Weather & Climate Definitions

o Weather: State of the atmosphere at a place over a short period of timeo Climate: Average state of atmosphere at a place over a long period of time

(usually over 30 years)

Layers of the atmosphereo Troposphere (0km – 15km)

Temperature decreases as altitude increases Rate of decrease is called the environmental lapse rate or the normal

lapse rate; decreases at 6.5℃ per 1kmo Stratosphere (15km – 50km)

Temperature increases as altitude increases Known as temperature inversion

o Occurs as upper layers absorb more UV lighto Mesosphere (50km – 90km)

Temperature decreases as altitude increases Air becomes rarefied and loses heat faster

o Thermosphere (90km – 600km) Temperature increases as altitude increases

Due to exposure to highly energetic solar radiation

Earth’s Energy Budgeto Definition

The balance between incoming solar energy and outgoing long wave heat energy on a global scale

o Solar energy is the main source of energy input into the earth-atmosphere system

o Only about half of incoming solar energy reaches the earth’s surface while the rest is lost by: Reflection and scattering by clouds and atmosphere (27%) Absorption by gases and particles in atmosphere (24%)

o Insolation occurs when earth’s surface absorbs the remaining shortwave radiation from the sun subsequently turned into long wave radiation and re-radiated as sensible heat energy

o Greenhouse effect Greenhouse gases cause the atmosphere to absorb heat energy as it is

being re-radiated to space from the earth’s surface, thereby increasing global average surface temperature

Goldilocks Principleo Earth’s atmosphere is composed of gases – like carbon dioxide and methane –

of just the right types and the right amounts to warm Earth to temperatures just right for life Comparing to Venus and Mars, Venus too hot and Mars too cold

Same types and % of gases but different atmospheric densities

Temperature Definition

o A measure of how hot or cold the atmosphere is at any point of time

Factors affecting temperatureo Latitude: temperature generally decreases from low-latitude (equatorial

regions) towards high-latitude (polar regions) Angle of the sun

The angle of the mid-day sun is higher at low latitudes, thus the greater the angle of incidence of solar radiation and the more concentrated the insolation received

Vertical or oblique sun’s rays Sun’s rays reach low latitudes more vertically, thus insolation

received is more concentrated and heating is more intense Thickness or distance of atmosphere travelled by Sun’s rays

Sun’s rays travel a shorter distance to reach low latitudes, thus less solar energy is lost by reflection, scattering and absorption by dust particles and water vapour in the atmosphere and more solar radiation reaches low latitudes

o Altitude: the greater the altitude, the cooler the air temperature Density of air

Air becomes less dense, more rarefied and contains less water vapour and dust particles heat escapes more easily

Air temperature decreases at a rate of about 6.5℃ per 1km, also called the normal lapse rate

Temperature inversion Temperature increases as altitude increases due to:

o Nocturnal cooling, where air near the ground surface is cooled rapidly while the air above remains relatively warm

o Katabatic winds, which blow down a valley such that air remains cool at the valley bottom

o Cold air mass undercutting warm air mass at a cold fronto Warm air mass overrides a cold air mass or occlusion

o Distance from sea: higher annual range of temperatures inland Different rates of heating and cooling of land and water surfaces

Water has a higher specific capacity than land Maritime climate (confined to coastal regions)

Onshore winds from sea lower summer temperatures and raise winter temperatures Smaller annual range of temperature

Continental climate Maritime effect is lost further inland, with hotter summers and colder

winters Larger annual range of temperatureo Aspect

Northern hemisphere, south-facing slopes warmer than north-facing ones and vice versa in the southern hemisphere Warmer slopes are so because they receive more direct solar radiation

and are protected from cold polar winds Less noticeable in tropical latitudes due to high angles of mid-day sun

Urban Heat Islands Definition

o Elevated temperatures in urban areas compared to rural surroundings

Factors affecting formation of UHIso Vegetation of area: Areas with reduced vegetation are warmer

Trees and vegetation provide shade, reducing surface temperatures Vegetation releases water to the air through evapotranspiration, thereby

dissipating ambient heat Urban areas are covered by dry and impervious surfaces that provide less

shade and moistureo Colour of surface

Darker surfaces are warmer (dark colours absorb more heat) Lighter surfaces are cooler (light surfaces reflect more heat)

o Heat capacities of surfaces Building materials, such as steel and stone, have higher heat capacities

than rural materials, such as dry soil and sand urban areas can store twice the amount of heat as compared to rural surroundings

o Surface Geometry Spacing of building

o Areas with buildings that are less spaced out are warmer More difficult for long wave radiation to escape at night

Height of buildings (sky view factor)o Provides shade less solar energy can reach the areao Conversely, more solar energy is absorbed and at night, long wave

radiation does not escape as easily temperatures increaseo Anthropogenic source of heat

Refers to heat produced by human activities (e.g. cars, industrial facilities, air conditioners)

Urban areas have a greater concentration of human activities Higher temperature

o Weather Calm winds and clear skies allow for maximum insolation Formation

of UHIs

UHI Singaporeo Effect of vegetation

“Hot” spots observed on exposed hard surfaces, such as the industrial area, airport and Central Business Districto Highest temperature of 28.3°C observed in a high density and high

rise commercial area – Central Business District “Cool” spots observed on large parks, landscape in-between housing

estates and the catchment areaso Lowest temperature of 24.3°C observed in a well-planted area – Lim

Chu Kang Difference of 4°C as compared to “hot” spots Temperatures vary from southern “urban” areas to northern “rural” areas

o Effect of colour Dark-colour materials can be up to 7℃ higher than air temperature

Light-colour materials only 2℃−3℃ higher than air temperature

Measures to mitigate effects of UHIo Urban greening: Reduced long wave radiation emitted Lower temperature

Parks Rooftop gardens Vertical landscaping: housing estates with 22% vegetation coverage had

lower ambient air temperature as compared to the one with only 7% o Use of lighter façade materials

Urban ambient temperature correlates to surface temperature of buildings Materials of lighter colours have higher albedo and can reduce solar heat

gain in daytimeo Reduction of 7.48% of cooling energy achieved when colour of

façade is changed from black to whiteo In narrow canyons where high albedo materials were used,

temperatures experienced were lower by up to 2.5℃o Strategic placement of tall buildings

High rise towers strategically placed around low rise buildings can enhance wind velocity within canyons

Strategic placement of tall buildings can cause strong airflow at the lower zone, thus dissipating large amounts of heat produced by asphalt roads

Wind velocity can be increased by up to 10 times and temperature reduced by up to 1.1℃ when wind flow is perpendicular to canyon

Humidity & Precipitation Definitions

o Humidity The amount of water vapour/moisture in the air

Air is unsaturated if it contains less moisture than the maximum amount it can hold at a particular temperature

When temperature falls and the amount of water vapour remains unchanged, until air is full of water vapour, air is saturated

o Absolute humidity The actual amount of water vapour in the air at a particular temperature

(measured in kg/m3)o Relative humidity

Relationship between the actual amount of water vapour in the air and the maximum amount of water vapour, which the air can hold at that particular temperature (expressed in %)

Forms of Precipitationo Rain

Relief Rainfall (Orographic Rain) Onshore winds from the sea bring moisture Reaching land, they are blocked by mountain ranges / relief barriers

o Air is forced to rise, expands due to decrease of air pressureo Due to expansion, air cools adiabatically

Air becomes saturated at dew point, relative humidity reaches 100% Water vapour condenses, forming water droplets

Continual condensation results in bigger droplets in clouds on the windward slope

The leeward side of a mountain forms a rain shadow areao E.g. in SE Australia, the Murray Basin is shadowed by the

Eastern Highlands from the SE onshore winds If the mountain is very high, after crossing the mountain, the

descending dry air is compressed and becomes warm adiabaticallyo Forms warm dry wind called fohn wind or chinook in leeward

side E.g. Calgary in the Rockies in Canada

Convectional Rainfall Commonly occurs in warmed or heated areas Air layer near the surface is heated up due to intense insolation Warm air expands, and, being lighter than the surrounding air, rises Rising air cools adiabatically to its dew point at its condensation

level Tall, tower clouds (cumulonimbus) may be formed rapidly by

uprising, moisture-laden currents Convection rain is usually associated with tropical depressions

(typhoons) and thunderstormso Also common in the intertropical convergence zone (ITCZ)

Characteristics

o Almost daily occurrence and even distribution throughout the year (in equatorial and tropical regions)

o Usually occurs in the afternoono Brings intense but short-lived showers which last for minuteso Raindrops are usually larger (i.e. heavy downpours)

Frontal Rainfall Occurs along fronts

o Fronts formed by the meeting of a tropical maritime air mass and a polar air mass moving in opposite directions (warm meet cold)

Cold air is denser and heavier; it sinks and remains close to the ground

Warm moist air rises over the cold air resulting in frontal uplifto Warm air expands and cools adiabatically to its dew pointo Becomes saturated with relative humidity reaching 100%o Condensation takes place and clouds formo Rain finally falls, known as frontal rain

Characteristicso Usually associated with temperate depressions (temperate

cyclones)

o Steady, persistent and lasts for several hours to several days Similarities amongst 3 types

Involveo Warm moist air rising cooling condensation

precipitation Differences amongst 3 types

The reason the air is risingo Relief: forced to rise by mountainso Convectional: air is being heatedo Frontal: warm air is rising over cold air

o Snow Formed by condensation of water vapour in the atmosphere below 0℃ Water vapour passes directly from gaseous to solid state Snow crystals are flat hexagonal plates with infinite variations in pattern Snow may either by dry and powdery or wet and compact

o Sleet Mixture of rain and snow Formed when snow falls through a layer of warm air, or when raindrops

have frozen in the atmosphere and partially melt as they fallo Hail

Ice-pellets with a diameter of 5mm or more Form of precipitation associated with thunderstorms Formed by the following process

Convection currents with moisture rise rapidly; condensation occurs Condensed droplets are carried upward by air currents; freezing

occurs around an “embryo” (frozen droplet in a cloud) As ice crystals fall, more water vapour condenses around them into

ice; Size of pellets increases Violent uprising air currents carry the ice crystals upward and

downward in the cumulonimbus cloud, until size increases and ice falls

Ice crystals do not melt by the time they reach the ground = hailstones

Air Pressure & Wind Definitions

o Air pressure The force exerted by atmospheric gases at a particular point

o Wind Air that is moving from high pressure to low pressure

Wind formationo Pressure gradient force

Force that oversees the movement of air from a region of higher pressure to one of lower pressure

o Coriolis force Wind is deflected as it moves across Earth due to Earth’s rotation

Deflection occurs to the right in the northern hemisphere and to the left in the southern hemisphere

Planetary wind beltso Equatorial Lows/Doldrums (5°N to 5°S)

Region of low pressureo Equatorial Sub-Tropical High / Horse Latitudes (30 °N∧30 ° S¿

Region of high pressureo Sub-Polar Lows (60 ° N∧60 ° S¿

Region of low pressureo Polar Highs (90 ° N∧90 ° S¿

Region of high pressure

Breezeso Sea Breeze (Day)

Specific heat capacity of water is higher than that of land Land heats up faster than sea Warm air at land expands due to heat, therefore pressure is low and

air rises Advection currents bring warm air over the sea As air contracts due to the cool sea, pressure is high and air sinks Air travels from high pressure (the sea) to low pressure (the land),

thus the sea breeze is formed

o Land Breeze (Night) Specific heat capacity of water is higher than that of land Land cools down faster than sea Warm air at sea expands due to heat, therefore pressure is low and air

rises Advection currents bring warm air over the land As air contracts due to the cool land, pressure is high and air sinks Air travels from high pressure (the land) to low pressure (the sea),

thus the land breeze is formed

Tropical Monsoons January (Winter Monsoon/Northern Monsoon)

o Occurs from November to Februaryo Sun is directly overhead the Tropic of Capricorn (23.5°S)

Southern hemisphere experiencing summer, higher temperatures which lead to expansion and rising of air Lower pressure region Large-scale movement of air from higher pressure regions in

North to lower pressure regions in South Results in Northeast Monsoon in northern hemisphere and

Northwest Monsoon in southern Hemisphereo Impact of rainfall on

Singapore Experiences more rainfall than usual, but less than the East Coast

of Peninsular Malaysia Peninsular Malaysia

Northeast Monsoon picks up moisture over the South China Sea and brings heavy rainfall to the East Coast of Peninsular Malaysia

West Coast experiences higher rainfall than usual but less compared to East Coast, since the Central Highlands shield the West Coast from much rainfall

India Little rainfall in most areas as Northeast Monsoon comes from

over Central Asia, a continental area with little moistureo Only Southeast India experiences heavy rainfall due to

Northeast Monsoon picking up moisture over Bay of Bengal

July (Summer Monsoon/Southern Monsoon)o Occurs from June to Septembero Sun is directly overhead Cape of Cancer (23.5° N)

Northern Hemisphere experiencing summer, higher temperatures which lead to expansion and rising of air Lower pressure region

Large-scale movement of air from higher pressure regions in South to lower pressure regions in North

Results in Southwest Monsoon in the northern hemisphere and Southeast Monsoon in the southern hemisphere

o Impact of rainfall on Singapore

Experiences more rainfall than usual, but less significant than Northeast Monsoon

Peninsular Malaysia Experiences more rainfall than usual, but less significant than

Northeast Monsoon India

Heavier rainfall on the West Coast of India due to Southwest Monsoon picking up moisture over the Indian Ocean

Northeast of India also experiences heavy rainfall as Southwest Monsoon picks up moisture over the Bay of Bengal