronts, troughs, ridges) to weather outcomes (wind direction/strength, precipitation, temperature changes).
• Relate atmospheric processes to real-world phenomena in Australia (heat waves, Antarctic outbreaks, monsoon effects in Townsville, and regional rainfall patterns).

Quick Equations and Critical Numbers (LaTeX)

• Pressure gradient and wind relationship (conceptual):
  • Wind speed V is proportional to the pressure gradient magnitude |∇P| and modulated by Coriolis and friction forces. In the upper atmosphere (minimal friction): $V \propto |\nabla P|,$ with wind directed along isobars in a geostrophic balance.
• Dry adiabatic lapse rate: $\Gamma_{dry} = 10\,\degree\mathrm{C}/1000\mathrm{m}.$
• Moist adiabatic lapse rate (approximate): $\Gamma_{moist} \approx 5\,\degree\mathrm{C}/1000\mathrm{m}.$
• Dew point lapse rate: $\Delta T_{dew} \approx 1.8\,\degree\mathrm{C}/1000\mathrm{m}.$
• Latent heat values (per gram):
  • Vaporization: $L_v \approx 540\ \text{cal g}^{-1}$
  • Condensation: $-L_v \approx -540\ \text{cal g}^{-1}$
  • Fusion: $L_f \approx 80\ \text{cal g}^{-1}$
  • Freezing: $-L_f \approx -80\ \text{cal g}^{-1}$
• Normal sea-level pressure: $p_{SL} \approx 1013.2\ \text{mb} = 29.92\ \text{inHg}.$
• Record low/high pressures (examples):
  • 940.2 mb (Canada, 1977); 882 mb (U.S., 2005); 870 mb (Earth, 1979).
  • 1079.6 mb (Canada, 1989); 1084 mb (Earth, 1968).
• Magnetic and rotational considerations (Coriolis) (illustrative):
  • Equatorial linear speed: $v_{eq} \approx 1675\ \text{km/h} (\approx 465\ \text{m/s}).$
  • At 60°N latitude: $v_{60} \approx 838\ \text{km/h} (\approx 232\ \text{m/s}).$

Remember to Review

• How pressure systems and airmass boundaries influence weather patterns.
• The roles of fronts, uplift processes, and topography in rainfall distribution.
• The interplay between energy budgets, wind, and currents in shaping climate patterns.
• australia-specific weather drivers including monsoon, MJO, ENSO, IOD, blocking highs, and regional fronts.
• how to read synoptic charts and interpret MSLP, isobars, fronts, troughs, and ridges for practical forecasting.