7.3 Earth, Weather & Climate
Key Takeaways
- The water cycle continuously circulates water between ocean, atmosphere, and land through evaporation, transpiration, condensation, precipitation, runoff, and infiltration.
- Weather conditions are measured using standardised meteorological instruments including thermometers, barometers, anemometers, rain gauges, wind vanes, and hygrometers.
- Cloud formations (cumulus, stratus, cirrus, nimbostratus, cumulonimbus) reflect atmospheric elevation, instability, and precipitation potential.
- Weather refers to short-term day-to-day atmospheric variations, whereas climate represents long-term statistical weather patterns over 30+ years (e.g., Caribbean tropical marine climate).
- Caribbean island nations are vulnerable to severe natural hazards—including hurricanes, volcanic eruptions, earthquakes, and storm surges—requiring robust emergency preparedness and safety protocols.
The Hydrologic (Water) Cycle & Atmospheric Processes
Definition: The water cycle (hydrologic cycle) is the continuous, endless movement of water between Earth's surface, oceans, underground reservoirs, and the atmosphere. Driven primarily by solar thermal radiation from the Sun and Earth's gravitational pull, the water cycle ensures the constant recycling of Earth's finite water supply.
ATMOSPHERE
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CONDENSATION PRECIPITATION
(Cloud Formation) (Rainfall to Earth)
^ |
| v
EVAPORATION / SURFACE RUNOFF &
TRANSPIRATION INFILTRATION
(Vapour from Sea/Plants) (Rivers & Groundwater)
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+---------------+---------------+
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EARTH SURFACE
Key Stages of the Water Cycle
1. Evaporation
Solar radiation heats liquid water in oceans, seas, lakes, and rivers. Water molecules absorb thermal energy, increase kinetic energy, and transform from liquid into gaseous water vapour, rising into the lower atmosphere.
2. Transpiration & Evapotranspiration
- Transpiration: Water absorbed by plant root systems travels up stems to leaves, where it evaporates as water vapour into the air through microscopic stomatal pores.
- Evapotranspiration: The combined total of evaporation from soil and water surfaces plus plant transpiration.
3. Condensation
As warm, moist air rises into the atmosphere, it expands and cools (since atmospheric temperature decreases with altitude). Water vapour loses thermal energy and condenses back into liquid water droplets or tiny ice crystals around airborne particles known as cloud condensation nuclei (dust, sea salt particles, pollen), forming clouds and fog.
4. Precipitation
When condensed water droplets inside clouds collide and coalesce into larger droplets, they grow too heavy to remain suspended by atmospheric updrafts. Gravitational pull pulls them down to Earth as precipitation (rain, drizzle, hail, sleet, or snow). In the warm tropical Caribbean, rain is by far the predominant precipitation form.
5. Surface Runoff
Precipitation falling on land flows over the soil surface under gravity into streams, rivers, wetlands, and ultimately returns to the ocean.
6. Infiltration and Percolation
- Infiltration: Process by which rainwater soaks directly into surface soil layers.
- Percolation: Deep downward movement of infiltrated water through porous rock strata and soil layers to replenish underground aquifers (groundwater storage reserves).
Weather Instruments & Cloud Classification
Definition: Weather describes the temporary, short-term state of the atmosphere at a specific location over a brief period (hours or days). Meteorology is the scientific study of weather and atmospheric phenomena.
Weather Measuring Instruments
Meteorologists utilise precise meteorological instruments to monitor atmospheric parameters:
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| METEOROLOGICAL INSTRUMENTS |
+-------------------+----------------------------+------------------------+
| Instrument | Parameter Measured | Standard Units |
+-------------------+----------------------------+------------------------+
| Thermometer | Air Temperature | Degrees Celsius (°C) |
| Barometer | Atmospheric Pressure | Millibars (mb) / hPa |
| Anemometer | Wind Speed | km/h, knots, mph |
| Wind Vane | Wind Direction | Compass directions |
| Rain Gauge | Liquid Precipitation | Millimetres (mm) |
| Hygrometer | Relative Humidity | Percentage (%) |
+-------------------+----------------------------+------------------------+
1. Thermometer
Measures atmospheric thermal energy (air temperature). Liquid-in-glass thermometers use expanding alcohol or mercury inside a graduated column. Digital weather stations utilise electronic thermistors.
2. Barometer
Measures atmospheric (barometric) pressure exerted by the weight of air molecules overhead. A falling barometer signals approaching low-pressure systems, clouds, and rainy weather. A rising barometer indicates fair, clear skies.
3. Anemometer
Measures wind speed. Features 3 or 4 hemispherical cups mounted on horizontal arms attached to a vertical rotating shaft; faster wind speeds rotate the cups more rapidly.
4. Wind Vane (Weather Vane)
Determines wind direction. Points into the oncoming wind. Crucial Rule: Wind is always named according to the direction from which it originates (e.g., a "Northeast Trade Wind" blows from the northeast toward the southwest).
5. Rain Gauge
Measures liquid rainfall depth. Consists of a funnel receiver that collects rainwater into a narrow inner measuring tube calibrated in millimetres (mm).
6. Hygrometer / Psychrometer
Measures relative humidity—the percentage of water vapour present in air relative to the maximum water vapour the air can hold at that specific temperature.
Cloud Classification
Clouds are classified according to their physical appearance, altitude, and weather characteristics:
- Cumulus Clouds: Puffy, cotton-like white clouds with flat bases observed on bright sunny days. Indicate fair weather, but can grow vertically during hot afternoons.
- Stratus Clouds: Uniform, low-level grey cloud sheets covering the entire sky like a dull blanket. Often produce light drizzle, mist, or overcast conditions.
- Cirrus Clouds: High-altitude clouds composed entirely of delicate ice crystals. Feature thin, wispy, feather-like appearances; signal approaching warm fronts.
- Nimbostratus Clouds: Dark, thick, low-to-mid level cloud layers that produce continuous, widespread heavy rainfall or steady showers.
- Cumulonimbus Clouds: Massive vertical thunderstorm clouds rising up to 12,000 metres with anvil-shaped tops. Produce intense downpours, severe lightning, thunder, hail, and violent wind gusts.
Weather vs. Climate & Caribbean Natural Hazards
Key Distinction: Weather represents day-to-day atmospheric conditions (e.g., "It is raining in Kingston today"). Climate is the long-term statistical pattern of weather conditions averaged over an extended timeframe (typically 30 years or more) for a geographic region.
The Caribbean Tropical Marine Climate
Caribbean island nations experience a Tropical Marine Climate, strongly regulated by surrounding ocean waters and constant Northeast Trade Winds.
- Temperature: Consistently warm year-round, averaging between 26°C and 31°C with narrow seasonal variations.
- Two Primary Seasons:
- Dry Season (December to May): Characterised by lower rainfall, moderate humidity, sunny skies, and cooler breezy evenings.
- Wet / Rainy Season (June to November): Warm, humid conditions with heavy convectional rainfall. Coincides directly with the Atlantic Hurricane Season.
Severe Weather & Natural Disasters in the Caribbean
Due to tectonic location and tropical maritime environment, the Caribbean is susceptible to severe natural hazards:
CARIBBEAN NATURAL HAZARDS
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METEOROLOGICAL HAZARDS TECTONIC HAZARDS
(Hurricanes & Storm Surges) (Volcanoes & Earthquakes)
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Tropical Storm Surge Volcanic Seismic
Storms (Ocean Rise) Eruptions Earthquakes
e.g. Category High winds Ash fall & Ground
1-5 Cyclones pushing sea pyroclastic shaking &
inland flows tsunamis
1. Tropical Cyclones & Hurricanes
Large low-pressure storm systems forming over warm ocean waters (surface temperatures exceeding 26.5°C). Rated on the Saffir-Simpson Scale from Category 1 (74–95 mph winds) to Category 5 (winds >157 mph).
- Eye: Calm, clear centre of the hurricane with low pressure and light winds.
- Eyewall: Ring surrounding the eye containing the most destructive winds and intense rainfall.
- Storm Surge: The most dangerous hurricane hazard; a massive, abnormal rise of ocean water pushed inland by high winds and low pressure, flooding coastal communities.
2. Volcanic Activity
The Eastern Caribbean arc (Lesser Antilles) sits on active tectonic subduction zones containing active volcanoes (e.g., La Soufrière in St. Vincent, Soufrière Hills in Montserrat).
- Hazards: Heavy ash fall (contaminating water supplies and collapsing roofs), pyroclastic flows (deadly avalanches of superheated gas, ash, and rock rushing down slopes at >100 km/h), and lava flows.
3. Earthquakes
Sudden slipping of tectonic plates along fault lines releases immense elastic energy as seismic waves, causing severe ground shaking, structural collapse, and potential tsunamis.
Emergency Preparedness & Safety Protocols
| Natural Hazard | Pre-Disaster Preparation | Safety Action During Event |
|---|---|---|
| Hurricane | Store emergency kit (canned food, bottled water for 3–7 days, flashlights, batteries, radio); board up glass windows; clear gutters. | Stay indoors in an interior windowless room; monitor emergency radio broadcasts. Never venture out during the eye calm. |
| Earthquake | Secure heavy furniture; locate safe indoor cover spots. | DROP, COVER, and HOLD ON beneath a sturdy table or desk. Stay away from glass windows and exterior walls. |
| Volcanic Eruption | Keep N95 dust masks and goggles in emergency kits; store covered fresh water supplies. | Wear dust masks outdoors; remain indoors with doors/windows sealed to avoid inhaling toxic ash particles. |
Which stage of the water cycle involves water vapour cooling as it rises high into the atmosphere and changing back into liquid droplets to form clouds?
A meteorologist observes dark, thick, low-lying cloud sheets covering the entire sky that produce steady, continuous rainfall over widespread island areas. What type of clouds are these?
What is the primary scientific distinction between weather and climate?
During a severe tropical hurricane in the Caribbean, what phenomenon causes an abnormal, dangerous rise in sea levels that floods coastal towns?