Urban heat island effect is caused by: urban surfaces (brick & tarmac) absorbing insolation during the day and releasing it at night, heat from combustion and blanket effect of pollution stops heat escaping. (size of urban area not important, density is = leicester 300,000 same urban heat island as london 6M)
It is greatest in summer because: more intense solar radiation, skies are clearer & high pressure is more common which stops heat rising and escaping.
It is greatest at night because: rural areas lose heat quickly but urban materials give off heat all night long.
Higher rainfall in urban areas because there are more condensation nuclei (dust/pollution particles) to allow rain to form. Tall buildings give orographic uplift to cool air. Urban surfaces create convectional uplift. There is lower humidity however.
Winds are generally less but are locally faster, funnelled winds caused by grid iron pattern (e.g. USA housing estates) and also by accelerated winds due to wind speeding up as it hits urban building (Croydon, St. Georges Walk)
Improving urban air quality
- London congestion zone = brought in in 2003 and it charges motorists £8 a day, initially designed to reduce congestion but this lead to less pollution.
- Improved public transport = croydon tramlink is new and environmentally friendley, now carries 22 million passengers a year.
- Car sharing
- Londons low emission zone
- Policies which encourage cycling = free Barclays sponsored bikes in London
Formation of Hurricanes
They require certain conditions, warm oceans with temperatures above 26C & between 5-20 degrees north and south of the equator.
The oceans heat and moisture is the hurricanes energy source so they die out inland, the spin is caused by the earths coriolis effect. Less than 5 degrees there is no coriolis and above 20 degrees the oceans are not warm enough.
- Warm moist air rapidly rises and water vapour condenses to form storm clouds & heavy rain
- Condensation releases latent heat used to evaporate water from ocean and this drives winds
- Rising air creates low pressure and more air rushes in to replace it and as it does so spins due to coriolis
- A cycle of rising air, heat releasing and air rushing in to replace it is established to form a hurricane
Monsoon (tropical) Climate
- temperatures are high enough for year round agriculture
- rainfall is high for 4 months of the year so farming is ver productive at this time
- with proper management monsoon rains can be stored and used during dry season (e.g. Indian Farraka Dam)
- monsoon rains are variable and can bring less rain than expected so farmers struggle
- flooding is common (e.g. Bangladesh) causing economic hardship and loss of life
- hurricanes can occur after onsoon period causing further destruction
- a lack of water mangement in most places means no agriculture can take place for most of the year