Urban Meteorology II

October 15, 2007

• Urban areas represent vast tracts of impervious surfaces: roads, sidewalks, driveways, parking lots, roofs, and walls

• These alter cycles of energy, water, and chemicals between the land and atmosphere

Urban Heat Island

• Temperature differences between urban and rural areas are attributed to– Urban geometry

• Size, shape, and orientation of buildings and streets

– The nature of urban surfaces• Albedo, heat capacity, thermal conductivity, and

wetness• Suburbs release more sensible heat, while rural

areas release more latent heat making them cooler

Urban Heat Island• The urban-rural difference in surface

temperature is generally greatest during the day whereas the air temperature difference is greatest at night

• Within urban canyons, canyon walls may absorb reflected radiation

• Canyon width– Small width means more longwave radiation

• So milder temperatures

– Larger widths means more shortwave radiation

• So larger diurnal temperature differences

Forested vs Urban

• Forested areas hold water and have more evaporative cooling

• Urban areas have higher runoff rates and lower evaporative cooling and more sensible heating

• Deforestation leads to warming and dehumidifying of the atmosphere

Urban Surface Temperatures7:15am 9:45am 1:30pm 3:30pm

Air Temp 80ºF 89ºF 92ºF 93ºF

Stone Patio 88.5ºF 112ºF 121ºF 112ºF

Exposed Soil 87ºF 128ºF 143ºF 107ºF

Watered Grass

78ºF 89ºF 85.5ºF 74ºF

Watered Flower Bed

63ºF 82.6ºF 90ºF 82ºF

Dry Grass 71ºF 94ºF 102ºF 97ºF

North Wall 92ºF 91.6ºF 89ºF 89ºF

East Wall 132ºF 129ºF 91ºF 88.5ºF

South Wall 75ºF 99.5ºF 113ºF 99ºF

West Wall 74ºF 82ºF 111ºF 142ºF

* Suburban Colorado during summer

Urban Heat Island

• North American instantaneous temperature difference

• ΔT = 5.21 log10 (population) – 11.24

• Atlanta: 18.32ºC difference

• Carrollton: 11.37ºC

Urban Heat Island

• In addition to radiative heating, heating from:– Automobiles– Industrial processes– Power plants– Heating systems

• Solutions

Increasing Evaporative Cooling

• Urban parks– The larger the urban park, the spatially larger

the cooling effect

• Water sources– Rice paddies outside Tokyo

• The urban-rural temperature difference for a city increases with greater impervious surface area and less vegetated area

Urban Soil

• Urban soils more polluted, compacted, and eroded than rural soils

• Less able to absorb water

• Kudzu decreases latent heating provided by trees

• Impermeable surfaces

Planning• To minimize climate stress in hot, humid

regions– Reduce solar radiation gain– Ventilation

• Widely-spaced buildings• Wind-permeable roofs

– Minimize flooding hazards• Structures on stilts• Well-built overhanging roofs

Planning• In hot, arid climates

– Minimize solar gain in the summer and maximize it in the winter

• High-mass construction materials that absorb heat during the day and release it at night

– Minimize wind exposure during both seasons– Use of wind towers for cooling– Build within smallest possible space

• Narrow streets to provide shade

Planning

• Cold regions– Maximize solar gain

• Minimal external surface area• Use of high-mass materials• Dark outer surfaces• South-facing windows

– Minimize wind exposure

Planning

• Midlatitudes (2-season challenge)– Coniferous trees (dense) on the N, NW side to

protect against winter winds– Deciduous trees (widely spaced) on the South

side to protect against the summer sun, but allow winter sun

– Consider shade on East side depending on latitude

Cloud Seeding

• Cloud seeding is an attempt to modify the amount or type of precipitation produced by clouds by increasing the amounts of condensation nuclei in the sky– Silver iodide and dry ice most popular

• Is effective in altering cloud structure and shape – not certain in ppt amounts altered

Cloud Seeding

• China uses silver iodide rockets to produce ppt over cities

• China plans to use cloud seeding before the Beijing Olympics to reduce atmospheric pollution

Cloud seeding

• In the US– Used in drought-stricken areas– To reduce hail size– Increase snow fall– Reduce fog around airports

GEC

• Increased global temperatures will raise the base temperature on which the heat island effect is superimposed

• Areas of increasing anticyclonic activity would experience heat island effect

• More people die in heatwaves in US tan hurricanes

• UV radiation reduced over cities with ozone and aerosols

GEC

• In future…– Increased air pollution will decrease air

quality, while increasing haze and respiratory diseases

– Unorganized city growth will lead to landslides, flooding, dust storms

– At street level, temperatures will rise and ventilation will increase