The generic pattern within climatic domes such as the UHI is the occurrence of higher precipitation over urban areas in contrast to the surrounding rural areas.
Key Words:
Convection - Vertical heat transfer whereby the expansion of a gas, liquid or solid via increased temperature causes the material to become less dense (Lower Pressure) and therefore rise.
So why is precipitation more frequent and more intense in UHI's?
- An important principle that one must apply to this course is the concept of convection. As temperatures rise, as do the prevalence of convection currents meaning the vertical heat transfer of low pressure air increases (relate this to the ITCZ between the Hadley cells and how precipitation is formed).
- Rising air through convection is enhanced by the geometry of buildings. For instance, the presence of tall buildings induces both air turbulence and a vertical motion of this air.
- Finally, the presence of industry can promote precipitation formation via producing water vapour and pollutants which act as hygroscopic nuclei and encourage raindrop formation.
Essentially, the creation of low pressure as a result of higher urban temperatures means convectional rainfall is more likely, as is the presence of thunderstorms and lightning.
And why is the frequency of fog more prevalent in urban areas?
Urban heat islands typically have a greater affiliation with fog formation in relation to their rural counterparts. Evidence of this was founded in the 1800s where the urban centres of London and Manchester saw the increasing prevalence of fog as a result of industrialisation. In the 1950s, it was discovered that these cities were so badly affected as a result of pollutants such as suspended particulate matter which acted as condensation nuclei and encouraged fog formation at night (which were able to stabilise if under anticyclonic temperature inversions).
In response, the 1950s saw legislation to counter fog which was epitomised in the Clean Airs Act. This statute reduced the amount of suspended particulate matter in the atmosphere over the urban areas which correlated to a decrease in the number of foggy days. This meant, in modern context, the UK as a developed country see's less fog as it did under industrialisation. In contrast however, the developing countries of Beijing are now seeing the same issues as a result of industrialisation which has resulted in fog creating major infrastructural damage and contributing to health conditions such as asthma and bronchitis.
Furthermore, why are thunderstorms more frequent within urban areas?
Thunderstorms develop in hot humid air and are associated with heavy precipitation, thunder and lighting. Their frequency is increased under summer months as a result of temperature increases which drive convection.
Thunderstorms are created by convectional uplift under conditions of extreme instability. Cloud formations typically associated with thunderstorms are the cumulonimbus clouds which can create towering columns reaching up to the height of the Tropopause. These clouds can stabilise particularly under temperature inversions.
The process: The updraught of low pressure air (as a product of convection) through the towering cloud causes rapid cooling and condensation of rising air into precipitation which then forms a down-draft toward the ground surface. Condensation produces latent heat (as we know from SALRs) which further fuels convectional uplift so the weather system can sustain. However, as raindrops split in the cumulonimbus cloud, positive electrical charge builds up until the charge is big enough to overcome resistance in the cloud. When this happens (charge overcomes resistance) discharge will strike a negatively charged area in the cloud on the ground surface which creates what is known as lightning.
Next time on 'Urban Heat Islands'.... Will Air Quality be able to amend itself through pollution reduction policies? Will you be able to list the types of pollution impacting urban areas across the world?
Stay tuned.
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