5 - Urban climate

What's a cities microclimate?

Cities with their own distinctive climatic dome that's different from surrounding rural areas

What has happened to the annual mean temp in inner cities?

0.5-0.8 degrees C increase

What has happened to the annual precipitation in inner cities? Why?

5-10% increase in quantity. Rain is more intense and there are more thunderstorms.

This is because UHI effect means the air in urban areas is warm, warm air can hold more water. As the air rises, it cools, condenses and rains. This is convectional rainfall.

What has happened to the annual humidity in inner cities?

Annual mean increase by 6%

How is wind speed different in inner cities?

20-30% increase

How is pollution different in inner cities? How is it different and why?

1000% increase. There is more particulate pollution from vehicle exhausts, burning of fuels or cigarettes eg sulphates, construction produces coarse particulates eg tiny fragments of brick.

What is the urban canopy?

Over the city having the hottest temperatures

What's the urban boundary?

Above the canopy and extends due to the prevailing wind downstream into neighbouring rural areas making a plume influencing the surrounding countryside

What's the urban heat island effect and when is it most noticeable?

When large cities are warmer than the surrounding rural areas.

It is most noticeable:

• at night when rural areas cool but urban areas can’t cool because urban surfaces continue to release the heat which they absorbed during the day

• Summer because there is more solar radiation in summer so urban areas can absorb more heat

• Anticyclone conditions. When there are clear skies and low winds so more solar radiatuon reaches the ground because there is no clouds blocking them and the warm air isn’t blown away from wind

What can cause the UHI effect?

• Absorption of heat by urban surfaces such as concrete and tarmac which have low albedos. They absorb heat during the day then at night slowly release it.

• Air pollution increases cloud cover over the cities creating a pollution dome over the city where heat radiation is trapped and reflected back on the surface

• Heat from human activity such as cars, heat being leaked from buildings

• Less evapotranspiration as there is less vegetation which uses heat energy

What are some risks from the UHI effect?

• Dust causing respiratory issues making conditions such as asthma worse

• Dehydration causing pressure on water and social conflict

• Hot anticyclonic air conditions causing more pollution and low visibility which can cause car crashes

• Chemical heat weathering on rock

How can roofs reduce the severity of the UHI effect?

Green roofs can reduce rooftop temperatures by 40 degrees as they reduce runoff and increase biodiversity

Also, cool surfaces built from materials with high solar reflectance can absorb heat

How does urban greening reduce the severity of the UHI effect?

Planting trees can increase shade and act as a carbon store. Also evapotranspiration uses heat energy reducing temperatures

How does sky view factor reduce the severity of the UHI effect?

By putting buildings perpendicular to the prevailing wind it can help disperse hot air throughout the city. But this is better with a high sky view factor (lots of open space)

How does cool cars reduce the severity of the UHI effect?

Lighter cars can reflect sunlight and reduce the need for air con which can leak and warm the atmosphere

What does UHI effect increase the chances of, by what percentage and why?

Thunderstorms by 25% because of increased convection. Convection creates thunderstorms by heating the ground, causing warm, moist air to rise rapidly (updraft), cool, and form towering cumulonimbus clouds

What causes fog in urban areas?

Dust and pollution in the air act as condensation nuclei encouraging clouds to form rather than allowing moist air to disperse. There is 10% higher cloud cover in urban areas which increases the frequency of fog.

What are 2 effects that skyscrapers have on wind in urban areas?

Urban canyon - relatively narrow streets bordered by high-rise buildings funnels and concentrates winds

Venturi effect - Violent form of gusting caused by narrow gaps by air rushing to replace low pressure

This means winds are slower due to frictional drag and more turbulent

What type of pressure does the windward and lee side have when discussing winds approaching a building?

Windward - High pressure due to air pushing against the building

Lee - Low pressure and eddying winds and a steep pressure gradient

What's a temperature inversion and what causes it?

When warm less dense air moves over dense cold air as a result of other weather conditions. This reverses the normal pattern where air gets colder with height, acting like a lid that traps pollutants, fog, and cold air near the ground, which is more common in cities due to their heat output, lack of dispersion.

What is smog? What are the effects of smog in Delhi?

Smog is where pollutants which come from car exhausts such as sulphur oxides come into contact with sunlight and the UV radiation causes them to break down into harmful chemicals forming photochemical smog. This is due to a temperature inversion which traps all the harmful chemicals at ground level.

In Delhi schools closed, road accidents due to poor visibility and enhanced respiratory issues.

Where is urban air quality worst? Why?

NEE's. Because they are industrialising too quickly to be able to find solutions to pollution

Name some pollution reduction strategies? (5 in UK, 3 elsewhere)

• Bike lanes (Oxford Road)

• car pool lanes (LA)

• London congestion charge

• green roofs (Singapore)

• bus lanes (reduced traffic by 21% in Dublin)

• Eco-driving test for new cab drivers so they take the shortest route and black cabs have 2 MOT's per year

• In Mexico they have a programme where cars with a last digit eg:5/6 can’t drive on Monday however wealthy people get around this y buying 2 cars.

• Environment Act 2021: Biodiversity Net Gain (BNG): The Act introduces a mandatory requirement for BNG, meaning developments must result in a 10% increase in biodiversity value. And nature recovery areas.

• Electric vehicles - however producing and disposing of the batteries can cause environmental hazards

What did Accra in Ghana LIC do to reduce urban air pollution?

Ban on burning plastic bags and 5% tax on plastic products (hard to enforce and very low tax but could damage weak economy)