Lecture 7 Notes
Outline of this lecture
Part 1 Thermal Winds
Part 2 Urban Heat Island
Part 3 Research: a) Trends in Temperature Time Series in the GTA
b) Day to day temperature variability and Chinooks
Part 1 Thermal Winds
1.1 Land/sea breezes
Land/sea breezes arise from differential heating of the earths surface on a daily
(diurnal) basis. Land surfaces heats more rapidly and cools more rapidly than water
surfaces. During the day, the land surface is typically warmer than the water surface.
Warmer air rises and so air is rising over the land and sinking over the water. To
complete the circulation air moves from the water area to the land area at the surface.
This forms a cool breeze at the surface and is called a sea or lake breeze. It is
particularly noticeable during the day in the summer. At the upper level there is a
reverse flow with air moving out over the water.
At night the opposite occurs, the water surface is warmer than the land. Air rises
over the water and sinks over the land. The induced surface flow is from land to water
or a land breeze.
Sea breezes are strongest in the summer when the temperature contrast is the
greatest. Even on seasonal scales, bodies of water warm up and cool down slower
than land masses. In the summer, therefore, the average temperature of the land over
the entire 24 hour period is warmer than the average sea temperature; the diurnal
heating of the land mass adds to this seasonal difference to create a larger temperature
contrast. Land breezes, for similar reasons, are stronger in the winter.
A monsoon is similar to a land/sea breeze except it occurs over a large spatial area
(continental) and a longer time span (seasonal rather than daily). South Asia (Sri
Lanka, India, Bengladesh, Pakistan) and Africa experience a monsoon each year
usually beginning in late May. During the winter months, the Indian Ocean is warmer
than the surrounding land mass and air rises over the ocean and sinks over the land
inducing a land breeze and dry conditions. During the summer, the land is warmer than the Indian Ocean. Air rises over land and sinks over the water. This induces a sea
breeze. This breeze has high moisture content. As the air moves over the land, it rises,
cools and water vapour condenses into water droplets (clouds) generating copious
amounts of rain. The monsoon persists into the fall.
1.3 Valley Breeze
Breezes develop in a valley on a diurnal basis. During the day the land heats faster
than the air above it. Air starts flowing up the sides of the valley causing an uphill
wind. At night, the land cools faster than the air above it. This air at the surface
becomes denser and tends to flow down the valley sides, a downhill wind or breeze.
This is especially important to hikers.
1.4 Katabatic wind
In glaciated regions of the world, katabatic winds can form. Air pools over the
glacier and becomes denser. When the air moves away from the glacier it can move
rapidly down hill (very dense air) and forms intense cold winds. Hikers need to be
aware of these winds as well. Often the local topography (the valley) will act to
tunnel the wind making it more intense.
This is a pre-eminently Canadian wind. It occurs on the lee side of mountains (eg.
Calgary, Alberta). Air traveling up the windward side of the mountain typically
reaches a point where the water vapour condenses and a cloud is formed. The
formation of clouds releases energy (latent heat) due to the phase change from vapour
to liquid. On the leeward side of the mountain the air warms rapidly and is quite dry.
This is a result of the latent heat release and cloud formation on the windward side.
This warm, drier air can occur in winter and cause a dramatic change in temperature
in a few hours. The wind is often referred to as a snow eater. In Germany the wind
is referred to as a Foehn and comes off of the Alps. These winds have been linked to
human health (Cooke et al., 2000). There is more on the effect of the chinooks in the
Part 2 Urban Heat Island
The urbanization of the earths surface has lead to a substantial change in the
surface micro-climate. Below is a list of observable differences between the
meteorological data collected in urban areas compared to surrounding rural areas.
Urban vs. Rural:
Pollution level Higher
Sunshine hours Lower
Relative humidity Lower
Wind speed Lower
Precipitation Higher Cloudiness Higher
Some aspects such as warmer temperatures (the urban heat island) are widely
recognized, however, some of the others may not be as obvious. Urban areas do
indeed have greater levels of pollutants and this is well known and leads to reduced
sunshine hours and reduced visibility as more radiant energy from the sun is scattered
by atmospheric pollutants. Relative humidity is lower because urban areas, due to the
paving of the ground surface, are cut off from evaporation sources. Urban surfaces
have sharper edges and are therefore are rougher than rural areas and this slows the
wind. However it also funnels the wind and thus it may seem windier. In spite of the
lower relative humidity urban areas have more precipitation, cloudiness and
thunderstorms. This results from two mechanisms. With more particulate in the
atmosphere it is easier for water droplets to form (cloudiness). The warmer
temperatures also cause the air to rise over urban areas. This rising air cools and
condenses into clouds and generates precipitation and thunderstorms.
2.1 What is the urban heat island?
The urban heat island is the warming of urban areas compared to surrounding
areas. It is most noticeable at night and in the winter months. It is also more intense
when winds are weak or non-existent. The urban core may be up to 10 C warmer than
surrounding rural areas. The effect is mitigated by green space and water surfaces.
2.2 What causes the urban heat island?
There are several identified sources of the urban heat island. These are the
reduction of evaporation (and transpiration) due to paved surfaces and less vegetation,
reduced albedo (reflectivity) due to snow removal, daytime heating of urban surfaces,
and the generation of heat by industry, commercial, residential buildings, and
2.2.1 Suppression of Evaporation
Radiant energy from the sun is used at the surface to either heat the surface or
evaporate water. Heating the surface increases the surface temperature. However,
evaporation partitions the energy into latent heat and does not heat the surface. In
urban environments more energy goes into heating the surface and less into
evaporation (latent heat), thus the heating is enhanced.
2.2.2 Albedo Changes
Albedo is a surfaces ability to reflect energy. It ranges from 0 to 1. 0 represents
no reflected energy and all is absorbed by the surface. 1 represents all the energy is
reflected and none is absorbed by the surface. Snow has a high albedo approaching 1.
In urban areas there is less snow due to the higher temperatures and the snow that
does fall is removed. Urban areas have a lower albedo and thus more energy is
absorbed causing the temperature to be higher. 2.2.3 Urban Structures
Tall buildings create a complex geometry which traps energy and alters air flow.
This increases the amount of energy available to increase surface temperatures.
2.2.4 Urban Generation of Heat
Industry, motor vehicles and domestic heating release heat causing urban areas to
warm. In addition, some types of urban pollution prevent the release of excess heat;
smog (ozone) can have a localized greenhouse effect.
2.3 Heat Island and Temperature Increase
In 1973, Dr. Tim Oke linked the heat island to city population. He found the
biggest difference is in daily minimum temperature which typically occurs at night.
The UHI has also been linked to electrical power load, likely due to the increase in air
conditioning. Electrical power load is linked to population size.
2.4 Torontos Heat Island
The first research study of Toronto heat island was done by Prof. Ted Munn of U
of T in 1967. It was the first thorough examination of Torontos micro-climate. He
uncovered a well defined heat island focused on the downtown. He found that it was
modified by wind and topography. The UHI is shaped by cool air intrusion from Lake
Ontario flooding up the Humber, Don and Rouge River Valleys.
A more recent analysis, using climate normals (thirty year averages of
temperature) was done at the Climate Lab at UTSC by Gough and Rozanov (2001).
The Urban Heat Island was assessed for Toronto, Ontario. Thirty year mean values of
maximum and minimum daily temperatures from Downtown Toronto and rural site
at Pearson Airport were compared. To assess the long term trends, Toronto data was
compared to another station in Vineland, Ontario located directly south of Toronto,
part of the Niagara Peninsula.
Once again, a well defined heat island was found. It was consistently about 3 C
warmer at night (minimum temperature) throughout the year. However it was less
during the day (maximum temperature) and in fact during the summer the downtown
station recorded cooler temperatures due to the presence of a lake breeze.