Intensities of the Urban Heat Island of Tehran under the Influence of Atmospheric Synoptic Patterns

Introduction
The Urban Heat Island is a phenomenon whereby cities become warmer than the surrounding suburbs. In other
words, there is a temperature difference between the cities and their surrounding areas. Generally, the UHI effect
is a result of excessive and unplanned growth of urbanization. The behavior of artificial urban texture in terms of
absorption of short-wave and long-wave radiation, transpiration, releasing of anthropogenic heat, and blocking
prevalent wind is significantly different from that of the rudimentary nature. Hence, the Bowen Ratio in the cities
alters and the sensible heat increases. Surface geometry, on the other hand, decreases wind speed in urban
regions that plays a significant role in formation of UHI. Since the energy balance inside a city is altered, UHI
intensity may change. This means UHI intensity is not spatially and temporally similar in different cities. It must
also be noticed that UHI formation in a city usually has diurnal or seasonal patterns which are mostly affected by
synoptic weather conditions. There are three main synoptic and local climatology parameters that affect UHI
formation: Air Pressure Systems, Cloudiness, and Wind Speed. Under stationary high-pressure system
conditions temperature differences between urban and rural areas become large. UHI intensity is largest in calm
air and cloudless sky conditions and tends to disappear in cloudy and windy weather. Generally, synoptic
patterns can be divided into three major conditions as stable, unstable, and mediocre. Unstable conditions reduce
the heat island intensity by making turbulences which mix the air. Stable conditions, on the other hand, increase
the heat island intensity as they are calm and without air movements. Mediocre conditions can play two roles
depending on their characteristics and wind properties.
The urban heat island can lead to urban temperatures being 2–5􀃛C higher than those in rural surroundings.
Studies have shown the difference in temperature between urban and rural regions (UHI Intensity: 􀀧TU􀀐R ) is
revealed in minimum temperatures rather than maximums. Henceforth, the Maximum UHI intensity should
usually occur after sunsets in urban areas. Other impacts of the Urban Heat Island could be intensifying pollutant
concentration over urban areas, altering local wind patterns, increasing humidity, forming cloud and fog, and
changing the precipitation rate over a city.
Material and Methodology
In this study, the influence of synoptic weather conditions on the intensities of the urban heat island of Tehran
was analyzed. Tehran is the largest and the most populated city of Iran, with an approximate area of 750 Km2
and a population of 8 Million during night time. The city lies almost in the middle of the Tehran Province (1882
Km2 of area) in the southern side of the Alburz Mountain and is limited to the highlands in northern and eastern
parts. On the southern and western parts, it is connected to the flat plains of Varamin, Shahriar and Karaj.
To investigate the effects of synoptic weather conditions on the intensities of the Urban Heat Island over
Tehran, after a literature reviews, 24 days were selected from the year 2006; two days of each month of the year,
one day with the highest and the other with the lowest air pressure over the urban area. After the homogeneity of
the data derived from the surface station with those of the midlevel atmosphere (850 HPa, 700 HPa, and 500
HPa) was examined, the climatologic data (including temperature, air pressure, wind direction and wind speed)
for each day were gathered from different data sources: 1- Iran meteorology Organization stations including
synoptic and climatology stations, 2- Air quality measuring stations including Air Quality Control Company
(AQCC) and Department of Environment (DOE) stations. The location and distribution of the stations is shown 
in Fig. 1. After data refinement, the measuring times were transformed from Greenwich Mean Time (GMT) to
the Local Time (LT) by adding 3:30 to the GMT.
The difference between the average temperature of the urban district (TU) and the rural area (TR) was
calculated by MS Excel 2007 for collecting the UHI intensity during different seasons and months and for all 24
days at each measurement hour. The midlevel atmospheric data were gathered from National Centers for
Environmental Prediction/National Center for Atmospheric Research of the United States (NCEP/NCAR). Then,
all patterns occurred at mentioned days were manually observed and investigated. Then, the data including geopotential
meter height, sea level pressure, wind characteristics, and temperature were analyzed using 2.5*2.5
geographical grids. At the final step, the 4 days were chosen to represent the influence of the synoptic conditions
on the heat island intensities. Two days, with the absolute maximum and minimum of UHI intensity, and two
days representing the total weather conditions of wintertime and summertime heat island.
Fig. 1. Spatial Distribution of weather stations in the study area
Results and discussion
According to the literature reviews, it was expected that during cyclonic condition the intensity of the UHI
would be reduced and inverse condition would be happened in anti-cyclonic condition. Figure 2 represents the
variation of the heat island intensities in the study days. As it could be seen, the absolute maximum intensity (8.9
Celsius degrees) has occurred in July while the absolute minimum intensity (1.1) has occurred in January. It can
also be seen that the difference between the maxima and minima of heat island intensities have seasonal changes.
While the difference between maxima and minima is the least in cold period, it is the most in the warm period. In
fact, in the summer the maxima intensities raises more than those of the minima making the difference bigger
than what it is in the winter. It should also be noticed that the behavior of the minima and maxima is
significantly simultaneous. The maxima and minima almost increase and decrease together. Even in the summer
in which the difference is bigger, the maxima and minima are closely correlated.
Fig. 2. The variation of maxima and minima intensities of the urban heat island of Tehran
In order to investigate the influence of the synoptic weather conditions on the heat island intensity for all four
days, as mentioned previously, the sea level pressure map,wind field and geo-potential height was calculated
(Fig. 3 and 4).
Fig. 3. The synoptic weather condition for the absolute maximum (29th of July: A and B) and minimum (4th of
January: C and D) time. A and C: sea level pressure (contours) and surface temperature in Celsius (colored
spectrum); B and D: wind direction (vectors) and wind speed in m/s (colored spectrum).
Fig. 4. These are representatives of cold period (A and B) and warm period (C and D). A and C: geo-potential height
of 500 Hpa (contours) and sea level pressure (colored spectrum) for the occurrence time of minimum intensity of heat
island; B and D: geo-potential height of 500 Hpa (contours) and sea level pressure (colored spectrum) for the
occurrence time of maximum intensity of heat island.
Conclusion
In this study the influence of synoptic weather conditions on the intensities of the urban heat island of Tehran
was investigated. The results indicated that the intensity of summertime heat island is higher than that of the
wintertime. Furthermore, the correlation between the minima and maxima of heat island intensities shows the
influence of the synoptic weather patterns on heat island intensity. In the combined maps it was revealed that the
correlation between the maximum and minimum times of heat island intensity is much more significant in the
warm period while there are some inconsistencies in cold period. The reason for this condition could be the
different patterns of the atmosphere of Iran. In summer, the edge of Azores' subtropical high pressure is located
in the midlevel atmosphere of Iran while there are several thermal low pressure cells near the ground. This
causes daytime turbulences due to the high radiation income and calm weather when the radiation effect is
lessened. However, the condition is almost the opposite in the cold period. In cold period, while there is a cold
high pressure condition near the ground, the midlevel atmosphere experiences a relatively active pattern. Due to
the passing of westerlies, many unstable synoptic systems pass through Iran's atmosphere. The instability and
variety of passing systems increases the wind speed by which the heat island intensity is reduced or undergone
variation. Henceforth, the difference between the low level and midlevel atmosphere is the main cause for the
variation of the intensities of the heat island of Tehran.