Weather and Climate | DSE - Wyatt's Notes

• Atmospheric circulation: The large-scale movement of air across the globe, driven by unequal solar heating and the Coriolis effect, creating a pattern of pressure belts and wind systems.
• Monsoon: A seasonal reversal of wind direction caused by differential heating of land and ocean, bringing distinct wet and dry seasons to South and East Asia.
• Tropical cyclone: A rotating, organised system of clouds and thunderstorms that originates over tropical oceans and has a closed low-level circulation. In the western North Pacific, these are called typhoons.
• El Nino-Southern Oscillation (ENSO): A coupled ocean-atmosphere phenomenon involving fluctuations in sea surface temperatures across the equatorial Pacific, affecting global weather patterns.
• IPCC: Intergovernmental Panel on Climate Change, the United Nations body responsible for assessing scientific information about climate change.

The Earth’s atmosphere circulates heat from equatorial regions toward the poles through a system of three circulation cells in each hemisphere:

At the equator, intense solar heating causes air to rise, creating a zone of low pressure known as the Intertropical Convergence Zone (ITCZ). The rising air produces heavy convective rainfall. At approximately 15 km altitude, the air diverges poleward. As it descends at around 30 degrees latitude, it creates a zone of high pressure known as the subtropical ridge. Surface winds return toward the equator as the trade winds, deflected by the Coriolis effect to become northeasterly in the Northern Hemisphere and southeasterly in the Southern Hemisphere.

A secondary circulation cell driven by the Hadley and Polar cells. Surface winds blow poleward as the prevailing westerlies. At approximately 60 degrees latitude, warm westerly air meets cold polar air at the polar front, producing mid-latitude cyclones and frontal rainfall.

Cold, dense air descends at the poles, creating high pressure. Surface winds blow equatorward as the polar easterlies. Where this cold air meets the warmer Ferrel Cell air at the polar front, ascending air produces frequent cloudiness and precipitation.

The global wind and pressure belt system explains the distribution of climate zones: equatorial regions experience hot, wet conditions; subtropical zones are dominated by high pressure and aridity; and mid-latitudes receive variable weather from passing cyclones and anticyclones.

Hong Kong’s climate is governed by the East Asian monsoon system, which produces distinct seasonal patterns:

Winter (October to March): The Siberian High (a semi-permanent high-pressure system over central Asia) generates cold, dry northerly to northeasterly winds. These winds cross the South China Sea, picking up moisture, so Hong Kong’s winter is relatively cool and dry but with periods of cloudiness and drizzle, particularly in March and April.

Summer (April to September): Intense heating of the Asian landmass creates the South Asian Low, drawing in warm, moist southwesterly to southeasterly winds from the South China Sea and western Pacific Ocean. This produces Hong Kong’s hot, humid summer with frequent convective rainfall and thunderstorms.

The monsoon transition periods (April-May and October-November) are characterised by variable conditions and occasional heavy rainfall.

The onset of the summer monsoon occurs in mid-May over the South China coast, bringing a marked increase in rainfall. The monsoon trough, a zone of low pressure along the southern flank of the subtropical ridge, is a key generator of rainfall over southern China. Persistent monsoon depression events can produce widespread, sustained rainfall, particularly in June and July.

Tropical cyclones require several conditions for formation:

1. Sea surface temperature (SST): Minimum of approximately $26.5\,^{\circ}\mathrm{C}$ to a depth of at least 50 metres, providing the energy source through evaporation and latent heat release.
2. Coriolis effect: Sufficient planetary vorticity to initiate rotation, generally requiring formation at latitudes greater than about 5 degrees from the equator.
3. Low vertical wind shear: Minimal variation of wind speed and direction with height, allowing the storm’s warm core to remain vertically aligned.
4. Atmospheric instability: Conditionally unstable atmosphere permitting sustained deep convection.
5. Pre-existing disturbance: An initial disturbance (e.g., easterly wave, monsoon depression) to serve as a focus for cyclogenesis.

Tropical cyclones are classified by sustained wind speed (Hong Kong Observatory scale):

• Tropical depression: winds less than $63\ \mathrm{km/h}$
• Tropical storm: winds $63$ to $117\ \mathrm{km/h}$
• Severe tropical storm: winds $118$ to $153\ \mathrm{km/h}$
• Typhoon: winds $154$ to $180\ \mathrm{km/h}$
• Super typhoon: winds exceeding $180\ \mathrm{km/h}$

Storm surge is the abnormal rise in sea level caused by a tropical cyclone’s low pressure and strong onshore winds. The surge height can be calculated approximately as:

$\Delta h \approx \frac{\Delta P}{\rho g}$

where $\Delta P$ is the central pressure deficit, $\rho$ is seawater density, and $g$ is gravitational acceleration. Storm surge is the greatest killer associated with tropical cyclones, particularly in low-lying coastal areas.

Tropical cyclones affecting Hong Kong follow one of several tracks:

1. West-northwest track: Forming near the Marshall Islands, curving northwest through the Philippines toward southern China or Hainan Island. Example: Typhoon Hato (2017).
2. recurved track: Forming in the western Pacific, moving northwest then curving northeast before reaching the South China coast. These often pass east or southeast of Hong Kong.
3. South China Sea formation: Cyclones forming within the South China Sea tend to be smaller but can intensify rapidly if conditions are favourable.

Hong Kong is most vulnerable to tropical cyclones from June to October, with a peak in July to September.

Scientific evidence indicates that climate change is already affecting South China:

• Temperature increase: Average temperatures in Hong Kong have risen by approximately $0.12\,^{\circ}\mathrm{C}$ per decade since 1947, with a total increase of roughly $1.6\,^{\circ}\mathrm{C}$.
• Sea level rise: Tide gauge records and satellite data indicate a rise in sea level of approximately $3$ to $4\ \mathrm{mm}$ per year around the South China coast, slightly above the global average.
• Changing rainfall patterns: While total annual rainfall has not shown a clear trend, there is evidence of increased intensity of extreme rainfall events and a shift in seasonal distribution.
• Tropical cyclone intensity: While the frequency of tropical cyclones may not increase, there is evidence that the proportion of intense storms (Category 4-5) is rising, consistent with warmer sea surface temperatures.

The IPCC Sixth Assessment Report projects the following for East and Southeast Asia under moderate emissions scenarios:

• Continued warming of $1.5$ to $2.5\,^{\circ}\mathrm{C}$ by 2100 depending on emissions pathway.
• Sea level rise of $0.3$ to $0.6$ metres by 2100 under moderate scenarios, with higher estimates under high-emissions pathways.
• Increased frequency and intensity of heatwaves, heavy precipitation events, and coastal flooding.
• Changes in monsoon patterns, potentially including delayed onset, increased variability, and more frequent drought-flood alternation.

Hong Kong has adopted several climate adaptation measures:

• Coastal defence: Construction and enhancement of sea walls, flood barriers, and drainage infrastructure. The Tseung Kwan O-Lam Tin Tunnel and associated drainage improvements address flooding vulnerability.
• Urban heat island mitigation: Expansion of urban greening, cool roof programmes, and improved urban ventilation through building design guidelines.
• Water resource management: Diversification of water supply sources, including the Dongjiang water supply scheme, desalination research, and expansion of rainwater harvesting.
• Health system preparedness: Surveillance systems for heat-related illness, dengue fever (which may expand its range), and waterborne diseases.
• Building standards: Revision of design standards for structures to account for increased wind loads and rainfall intensity.

flowchart TD
    A[Climate] --> B[Key Concepts]
    A --> C[Core Principles]
    A --> D[Practical Applications]
    B --> E[Fundamental definitions]
    C --> F[Design patterns]
    D --> G[Real-world usage]

Climate is the personality of a place — it determines what grows, what people wear, and how they build their homes. South China’s climate is shaped by the monsoon, which is like a seasonal heartbeat — the land breathes in moisture from the ocean in summer and exhales dry air in winter. Tropical cyclones are nature’s heat engines, converting warm ocean water into spinning winds. Climate change is like turning up the thermostat on the entire planet — small changes in average temperature produce big changes in extreme weather. For South China, this means more intense typhoons, rising sea levels, and shifting agricultural zones.

1. Link circulation to local climate: Explain how the Hadley Cell creates Hong Kong’s subtropical high-pressure influence in winter and how the monsoon disrupts this pattern in summer.

2. Use diagrams: Draw cross-sections of the three circulation cells, showing wind directions and pressure zones. Sketch the seasonal monsoon reversal with arrows indicating wind direction changes.

3. Be specific about tropical cyclones: Reference the Hong Kong Observatory warning signals (Signal 1 through Signal 10) and explain what each means. Mention specific cyclones as case studies (e.g., Typhoon Mangkhut 2018, Super Typhoon Hato 2017).

4. Evaluate adaptation critically: When discussing adaptation strategies, assess their effectiveness, cost, and limitations. Coastal defences cannot protect against all storm surge scenarios; urban greening has limited effect on the urban heat island compared to structural changes.

5. Distinguish observation from projection: Be clear about what is observed versus what is projected. Observed changes are based on measured data; projections involve uncertainty and depend on emissions scenarios.

6. Connect to other topics: Climate change links to agriculture (shifting growing zones), water resources (altered precipitation), and population (climate migration and coastal vulnerability).

Confusing weather with climate: Weather is short-term (days to weeks), climate is long-term average (30+ years). Don’t describe a single cold day as evidence against climate change.

Assuming tropical cyclones form at the equator: They need the Coriolis effect to rotate, which is zero at the equator. Most form between 5° and 20° latitude where there’s sufficient Coriolis force.

Mixing up Hadley, Ferrel, and Polar cells: Hadley cells are at the equator (0-30°), Ferrel cells at mid-latitudes (30-60°), Polar cells at high latitudes (60-90°). Each produces different wind patterns and pressure zones.

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