Outdoor Thermal Environmental Measurement
By Tsz Fung Wong
13th January 2018
Daniyal Ahmad, Robin Craft, Alexander Scruton
On Tang, Oliva Thomas, Conor Todd, Ali Uddin
Hamish Walker-Kerr, Toby Warburton, Mael Williams
Rekaine Wilson, Michael Winn, Cheuk Wong
Lok Ching Wong, Joe Wood, Yee Yeung
The construction of a building and its surrounding environments has always been highly connected. The design of buildings must take in many environmental parameters for maximizing human comfort. Factors such as temperature, humidity and solar radiation affect the use of heating, cooling and lighting. The building itself affects these parameters of the immediate environment as well. Microclimates can be formed in the built environment influencing the temperature and the wind. These factors will affect pedestrian comfort and the green environment. As such we want to investigate further into the relationship between buildings and the outdoor environment.
This experiment is designed to find out the how outdoor environmental parameters respond to their surroundings. We wish to find out:
· Outdoor environmental parameters such as solar radiation, air temperature and velocity, relative humidity in different surroundings.
· The relationship between the above parameters with the exchange with the surroundings.
The instruments used in this experiment are:
· Davis Vantage Pro2 Weather station (x2)
This weather station records environmental parameters including solar radiation, wind speed, air temperature, relative humidity, wind direction, rain gauge etc.
Figure 1. Weather Station
· Landscape site
Two landscape sites are investigated:
1. Man-made urban built environment
2. Open green space
Open green space Urban built environment
Figure 2. Different landscape sites
First, we obtained values of these environmental parameters from two different landscapes in the UoR campus:
1. Solar radiation intensity (R)
2. Air temperature (T)
3. Air velocity (v) and wind direction
4. Relative humidity
These steps are followed to obtain the values.
1. The two weather stations are cross-validated in the same environment
2. A weather station is installed at each site and is orientated to the north.
3. Each parameter is measured for two hours from 10:30 to 12:30.
4. Data is recorded on a table in 5-minute intervals.
5. Recorded data is adjusted for instrumental error.
The following are notable assumptions are made in the experiment:
1. There is no instrumental error after calibration.
2. There is no human error in recording measurements.
3. The data recorded is of normal distribution in each interval.
4. There is linearity between each interval of recorded data.
These are the results obtained in the experiment:
From this graph, we can see that the solar radiation recorded in the open green space and the urban built environment follow the same trend, but it is consistently 10-20 W/m² lower in the urban built area. There is an upward trend over time.
This graph shows that the air temperature of the open green space and the urban built environment is about the same. It shows that there is a rapid decrease in temperature in the beginning 25 minutes, then the temperature stays in a stable slight upward trend.
This graph shows that there is no recorded wind at the open green space, while the air is also still at the urban built environment the majority of the time, with slight wind only recorded 3 times over the 2 hours.
From the first chart, we can observe that the wind direction in the open green space is mostly north-northeast, with the wind sometimes changing to the north, northeast and east. The second chart shoes that the wind in the urban built area is mostly blowing to the north, with occasional north-northwest and northwest winds.
In this graph, both the recorded relative humidity of the open green space and urban built environment rises in the initial 20-25 minutes, then they stabilise and vaguely hover around the same area.
Most of the results are directly taken from the instrument and need no further calculation, however, adjustment for instrumental error is needed. In cross-validation, we found that both instruments measured the same air temperature and relative humidity, however, the measured solar radiation value was different (3.599/0.345 W/m²). In this instance, we treated the average value between two stations as the actual solar radiation (1.972 W/m²). The difference of the recorded value to the actual value (1.627W/m²) was added/subtracted from subsequently recorded values.
The most notable finding in this experiment is the variation between solar radiation measured between the two sites. The solar radiation in the urban built site was consistently lower than the open green space. This result is expected since there are taller structures present in the urban environment, often obstructing direct sunlight, thus reducing solar radiation. This can bring both benefit and harm to people in urban environments. Excess exposure to solar radiation have adverse effects such as skin ageing, immunosuppression or even skin cancer, however, lack of solar radiation exposure can also lead to vitamin D deficiency. The lower solar radiation may also impede the growth of vegetation in urban environments.
A huge setback in this experiment is the lack of significant wind on the day. Therefore, it is hard to investigate the relationship of wind in the different environments. However, we can still see that the wind direction of both sites is mostly different. We can deduce that the structure in the urban environment might cause changes in wind directions and might even have a wind funnelling effect, increasing the wind speed in certain areas. This effect may cause discomfort to pedestrians in urban environments, as they might experience sudden strong winds and changing wind directions when moving within the environment.
The temperature and relative humidity data we recorded was very unexpected as they were largely the same between the two sites. We expected that the temperature and humidity to will be higher in an urban environment due an effect known as the “urban heat island effect”. A report by the EPA suggests that urban environments are often 1-3°C higher than a rural area. This effect is caused by the properties of modern construction materials, as they have a high heat capacity and low thermal emittance, solar energy is stored in the materials and released slowly, continuously warming up its surroundings. The abundance of structures in an urban environment also contributes to the heat island effect, as it reduces wind flow and surface radiation from removing heat from the environment. This higher temperature along with the lack of vegetation transpiration also causes the lack of humidity in urban environments.
Figure 3. Mean air temperature in Paris (Summer 2003)
From these findings, we can see some simply improvement through design that can be made to improve general comfort in urban built environments. Like not designing buildings that will act like a “wind tunnel” or decreasing the density of urban structures to decrease the “urban heat island effect” etc.
Limitations and Errors
I believe there are many limitations and potential errors in this experiment, which might lead to inaccurate results.
1. There might be an error due to the separation of time between data recordings. Slight changes in the environmental parameters cannot be recorded between the intervals. We are forced to assume that we the data we recorded have normality and there is linearity between each data recording. To improve, we should use an instrument with data logging function for the continuous recording of data.
2. The proximity of the selected sites is a limitation of the experiment. As it might diminish the effects an urban and a green site has on its environmental parameters. Ideally, a more suitable site would be an urban built environment not near any open green site or vice versa.
3. In the recorded temperature and humidity data, there is a rapid change in values initially. This might be due to a slow response time from the instrument’s sensors. To minimize this error, we could set up the instruments at their sites before the recorded session, to let the instrument calibrate to its surroundings.
In conclusion, this experiment is flawed by multiple limitations and errors, which might cause results not indicative of the environments we aim to investigate further. Nevertheless, we can say that buildings in the urban built climate will have an effect on multiple environmental parameters, including decreasing solar radiation, changing the wind speed and direction, as well as increasing temperature and decreasing humidity through microclimate effects. These findings will be helpful for building designers and urban planners to maximize the comfort of people living in these environments.
United States Environmental Protection Agency. (2014). Reducing Urban Heat Islands: Compendium of Strategies. Retrieved from https://www.epa.gov/sites/production/files/2014-06/documents/basicscompendium.pdf