Urban thermal plume
Encyclopedia
An urban thermal plume describes rising air in the lower altitudes of the Earth's atmosphere caused by urban areas being warmer than surrounding areas. Over the past thirty years there has been increasing interest in what have been called urban heat island
Urban heat island
An urban heat island is a metropolitan area which is significantly warmer than its surrounding rural areas. The phenomenon was first investigated and described by Luke Howard in the 1810s, although he was not the one to name the phenomenon. The temperature difference usually is larger at night...

 (UHI), but it is only since 2007 that thought has been given to the rising columns of warm air, or ‘thermal plumes’ that they produce. We are all familiar with on-shore
Sea breeze
A sea-breeze is a wind from the sea that develops over land near coasts. It is formed by increasing temperature differences between the land and water; these create a pressure minimum over the land due to its relative warmth, and forces higher pressure, cooler air from the sea to move inland...

 breezes at the seaside on a warm day, and off-shore breezes at night. These are caused by the land heating up faster on a sunny day and cooling faster after sunset, respectively. Our personal experience of on-shore breezes shows us that the thermals, or warm airs, that rise from the land and sea respectively have a sensible effect on the local microscale meteorology
Microscale meteorology
Microscale meteorology is the study of short-lived atmospheric phenomena smaller than mesoscale, about 1 km or less. These two branches of meteorology are sometimes grouped together as "mesoscale and microscale meteorology" and together study all phenomena smaller than synoptic scale; that is they...

; and perhaps at times on the mesometeorology. Urban thermal plumes have as powerful although less localized an effect.

London
London
London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...

 is generally 3 to 9 Celsius hotter than the Home Counties
Home Counties
The home counties is a term which refers to the counties of South East England and the East of England which border London, but do not include the capital city itself...

. London’s
London
London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...

 meteorological aberrations were first studied by Luke Howard, FRS
Luke Howard
Luke Howard FRS was a British manufacturing chemist and an amateur meteorologist with broad interests in science...

 in the 1810s, but the notion that this large warm area would produce a significant urban thermal plume was not seriously proposed until very recently.

Microscale thermal plumes, whose diameters may be measured in tens of metres, such as those produced by industrial chimney stacks, have been extensively investigated, but largely from the point of view of the plumes dispersal by local micrometeorology. Though their velocity is generally less, their very much greater magnitude (diameter) means that urban thermal plumes will have a more significant effect upon the mesometeorology and even continental macrometeorology. In 2007, Anthony Rail, postulated that London’s
London
London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...

 urban thermal plume, say, will generally have a detectable impact, and at times have a significant impact on the macrometeorology of Northern Europe, and may play as significant a role in Climate change
Climate change
Climate change is a significant and lasting change in the statistical distribution of weather patterns over periods ranging from decades to millions of years. It may be a change in average weather conditions or the distribution of events around that average...

 as the anthropogenic augmentation of atmospheric CO2.
A thermal plume from a large urban area is much more complex than that from, say, a large chimney stack. It is better regarded as a concatenation of several separate thermal plumes, their turbulence increased by the presence of cool-spots such as parks and, especially, lakes within the urban environment. A geographical area such as the British Isles has a thousand urban areas and many industrial sites, all of which will at times produce thermal plumes of different magnitudes. Their effect on the wider meteorology will be cumulative, through interaction between plumes, interaction between local wind variations as affected by plumes, and the influence of other topographical effects, not least of which are the cool areas of estuaries, lakes and man-made reservoirs. Absolute modelling of these collective effects is in a sense impossible, in that we cannot know the dynamics of the native geometeorological system in, for example, the British Isles. We can’t model the innate wind patterns, because the innate wind patterns no longer exist. We can’t ‘turn off’ all the anthropogenic thermal influences to rediscover the geometeorological systems before industrialization, urbanization, and large-scale water management; and then turn man’s influence back on to observe the differences.

Urban thermal plumes and climate change

Decreasing Arctic sea ice cover is one of the most visible manifestations of climate change, often linked to rising global temperatures. However, there are several reports that shrinking polar ice is due more to changes in ambient wind direction than to increasing environmental temperatures per se.

In 2006-07, a team led by Son Nghiem of NASA Jet Propulsion Laboratory, Pasadena, California
Pasadena, California
Pasadena is a city in Los Angeles County, California, United States. Although famous for hosting the annual Rose Bowl football game and Tournament of Roses Parade, Pasadena is the home to many scientific and cultural institutions, including the California Institute of Technology , the Jet...

, studied trends in Arctic perennial ice cover by combining data from NASA's QuikSCAT
QuikSCAT
The QuikSCAT is an earth-observing satellite that provided estimates of wind speed and direction over the oceans to National Oceanic and Atmospheric Administration and others. This "quick recovery" mission intended to replace the NASA Scatterometer , which failed in June 1997...

 satellite, which can identify and map different classes of sea ice, including older, thicker perennial ice and younger, thinner seasonal ice. The scientists observed that the Arctic Ocean was dominated by thinner seasonal ice that melts faster. This ice is more easily compressed and responds more quickly to being pushed out of the Arctic by winds. Those thinner seasonal ice conditions facilitated the ice loss, leading to this 2007’s record low amount of total Arctic sea ice. Nghiem concluded that the rapid decline in winter perennial ice the past two years was caused by unusual wind patterns that compressed the sea ice, loaded it into the Transpolar Drift Stream
Transpolar Drift Stream
The Transpolar Drift Stream is a major ocean current of the Arctic Ocean. Discovered in 1937 by Pyotr Shirshov at the Soviet drift ice station North Pole-1, it conveys water in roughly two major routes to the northern Atlantic Ocean at a rate of about per day. Primarily wind-driven, it flows...

 and then sped its flow out of the Arctic, where it rapidly melted in the warmer waters at lower latitudes.

It has been severally reported that in a stratified atmosphere cross-stream exchange occurs above the Planetary boundary layer
Planetary boundary layer
The planetary boundary layer , also known as the atmospheric boundary layer , is the lowest part of the atmosphere and its behavior is directly influenced by its contact with a planetary surface. On Earth it usually responds to changes in surface forcing in an hour or less...

 when there is a vertical motion of significant moment. While recognising that the steady lessening of vertical motion towards the edges of urban thermal plumes will have an ameliorating effect, Rail proposed that such urban thermal plumes play a critical part in producing the changes in ambient wind direction over the Arctic and have had a direct impact on Arctic shrink. The impact of urban thermal plumes will vary depending on a large variety of factors including the diameter and temperature gradient of the Urban heat island
Urban heat island
An urban heat island is a metropolitan area which is significantly warmer than its surrounding rural areas. The phenomenon was first investigated and described by Luke Howard in the 1810s, although he was not the one to name the phenomenon. The temperature difference usually is larger at night...

, the latitude, the thermal stability of the stratiform, the synoptic wind
Synoptic scale meteorology
The synoptic scale in meteorology is a horizontal length scale of the order of 1000 kilometres or more. This corresponds to a horizontal scale typical of mid-latitude depressions...

, &c. Thus, for example, urban thermal plumes will have far greater impact at higher latitudes (above 40°N and above 40°S), where the Earth-atmosphere system undergoes net cooling by radiation.
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