Variation in solar energy received from places results in movement of air from one place to another, resulting in atmospheric circulation and oceanic circulation
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
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
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
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
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
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
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
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
Regional Temperature Variations
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
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
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
Cloud Cover
Cloud cover decreases insolation reaching surface and amount leaving surface
Due to reflection, high albedo of white clouds
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
Global Atmospheric Circulation
Unequal heating of Earth’s surface leads to thermally direct circulations
The Tri Cellular Model
Proposed by William Ferrel, incorporating Hadley’s ideas.
Forces Influencing Air Movement
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.
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
Affected by wind speed – stronger the wind, greater the deflection
Friction
Opposes pressure gradient to moderate and slow airflow
Significant influence near surface, but negligible a few km above
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
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
The Major Wind Belts
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
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
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
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.
Observed Pressure and Wind Patterns
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
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
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
Global Oceanic Circulation
Accounts for ¼ of total heat transport
Atmosphere and ocean in contact, energy passed from moving air to water
Seasonal Climatic Variations
Latitudinal differences, tilt and revolution of Earth around sun causes seasons
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
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.
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
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
Jet Streams
Narrow ribbons of high-speed winds
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
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
Jet Stream and Weather
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
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
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
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
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
Condensation
Air should be saturated for condensation to take place
There generally must be a surface on which water vapour condenses
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.
Role of the Condensation Nuclei
At dew point, condensation occurs around particles in the air known as condensation nuclei
Some nuclei particularly attract water, such as sea salt. Hygroscopic nuclei. Air can condense even when not saturated
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
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
Air Lifting Mechanisms
Rising air expands and cools adiabatically, reaches lifting condensation level, cools to dew point and condenses into clouds
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
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
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
Convergence
When low pressure cell is at surface winds converge on centre of low from all directions, rising and cooling
Based on vegetation, monthly mean temperature and precipitation
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.
The Sub-Climatic Groups
Af, Am, Aw, and BWh
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)
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
High total annual rainfall, distributed nearly uniformly
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
Natural Vegetation
Prevalence of tropical rainforests, with dense tree cover and species diversity
Tropical Monsoon Climate (Am)
Transition between Af and Aw. Occur along tropical, coastal areas and predominant onshore winds supply warm, moist air to region.
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
Temperature Characteristics
All months warm, little variation. Variation associated with timing of precipitation. Warmest months just before precipitation season due to lesser cloud cover
Natural Vegetation
Dense forests, lush vegetation. Diversity not as abundant as Af, but much more living matter
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
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
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
Natural Vegetation
Savanna – grasses with widely separated trees
Recurrent fire, water logged soil and hard layers developing in soil
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
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
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
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
