Weather and Climate Flashcards

The Great Storm of 1987

• Occurred on October 16, 1987.
• Originated as a small wave on a cold front in the Bay of Biscay on October 15.
• Caused by the interaction of warm air from Africa and cold air from the North Atlantic.
• Initially appeared to be a typical depression.
• Unexpectedly deepened around 1800 hrs on October 15, reaching a central pressure of 964 mb.
• The steep pressure gradient caused severe winds.
• The cause of the rapid deepening is believed to be a combination of:
  • A strong jet stream initiated by Hurricane Floyd off the east coast of North America on October 13.
  • Extreme warming over the Bay of Biscay.
  • These factors may have led to a significant release of latent heat energy.
• The storm's change in direction towards the Midlands caught experts off guard.
• Moved rapidly across southern England in six hours.
• Winds were light near the center (e.g., Birmingham at 13 km/hr).
• Strong pressure gradient on the southern flank caused severe winds:
  • Portland Bill: 102 km/hr, gusting to 141 km/hr.
  • Dover: 115 km/hr, gusting to 167 km/hr.
• The storm resulted in:
  • 16 deaths.
  • Collapsed houses and damage to property.
  • Approximately 15 million trees blown over.
  • Disruption of transport and power lines.
• The return period for winds of this strength in Southeast England is measured in centuries.
• In comparison, a storm on March 10, 2008, brought winds of 150 km/hr to the Isle of Wight.

Anticyclones

• An anticyclone is a large mass of subsiding air, leading to high pressure on the Earth's surface.
• Originates from the upper atmosphere with limited water vapor.
• Air warms at the DALR (Dry Adiabatic Lapse Rate) during descent, resulting in dry conditions.
• Pressure gradients are gentle, causing weak winds or calm conditions.
• Winds blow outwards and clockwise in the Northern Hemisphere.
• Anticyclones can be 3000 km in diameter.
• Can persist for days or weeks, leading to settled weather.
• Weather conditions differ between summer and winter anticyclones.

Weather conditions over Britain

• Summer:
  • Intense insolation due to absence of cloud cover, leading to hot, sunny days (up to 30°C in southern England).
  • Absence of rain.
  • Rapid radiation at night can cause temperature inversions, dew, and mist.
  • Coastal areas may experience advection fogs and land/sea breezes.
  • Highlands experience mountain and valley winds.
  • Tc air mass (from North Africa) can lead to heatwaves and thunderstorms (Spanish plume).
• Winter:
  • Cloudless skies but little incoming radiation due to low sun angle.
  • Low temperatures and development of fog and frost at night.
  • Fog and frost may persist during the day.
  • Pc air mass (from central Asia) is cold, dry, and stable until it reaches the North Sea, causing heavy snowfalls on the east coast.

Blocking Anticyclones

• Occur when high-pressure cells detach from major subtropical or polar high-pressure areas.
• They can last for several days and block eastward-moving depressions.
• This leads to anomalous weather conditions, such as extremes in temperature, rainfall, and sunshine.
• Examples: Summer of 1995 and winter of 1987 in Britain.

Tropical Cyclones

• Intense low-pressure systems known as hurricanes, typhoons, and cyclones.
• Characterized by extreme winds and torrential rainfall.
• Develop over warm tropical oceans (sea temperatures exceeding 26°C).
• Occur in autumn when sea temperatures are highest.
• Form in the trade wind belt between latitudes 5° and 20° north or south of the Equator.
• Move westwards, often erratically, and then poleward upon reaching land.
• Dissipate rapidly over land due to energy dissipation.
• Another mechanism to transfer surplus energy away from the tropics.

Hurricanes

• Tropical cyclones of the Atlantic.
• Form after the ITCZ moves to its northernmost extent.
• Can have a diameter of up to 650 km.
• Occur when temperature, pressure, and humidity are uniform over a wide area in the lower troposphere.
• Anticyclonic conditions exist in the upper troposphere.
• Require a continuous heat source and a large moisture supply.
• A single hurricane can release energy equivalent to 500,000 atomic bombs (Hiroshima size) in a day.
• The central eye is an area of subsiding air with light winds, clear skies, and anomalous high temperatures (30-50 km in diameter).
• Decline rapidly when the heat source is removed (colder water or land).
• Average lifespan is 7 to 14 days.
• High winds (exceeding 160 km/hr, sometimes 300 km/hr).
• Ocean storm (tidal) surges can inundate coastal areas.
• Flooding from storm surges or torrential rainfall.
• Landslides can result from heavy rainfall on steep slopes.

Typhoon Warning System, Hong Kong

• Hong Kong uses a typhoon warning system to alert the public to the threat of tropical cyclones.
• The warning system consists of different signals, each representing a different level of threat.
• Signal 1: A tropical cyclone is centred within about 800 km of Hong Kong and is a potential threat.
• Signal 3: Strong winds are expected or blowing, with sustained speeds of 41-62 km/hr and gusts that may exceed 110 km/hr.
• Signals 4-8: Gale or storm expected or blowing, with sustained wind speeds of 63-117 km/hr and gusts that may exceed 180 km/hr.
• Signal 9: Gale or storm expected to increase significantly in strength, with sustained wind speeds expected to reach 88-117 km/hr in the next few hours.
• Signal 10: Hurricane-force winds expected or blowing, with sustained wind speeds reaching upwards from 118 km/hr and gusts that may exceed 220 km/hr.

Hurricane Ivan, September 2004

• Grenada: Hit on September 5, 2004, with 34 deaths and two-thirds of the island's residents left homeless.
• Jamaica: Hit on September 11, 2004, with 20 deaths.
• Cayman Islands: Hit a day later as a category 5 event with winds reaching 260 km/hr, but with no reported deaths.
• Cuba: Veered at the last minute, passing just to the west of the island after 2 million people were evacuated.
• USA: Made landfall between Mobile (Alabama) and Pensacola (Florida) on September 16, 2004, with wind speeds of 210 km/hr and a tidal surge of 4 m, resulting in 12 deaths.

Tropical Cyclone Nargis, Myanmar, May 2008

• Hit the Irrawaddy delta with wind speeds of 200 km/hr.
• A tidal surge created devastation of tsunami proportions.
• Crops, shrimp farms, and fishing boats were destroyed.
• Huge areas were left without fresh water, electricity, or transport.
• The death toll was estimated to be up to 200,000.

The Monsoon

• Derived from the Arabic word for 'a season'.
• Denotes a seasonal reversal of wind direction.
• Major monsoon occurs in south-east Asia and results from:
  • Extreme heating and cooling of large land masses.
  • Northward movement of the ITCZ.
  • Uplift of the Himalayas.

The South-West or Summer Monsoon

• As the sun moves northwards, the ITCZ follows.
• Insolation over northern India, Pakistan, and central Asia causes heated air to rise, creating low pressure.
• Warm, moist Em and Tin air from the Indian Ocean is drawn northwards and then north-eastwards due to the Coriolis force.
• Air is humid, unstable, and conducive to rainfall.
• Precipitation is substantial on India's west coast and on the windward slope of the Himalayas.
• Bombay has 2000 mm and Cherrapunji 13 000 mm in four summer months.
• Monsoon storms allow rice planting.
• 'Wet' monsoon is maintained by the release of substantial amounts of latent heat.
• Average arrival date is 10 May in Sri Lanka and 5 July at the Pakistan border.

The North-East or Winter Monsoon

• During the northern winter, the sun, the ITCZ, and the subtropical jet stream move southwards.
• Central Asia experiences intense cooling, allowing a large high-pressure system to develop.
• Airstreams moving outwards from this high-pressure area are dry.
• They become even drier as they cross the Himalayas and adiabatically warmer as they descend to the Indo-Gangetic plain.
• Bombay receives less than 100 mm of rain during these eight months.
• The south-west monsoon retreats from the extreme north-west of India on 1 September and takes until 15 November to clear the southern tip.
• Monsoon rainfall is unreliable.
• Failed rains lead to drought and famine.
• Excessive rainfall leads to extreme flooding.

Meso-scale: Local Winds

The Land and Sea Breeze

• A circulation system resulting from differential heating and cooling between land and sea.
• Pressure differences produce gentle breezes affecting coastal areas during calm, clear anticyclonic conditions.
• During the day, the land heats up rapidly, leading to lower pressure and a gentle breeze from the sea to the land.
• At night, the sea retains heat longer than the land, resulting in a reversal of the pressure gradient and wind direction.
• The land breeze is gentler than the sea breeze.

The Mountain and Valley Wind

• Occurs in mountainous areas during calm, clear, settled weather.
• During the morning, valley sides are heated by the sun, causing the air to heat, expand, and rise (anabatic wind).
• In the evening, the valley loses heat through radiation, causing the surrounding air to cool and become denser.
• Cold air drains down the valley sides and along the valley floor as a katabatic wind.
• Maximum wind speeds are generated just before dawn.

Föhn

• A strong, warm, and dry wind that blows to the lee of a mountain range.
• Occurs when a depression passes to the north of the mountains and draws in warm, moist air from the Mediterranean.
• As the air rises, it cools at the DALR of 1°C per 100 m.
• If condensation occurs, latent heat is released, and the air cools more slowly at the SALR of 0.5°C per 100 m.
• The descending air is compressed and warmed at the DALR.
• Temperatures may rise by 20°C within an hour, and relative humidity can fall to 10 per cent.

Microclimates

• The study of climate over a small area.

Urban Climates

• Large cities and conurbations experience climatic conditions that differ from those of the surrounding countryside.
• Generate more dust and condensation nuclei.
• Create heat.
• Alter the chemical composition and moisture content of the air.
• Affect albedo and airflow.

Temperature

• Urban heat island effect: Temperatures are highest in the city center and decrease towards the suburbs.
• Daytime temperatures are, on average, 0.6°C higher.
• Night-time temperatures may be 3° or 4°C higher.
• Mean winter temperature is 1° to 2°C higher.
• Mean summer temperature may be 5°C higher.

Sunlight

• Cities receive less sunshine and more cloud.
• Dust and other particles may absorb and reflect up to 50 per cent of insolation in winter and 5 per cent in summer.
• High-rise buildings also block out light.

Wind

• Wind velocity is reduced by buildings.
• Mean annual velocities may be up to 30 per cent lower than in rural areas.
• High-rise buildings create 'canyons' through which wind may be channeled.

Relative Humidity

• Relative humidity is up to 6 per cent lower in urban areas.