urban climate

While fog and mist are most associated with winter, urban convection is especially powerful in summer.

The rising heat (including water vapour and condensation nuclei from power stations, industry and, to a lesser extent, vehicles) can trigger heavier and more frequent later afternoon and early evening thunderstorms.

The more intense the heating, the more violent the storm. This is because updraughts of hot, humid air can rise higher in the atmosphere – and more quickly.

The air cools and condenses rapidly, forming water droplets, hail and ice which charge the thundercloud and discharge as lightening.

Urban microclimate - Human activity chemically and physically alters air and weather characteristics over and around urban areas, making it different from air and weather over rural areas

Microclimate - Climate within a small area that differs significantly from the climate of the surrounding are

higher temperatures of urban areas mean that the likelihood of thunderstorms is increased by 25%. Thunderstorms develop in hot humid air and are accompanied by violent rain, lightning and thunder. They are particularly common in the late afternoon when heat energy has had the chance to build up in the atmosphere. Thunderstorms are created by;

1. Heating of the air causes uplift of the air which rises through the troposphere
2. The air cools rapidly as it rises, causing condensation and rapid cloud formation
3. This results in the formation of towering cumulonimbus clouds
4. Rapid cooling leads to the formation of water droplets, hail and ice
5. Coalescence of water droplets or fusion of ice crystals makes them bigger when they will fall
6. As raindrops are split in the uplift of air, positive electric charge is released into the air
7. This electrical charge builds up until high enough to overcome resistance in the cloud, released to areas of negative charge on the ground or in the cloud.
8. This is known as lightning.
9. Thunder occurs as a result of the sudden increase in pressure and temperature from lightning which produces rapid expansion of the air surrounding it.

Intensity, frequency and length of fogs are much greater in urban areas particularly under anticyclone conditions.

For example, Kew in the middle suburbs of London has 79 hours of very dense fog, with visibility being less than 40metres.

Whereas, London Airport on the outer suburbs has only 46 hours, and south east England (the mean of 7 weather stations) has 20 hours.

This shows that further away from the urban areas of a city, towards rural areas, fog density decreases.

Obviously, the larger the city and the greater the quantity of urban structures and materials the greater the impacts of these microclimatic changes.

Urban microclimate management strategies

Trees and Vegetation - Increasing tree and vegetation cover lowers surface and air temperatures by providing shade and cooling through evapotranspiration. Trees and vegetation can also reduce storm water runoff and protect against erosion.

Green Roofs - Growing a vegetative layer (plants, shrubs, grasses, and/or trees) on a rooftop reduces temperatures of the roof surface and the surrounding air and improves storm water management. Also called “rooftop gardens” or “eco-roofs,” green roofs achieve these benefits by providing shade and removing heat from the air through evapotranspiration.

Cool Roofs - Installing a cool roof – one made of materials or coatings that significantly reflect sunlight and heat away from a building – reduces roof temperatures, increases the comfort of occupants, and lowers energy demand.

Cool Pavements - Using paving materials on pavements, car parks, and streets that remain cooler than conventional pavements (by reflecting more solar energy and enhancing water evaporation) not only cools the pavement surface and surrounding air, but can also reduce storm water runoff and improve night time visibility.

Smart Growth - These practices cover a range of development and conservation strategies that help protect the natural environment and at the same time make our communities more attractive, economically stronger, and more liveable.

Building design – needs to be considered to limit the impact of winds on buildings. The Burj Khalifa stands 828m tall and has curved sides to deflect wind around the building and prevent the formation of whirlpools or vortices. It is also orientated towards the prevailing wind direction. The building is known to sway 2m at the very top

• it rains more often in urban areas than in the surrounding countryside
• the rain is more intense and there are more thunderstorms
• the higher concentration of condensation nuclei in urban areas also increases the frequency of fog

there are two main reasons for these:

• the UHI effect means the air in urban areas is warm, and warm air can holds more water. The warm, moist air rises - this is called convection uplift - as it rises, it cools, the water vapour condenses and it rains -> this type of rain is called convectional rainfall

• urban areas generate huge amounts of dust and pollution. Particles of dust and pollution floating about in the air act as condensation nuclei - this encourages clouds to form, rather than allowing the warm moist air to disperse 

Due to human activity, the temperature in an urban microclimate is higher than that of the surrounding areas.

Urban areas are said to be urban heat islands as under calm conditions, temperatures are highest in the built up city centre and decrease towards the suburbs and countryside.

There are several reasons why this pattern occurs. In urban areas, the building materials are non-reflective and therefore absorb heat.

Also road surfaces such as tarmac and concrete have a high thermal capacity therefore also absorb large amounts of heat due to their dark colour.

This heat is absorbed during the day and then released slowly at night, increasing the temperature. Further heat is given off by the presence of factories and increased car use within the city, causing pollution which causes smog and a pollution dome to form.

This pollution dome allows short-wave insolationto enter, but traps outgoing terrestrial radiation due to its longer wavelength, therefore increasing the amount of heat obtained