Atmospheric Processes, Hazards and Management Structure and Composition of the Atmosphere



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Atmospheric Processes, Hazards and Management

Structure and Composition of the Atmosphere

  1. Vertical Stratification

    1. Troposphere

  • Bottom most layer, 80% of atmosphere, most of water vapour and dust

  • Vertical mixing of air, all weather occurs here

  • Temperature decreases with height, 15C near ground, -57C at tropopause

  • Due to lesser terrestrial radiation and less dense air

  • Reaches 16 km in tropics and 9km in polar regions

    1. Stratosphere

  • 19.9% of atmosphere

  • Temperature constant until 20km, starts to increase until stratopause at 50km

  • Due to ozone layer, which absorbs UV rays. Lesser UV with decreasing height.

  • No precipitation – particulates and aerosols stay for long time

  • Strong winds (jet streams) distribute aerosols all over the world

    1. Mesosphere

  • Temperature decreases with height because of decreasing density.

  • At 80km, about -90C

    1. Thermosphere

  • Minimal mass of atmosphere, pressure about one-millionth of at sea level

  • Temperature increases with height due to insolation

  1. Atmosphere’s Gaseous Components

  • Atmosphere composed of mixture of gases and microscopic solid particles and water droplets

    1. Permanent Gases

  • Nitrogen and oxygen make up 99% of clean, dry air

    1. Variable Gases

      1. Water Vapour

  • Source is evaporation from Earth’s surface, decreases rapidly with height

  • Most found in lowest 5km of atmosphere

  • Source of moisture to form clouds, also a greenhouse gas

      1. Carbon Dioxide

  • 0.037% of the atmosphere

  • Supplied via the carbon cycle – decomposition, respiration, photosynthesis

  • Absorbs radiation, greenhouse gas

Earth’s Energy Budget and Regional Temperature Variations

  1. Earth’s Energy Budget

  • Variation in solar energy received from places results in movement of air from one place to another, resulting in atmospheric circulation and oceanic circulation

    1. Global Energy Balance

  • Balance exists between insolation and outgoing terrestrial radiation so that the net radiation is zero

  • Insolation is scattered by gas and dust particles in the atmosphere, reflected by clouds and Earth’s surface, and absorbed by atmosphere and Earth’s surface

  • Outgoing radiation is released from the atmosphere and Earth’s surface, and removed by wind and condensation

      1. Scattering

  • Some insolation will be scattered in all directions until it reaches Earth’s surface or returns to space

  • This occurs because radiation travels in a straight line

      1. Reflection

  • Most important loss of insolation

  • Light coloured and smooth, shiny surfaces will reflect more radiation

  • Albedo – snow is about 90%, at low angles water is nearly 80%

  • In general albedo of sea lower than land

      1. Absorption

  • Some gases in atmosphere absorb certain wavelengths of radiation, which gain energy and heat up

  • Insolation is converted to long-wave radiation to warm atmosphere

  • Greenhouse gases, oxygen, ozone, carbon dioxide, water vapour

      1. The Greenhouse Effect

  • Terrestrial radiation has longer wavelengths due to being cooler that sun

  • Atmosphere absorbs long wave easier than short wave – greenhouse effect, gases absorb and re-emit radiation to warm the Earth

    1. Poleward Heat Transfer

  • Net radiation for Earth is zero, but not in all latitudes. Most energy received at tropics.

  • In poles, low angle of sun, greater albedo of snow and ice, thicker atmosphere for sun to pass through, insolation is much smaller.

  • Major equalising factor is transfer of heat by air movement, creating air currents, winds and ocean currents and weather

      1. Latitudinal Difference in Insolation amount

  • Beam spreading: at tropics, sun directly overhead, insolation more concentrated, more radiation per unit area. With increasing latitude, solar radiation reaches surface at lower angle, spreading out more, reducing insolation per unit area.

  • Passing through atmosphere: at higher latitudes, lower angle of approach means travelling through more atmosphere. More energy is absorbed, scattered or reflected at polar areas than equatorial areas.

  • Albedo: higher at poles due to snow, ice, and low angle over water

      1. Latitudinal Radiation Balance

  • Difference between incoming and outgoing radiation is radiation balance

  • Positive budget in tropics (heat surplus) and negative budget in poles (heat deficit)

  • Redistribution of heat via advection of winds and ocean currents

  1. Regional Temperature Variations

    1. Proximity to Sea

  • Maritime / oceanic areas have less seasonal contrasts, milder seasons

  • Moderating effect of oceans – cooler summers and warmer winters

  • Water has 5 times specific heat capacity than land, heats and cools slower

  • Radiation can penetrate water up to several meters, distributing throughout larger mass. In land, restricted to a thin, opaque layer

  • Warming of water reduced considerably due to energy used for evaporation

  • Water can be mixed vertically and horizontally

    1. Ocean Currents

  • Warm currents move poleward in western portions of ocean basins near east coasts of continents. e.g. Gulf Stream

  • Eastern margins of oceans, cold ocean currents travel equatorwards. e.g Labrador Current

    1. Altitude

  • Temperatures decrease with altitude – every 1km = 10C drop

  • Low altitude – heat escapes slowly due to dense air, containing dust and water vapour, retaining hear

  • High altitude – thinner air, heat escapes rapidly

    1. Cloud Cover

  • Cloud cover decreases insolation reaching surface and amount leaving surface

  • Due to reflection, high albedo of white clouds

    1. Aspect

  • Direction a place faces, noticeably in temperate latitudes.

  • South facing slopes / adret slopes are warmer than north facing slopes / ubac slopes in the Northern Hemisphere. Vice versa in Southern.

  • The warmer slope receives midday sunlight at a more direct angle

Atmospheric/Oceanic Circulations and the Occurrence of Seasons

  1. Global Atmospheric Circulation

  • Unequal heating of Earth’s surface leads to thermally direct circulations

    1. The Tri Cellular Model

  • Proposed by William Ferrel, incorporating Hadley’s ideas.

    1. Forces Influencing Air Movement

      1. Pressure Gradient Effect

  • In denser atmosphere, lateral pressure is great. Less dense, lesser pressure. Horizontal pressure difference, air pushed from high pressure to lower pressure, called the pressure gradient effect.

  • Greater the difference, greater the wind speed. Unequal heating of land-sea and different latitudes creates this difference.

  • Magnitude determines by spacing of isobars. Direction always high to low pressure at right angles to isobars.

      1. Coriolis Effect

  • Causes winds to appear to deflect to the right in the Northern Hemisphere and to the left in the Southern Hemisphere

  • Greatest at poles, weakens equatorward where it becomes nonexistent

  • Always deflected at right angles to direction of air flow

  • Only affects direction, not speed

  • Affected by wind speed – stronger the wind, greater the deflection

      1. Friction

  • Opposes pressure gradient to moderate and slow airflow

  • Significant influence near surface, but negligible a few km above

      1. Geostrophic Wind

  • Balance between Coriolis effect and pressure gradient

  • Pressure gradient perpendicular to isobars at starting point. Coriolis no effect because it is stationary.

  • As air accelerates perpendicular to isobars, Coriolis intensifies since it depends on wind speed. Deflects it until it is parallel to isobars, balance

  • Geostrophic winds flow straight, parallel to isobars, velocity proportional to pressure gradient

  • An idealised model which approximates airflow aloft

      1. Surface Wind

  • At surface, friction slows movement of air, reducing Coriolis as well.

  • Pressure gradient not affected by friction, so stronger than Coriolis

  • Air moves at an angle across isobars

    1. The Major Wind Belts

      1. Hadley Cells and the Trade Winds

  • Between equator and 30 latitude. Produced thermally, stronger in winter when temperature gradients are stronger

  • At equator, warm rising air releases latent heat, provides energy to drive cell. At upper flow, air moves poleward, acquiring westerly movement due to Coriolis.

  • At subtropical highs, air subsides due to radiation cooling away from the equator, increasing density

  • Air is very dry, and adiabatic heating reduces relative humidity further. Dry subsidence zones create the world’s subtropical deserts

  • Surface flow splits into poleward and equatorward branches. Equatorward turns right by Coriolis to from trade winds.

  • Both Hadley Cells converge at the Intertropical Convergence Zone (ITCZ)

  • ITCZ shifts north and south of the equator, but north more (30-40N compared to 5S) due to more land in north hemisphere

      1. Ferrel Cells and the Westerlies

  • Poleward from subtropical highs is the Westerlies

  • Prevailing westerlies, west only on average. Cyclones and anticyclones disrupt the westerly movement

  • Thermally indirect cells caused by turning of the other two

      1. Polar Cells and the Polar Easterlies

  • Subsidence near poles at polar highs produce surface flow which moves equatorward and deflected into polar easterlies

  • They meet the warmer westerly wind at the subpolar lows, or polar fronts

    1. The Tri Cellular Model vs Reality

  • The Hadley model provides a good account of low-latitude motions, but the Ferrel and Polar cells not very well represented in reality, due to local factors

  • With regards to upper-level motions, not realistic.

    1. Observed Pressure and Wind Patterns

      1. Zonal vs Cellular Distribution of Pressure

  • Due to variations in land-sea surfaces, pressure systems rarely form continuous belts around the earth.

  • In northern hemisphere, with higher proportion of land, the zonal pattern is replaced by semi-permanent cells of high and low pressure.

  • High pressure cells are anticyclones, low pressure cells are cyclones

  • Air spirals clockwise out of anticyclones and anticlockwise into cyclones in the northern hemisphere, vice versa for south.

  • Changes in highs and lows affected by seasonal temperature changes

      1. Seasonal Changes in Global Pressure/Wind Patterns

  • Seasonal change in pressure more noticeable in northern hemisphere

  • Subtropical anticyclones: centred between 20 and 35 latitude over larger oceans, like Hawaiian and Bermuda-Azores highs

  • More to east of oceans because of the cold currents

  • Seasonal fluctuations: January, strong Siberian High. Subtropical anticyclones weaken, and two intense cyclones (Aleutians Low and Icelandic Low) dominate

  • In summer, Siberian High replaced by Tibetan Low. Icelandic low weakens, Bermuda-Azores High strengthens due to change in temperature contrasts

    1. Monsoon Circulation

  • Pronounced seasonal reversal in wind direction

  • January – ITCZ in southern hemisphere, Siberian High produces cool, dry, continental air

  • July – ITCZ in northern hemisphere, Tibetan Low draws warm moist air from oceans onto Asia, producing summer rains

  1. Global Oceanic Circulation

  • Accounts for ¼ of total heat transport

  • Atmosphere and ocean in contact, energy passed from moving air to water

  1. Seasonal Climatic Variations

  • Latitudinal differences, tilt and revolution of Earth around sun causes seasons

    1. Revolution of the Earth and Seasonal Climatic Changes

  • Earth’s axis is tilted 23½ from perpendicular in relation to orbit

  • Change in orientation throughout orbit causes the subsolar point to migrate annually from Tropic of Cancer in N to Tropic of Capricorn in S.

  • Causes angle, and solar radiation to vary within a year, causing seasons in mid-latitudes

    1. Solstices and Equinoxes

  • June 21/22 and December 21/22 are solstices, when the subsolar point is at Tropic of Cancer or Capricorn

  • March 21/22 and September 22/23 are equinoxes which occur halfway between the solstices. the subsolar point hits the equator.

    1. Seasonal Variation in Daylight Hours

  • Length of daylight vs darkness at a latitude depends on the position of Earth in orbit

  • June solstice, all locations in northern hemisphere experience longer periods of daylight than darkness, further north = longer daylight. Vice versa in December solstice. Southern hemisphere is other way round.

  • Equinox – all latitudes experience 12 hours of daylight and darkness

Upper Atmosphere Wind and Jet Streams

  1. The Westerlies

  • Airflow in the middle and upper troposphere is westerly. As cold air is more dense and compact, air pressure decreases more rapidly when cold than warm

  • At same altitude over Earth’s surface, higher pressure exists over tropics than at poles

  • Pressure gradient directs air from equator to poles, turned by Coriolis to generate westerly winds (geostrophic winds)

  • Pressure gradient increases with altitude, and decreasing friction also contributes to fast wind speed

  1. Jet Streams

  • Narrow ribbons of high-speed winds

    1. Origin of Jet Streams

  • Greater temperature contrasts at surface produce greater pressure gradients aloft and faster upper air winds

  • Large wintertime temperature contrasts occur along fronts, resulting in very fast westerly winds

    1. The Polar Jet Stream

  • The polar jet stream often meanders

  • Travels at 125km/h in winter and about 60km/h in summer, due to stronger temperature contrasts in winter

  • Position also changes – during winter, the jet stream may extend south till 30N

  • Polar jet migrates between latitudes of 30 and 79, mid latitude jet stream

    1. Jet Stream and Weather

      1. Jet Streams and Seasonal Cyclonic Activities

  • As polar jet shifts northwards from winter to summer, regions where thunderstorms and tornadoes also shift northwards

  • More cyclones are generated in late summer and early autumn when temperature contrasts are greater

  • Jet streams supply energy to circulation of surface storms

      1. Jet Streams and Short Term Cyclonic Activities

  • Can change positions on shorter time scales as well

  • When jet stream does not meander, relatively mild temperatures occur and few disturbance south of jet stream

  • When meander, large amplitude waves form and a general north-south flow occurs, allowing cold air to advance southward, intensifying temperature gradient and strengthening flow aloft

  • Cyclonic activities redistribute heat, resulting in weaker temperature gradient, weaker flow and less intense weather at surface

  • Cycles of calm and stormy weather can last 1 to 6 weeks

  • When air moves out of trough, accelerates, drawing air from lower atmosphere causing reduction in surface pressure. Air converges at ground within depression

  • If divergence exceeds convergence = low pressure, if convergence exceeds divergence = high pressure

  • In N hemisphere, troughs and ridges tend to favour certain locations

      1. Jet Streams and Temperature Variations

  • If jet stream south of location, temperature will be colder and stormier

  • If north, warmer and drier

  • Wave development affects local temperatures

  • If position of jet stream remains fixed for long periods, weather extremes can result

  1. Rossby Waves

  • Westerlies follow wavy paths with long wavelengths and varying amplitudes

  • A wave effect as a result of the polar jet stream

  • Air flows eastward, but occasionally remain stationary or even slowly from west to east

  • Flow aloft is not perfectly circular, amplitudes distribute air temperature

  • Surface features also play a role in the development of Rossby Waves

Precipitation Formation

  1. Evaporation

  • Energy required to excite water molecules is latent heat of vaporisation

  • Depends on the vapour pressure deficit, the difference between saturation and actual vapour pressure

  • When air is saturated, no evaporation can occur

  1. Condensation

  • Air should be saturated for condensation to take place

  • There generally must be a surface on which water vapour condenses

    1. Saturation of an Air Parcel

  • Air needs to reach saturation point, either by cooling below dew point, or add water to air parcel

  • Cooling of air by expansion (adiabatic) to form clouds is more common. Radiative cooling at a cold ground surface forms fog.

    1. Role of the Condensation Nuclei

  • At dew point, condensation occurs around particles in the air known as condensation nuclei

  • Sources are dust, volcanic material, smoke and sea salt

  • Some nuclei particularly attract water, such as sea salt. Hygroscopic nuclei. Air can condense even when not saturated

  1. Environmental Lapse Rate

  • Normally, air temperature falls with increasing height in the troposphere, since heating is greatest at ground surface

  • Rate at which temperature falls is the environmental lapse rate (ELR)

  • ELR depends on atmospheric and surface conditions

  • Strong winds can mix air and lower ELR. Strong surface heating can cause ELR to be steeper.

  • ELR influences air movement and determines if atmosphere is stable

  • Inversions exist where temperature increases with altitude, and are thus extremely stable and resist vertical mixing

  1. Atmospheric Stability/Instability

  • Parcel of air, Dry Adiabatic Lapse Rate (DALR) and Wet Adiabatic Lapse Rate (SALR)

  • Stable ELR falls slower than DALR and SALR

  • Unstable if ELR falls faster than DALR and SALR

  • Conditionally stable if ELR falls slower than DALR but faster than SALR

  1. Air Lifting Mechanisms

  • Rising air expands and cools adiabatically, reaches lifting condensation level, cools to dew point and condenses into clouds

    1. Convective Lifting

  • When air heated from below rises. Unstable air found in areas with strong surface heating such as the humid tropics

  • Solar heating and irregular surfaces cause irregular heating on ground and some areas to heat up more than others. These warm, unstable parcels of air are buoyed upward. If reach condensation level clouds form

  • Height of clouds produced is limited, as instability of surface heating is confined to first few km of atmosphere

    1. Orographic Lifting

  • Air mass forced to ascend elevated terrains such as mountains

  • Adiabatic cooling encourages condensation, generating clouds and precipitation on the windward slope

  • On leeward side, most moisture has been lost, and with further adiabatic heating upon descending, condensation and precipitation is less likely, and can cause rainshadow deserts

    1. Frontal Lifting

  • Transition zones where great temperature differences occur across short distances are called fronts

  • Cold air advances towards warm air, displacing it up

  • Warm air advances towards cold air, forced upwards

    1. Convergence

  • When low pressure cell is at surface winds converge on centre of low from all directions, rising and cooling

Climatic Zones of Tropical Africa and Asia

  1. The Koeppen Classification System

  • Based on vegetation, monthly mean temperature and precipitation

    1. The Main Climatic Groups

  • A – Tropical. Climates in which temperature for all months is greater than 18C

  • Almost confined between Tropics of Cancer and Capricorn

  • B – Dry

  • Potential evaporation exceeds precipitation

  • Only B considers precipitation, designating deserts and semi-deserts. The rest are classified by temperature.

    1. The Sub-Climatic Groups

  • Af, Am, Aw, and BWh

  1. Features of Climatic Zones in Africa and Asia

  • A climates are warm year round with minor variations in temperature. They are distinguished by their degrees of precipitation seasonality. Af has no dry season, Am are monsoonal with short dry season of 1-3 months, Aw have a distinct dry season coinciding with subtropical high pressure of the Hadley cell

  • B are divided into BW (true deserts) and BS (semi-deserts)

    1. Tropical Wet/Rainforest Climate (Af)

  • Found in equatorial regions, around 10 N and S. Constant influence of ITCZ, with a lot of convectional rain due to strong solar heating

      1. Precipitation Characteristics

  • High total annual rainfall, distributed nearly uniformly

      1. Temperature Characteristics

  • Uniformly high temperatures, but not the highest on earth, due to moisture available for evaporation and cumulus clouds reflecting and scattering insolation

  • Diurnal range is lower due to high humidity and cloud cover. Diurnal range normally exceeds annual range

      1. Natural Vegetation

  • Prevalence of tropical rainforests, with dense tree cover and species diversity

    1. Tropical Monsoon Climate (Am)

  • Transition between Af and Aw. Occur along tropical, coastal areas and predominant onshore winds supply warm, moist air to region.

      1. Precipitation Characteristics

  • Not very far inland due to offshore air required. Rainfall enhanced by orographic uplift. Surface heating is less a factor. Hurricanes can also bring heavy rainfall

  • Precipitation not as steady throughout year. Annual precipitation are among highest in world due to very heavy rainfall during wet months

      1. Temperature Characteristics

  • All months warm, little variation. Variation associated with timing of precipitation. Warmest months just before precipitation season due to lesser cloud cover

      1. Natural Vegetation

  • Dense forests, lush vegetation. Diversity not as abundant as Af, but much more living matter

    1. Tropical Wet and Dry/Savanna Climate (Aw)

  • Along poleward margins of the tropics, most extensive in southern Africa.

  • Farther from Equator, more seasonality in precipitation and temperature

      1. Precipitation Characteristics

  • When ITCZ is near in summer, convectional rain is favoured

  • When ITCZ shifts south in winter, subtropical high brings descending air, stable atmosphere and lack of precipitation.

  • Being further from Equator means being closer to the subtropical high. Occasional hurricanes also brings precipitation

  • Annually, lesser precipitation than Af or Am

  • Considerable year-to-year variability in precipitation. Droughts or flooding possible

      1. Temperature Characteristics

  • Monthly mean temperatures more variable, about 3-10C

  • Diurnal variations are greater as well, especially during dry season due to lack of cloud cover

      1. Natural Vegetation

  • Savanna – grasses with widely separated trees

  • Recurrent fire, water logged soil and hard layers developing in soil

    1. Subtropical Deserts (BWh)

  • Subtropical deserts – subsidence of the Hadley cell at the subtropical highs

  • Adiabatic heating with descending air increases air temperature, reduces relative humidity and a stable atmosphere which impedes condensation and cloud formation

  • Some coastal deserts such as the Atacama Desert in Chile occur, along cold ocean currents on west coast of continents. Air flows out of subtropical high pressure system, cooling over the Peruvian Current, bringing air temperature down to dew point and creating fog via contact cooling, lowering the ELR and causing stability, which suppresses uplift and cloud and precipitation formation

      1. Precipitation Characteristics

  • Very little precipitation over the year – any rainfall occurs during the winter months

  • Greatest amount of inter-annual variability of rainfall – annual average is poor indicator of rainfall in a single year

      1. Temperature Characteristics

  • Diurnal temperature range can be very large, due to low humidity, high sun and clear skies allowing solar radiation to be relatively unhindered

  • Low soil moisture means little heat is lost in evaporation

  • Similarly, no cloud cover and low humidity means heat is lost quickly at night

  • The same applies to the annual temperature range

  1. Delineating the Boundaries of the Climatic Zones

  • A strength of the Koeppen system is that boundaries are determined easily, but these boundaries should not be seen as fixed and unchanging

  • All boundaries shift their positions constantly – current boundaries are only averages based on data collected. Thus, they should be viewed as broad transitions zones instead of sharp lines


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