1. Urban Heat Islands
An urban heat island is a metropolitan area which is significantly warmer than its surrounding rural areas.
Urban fabric

2. Bare earth and vegetation
Water is constantly being lost from bare earth (evaporation) and from all sorts of vegetation (evapotranspiration). This moves energy (heat) from the surface of the ground up into the atmosphere.
In big towns and cities, there is less bare earth and vegetation than in rural areas.

3. Water
Evaporation from water on the ground moves heat from the ground up into the atmosphere.
In rural areas, there tend to be more lakes, ponds, streams and river than in urban areas. Also, when it rains, water tends to be quickly channelled into underground drains and sewers and is not left on the urban surface to cool it down.

4. Albedo
The lighter or shinier a surface, the more of the Sun’s energy gets reflected straight back out to space, without heating up the ground.
Many urban surfaces are very dark – tarmac, roofs etc. more of the Suns energy tends to get reflected in rural areas – although it depends a lot on what sort of rural environment you are looking at. Sand and water are much more reflective than thick woodland.
Albedo is measured on a scale of 0 (nothing reflected) to 1 (everything reflected)

5. Buildings and the Ground Store Heat
Any of the Sun’s energy that isnt reflected back into space, is absorbed, heating the object up. So, the lower the albedo, the hotter the object will get.
To make matters worse, many buildings are designed to store heat – insulation etc means that, once they warm up, its very hard to cool them down at night.
This means that rural areas cool down faster than urban areas at night.

6. Sources of Heat
Vehicles, heating systems, air conditioning etc all release extra heat into the urban environment.

7. Cityscapes v landscapes
The complex three dimensional structure of the urban landscape means that they are less well ventilated than rural areas. Heat (and pollution) can literally get trapped in streets.

8. Energy in and out
From physics world august 2010 and
The difference in the energy balance between urban and rural areas – the wider the arrow, the larger the flow of heat.
In rural areas, most of the energy (‘radiation’) we get from the sun is reflected. There is also much more evaporation than in urban areas. In urban areas, all the leftover energy is stored – heating up buildings, roads etc.

9. Towns and cities don’t heat up the same amount everywhere
Towns and cities don’t heat up the same amount everywhere. The more densely developed an area is, the more it heats up.
Fine scale features can be very important – a small local park can reduce the local temperatures by a couple of degrees.

10. London, 2000
Richmond Park
From Ken Livingston’s report
British Museum

11. London, Summer 2000 sunset Sunrise
Huge amount of variability depending on weather conditions – country can even be warmer than the city in the middle of the day.
This graph shows difference between RHS Wisley (countryside) and central London
Urban heat island dissipates rapidly at sunrise and develops rapidly at sunset
It’s the rate of cooling after sunset that sets up the UHI as well as the rate of heating after sunrise.
So the urban heat island effect is biggest at night.
sunset
Sunrise

12. Chicago
Chicago (very old figure!) – time in months on the vertical axis and time in hours through the day on the horizontal axis.
The fine dotted line is sunrise/ sunset. The dashed line is the zero contour (no temperature difference between urban and rural areas).
Falls off at sunrise/ persists longer in winter than it does in summer, because as soon as the sun rises, the urban heat island dissipates.
The urban heat island effect is greatest in the middle of the night in summer

13. The urban heat island is biggest…
At night (before sunrise)
In the summer
When there is no wind
When the sky is clear
When the weather doesn’t change through the night

14. Daily mortality in Baden-Württemberg
Excess mortality rates in early August 2003 indicate about 40,000 heat-related deaths
Why do we care about urban heat islands?
This graph shows mortality in one state in southern Germany. You can see that, in general, more people die in winter than in summer. Flu outbreak in Feb 03 followed by a dip in mortality – most people who died, would have died soon anyway. Very obvious peak in August 03 – heat wave – not followed by a corresponding dip. Most of the people who died would not have died in the next 12 months.
People are affected most by night time temperatures – we can cope fine with warm days as long as we have the opportunity to cool off at night. The UHI therefore makes heat waves particularly dangerous for city dwellers.
Daily mortality in Baden-Württemberg

15. European 2003 summer temperatures
observations
HadCM3 Medium-High
2003
2050s
Temperature anomaly (wrt ) °C
“Right now, we average about 14 days each summer above 90 degrees in New York. In a couple of decades, we could be experiencing 30 days or more,”
The summer of 2003 in Europe was very unusual, but by 2050 it will be ‘normal’ and by the end of the century, it will be as unusually cool as it was unusually warm at the start of the century.
Given the long lifetimes of buildings, urban areas etc. we need to plan urban areas now to anticipate the summer temperatures of the future.

16. What’s that got to do with the UHI?
London – 600 heat related deaths in August 2003
Up to 9 °C warmer than surrounding rural areas
Mortality increases with temperature when temperatures are over 22°C
Mortality particularly linked to warm night time temperatures
Mortality linked to age, sex, deprivation etc.
Challenge now to design buildings and urban areas for the future – which will see more people living in cities, and will be warmer.

17. Other Consequences
The growing season can be up to 15 days longer in a city!
Warmer in a city in winter
Rainfall
Cities – pollutants act as cloud condensation nuclei – more, smaller raindrops in clouds which are less likely to rain. Less pollution at weekend – most likely to rain on Sunday.

18. How to reduce urban heat islands
Cool surfaces – paint building and pavements light, shiny colours – but you have to keep them clean
Green roofs
Plant vegetation - Large trees on the South/ West of buildings can reduce air conditioning costs by 30%
Urban planning – make sure streets are well ventilated