Climate Data

04 03

CLIMATIC CONTEXT

SHARJAH, UAE

Sharjah is one of the most populous emirates and a cultural capital of the United Arab Emirates. Located at approximately 25.35°N and 55.42°E (Figure 3.1), it can be classified as a Hot Desert Climate (Köppen BWh). However, sharing a coastal stretch of the Persian Gulf on the north and the Gulf of Oman on the east, it can experience an above-nominal humidity level at specific periods of the year.

Figure 3.2 shows the climatic data of Sharjah obtained from the CBE Clima Tool, representing a 60-year average of the data files from 1956 to 2021. Based on the monthly variations of the dry bulb temperature of Sharjah, the annual cycle can be divided into three distinct periods – a four-month period of mild weather (December to March), a warm period (November and April), and a hot period (May to October) (Yannas 2008).

3.1 Climate Data

Dry Bulb Temperature

Figure 3.2 illustrates monthly mean, minimum, and maximum dry-bulb temperatures, and wet-bulb temperatures for Sharjah. The annual cycle can be divided into three discrete periods, as mentioned before:

Hot Period | A period covering the months from May to October that can be very hot, with monthly mean dry-bulb temperatures above 30°C, reaching daily mean maximums of around 36°C in May and October and around 41°C in June, July, August, and September. These temperatures often exceed the upper

Figure 3.1 Location of Sharjah, UAE

Figure 3.2 Summary and Graphical Representation of the Monthly Averages of Sharjah with Three Distinct Periods (Source: CBE Clima Tool 2021)

limit of thermal comfort (based on ASHRAE Standard 55-2017), indicating that there is a risk of overheating indoors and that natural daytime ventilation alone may not always be an adequate cooling strategy since outdoor temperatures can exceed acceptable comfort levels.

Warm Period | Two warm months (in comparison to the previous period), which are April and November, with monthly mean dry-bulb temperatures of 25-27°C. This period lends itself well to passive techniques, and outdoor spaces can be pleasantly comfortable if well-shaded and exposed to the breeze coming from the Gulf.

Mild Period | A mild-cool period of 4 months from December to March with monthly mean dry-bulb temperatures well within 18-23°C. This is the most comfortable period when no additional conditioning or tempering of the microclimate is required.

Occurrence Pattern

Figure 3.3 illustrates how many days per month in Sharjah reach certain maximum dry-bulb temperatures. For instance, from May to September, most days have maximum dry-bulb temperatures above 40°C, and especially July, it has almost all days where the maximum temperature exceeds 40°C.

Diurnal Temperature Swings

Figure 3.4 illustrates the difference between the monthly mean minimum and maximum dry-bulb temperature in Sharjah. It can be noted that nighttime temperatures are relatively low in comparison to daytime temperatures, with a large daily swing of 10-12K. As discussed in the previous chapters, it is essential to know these differences to make informed choices about the passive strategies that can be implemented during different occupant hours (day 6AM-6PM and night 6PM-6AM).

During the hot period (June-August), the high daily temperatures leave the nights reasonably warm. However, during the warm and mild period (October to April), the diurnal swing involves night-time ambient temperatures, varying from 14°C to 20°C, that are low enough for nocturnal convective cooling of

Figure 3.3 Displaying how many Days in a Month in Sharjah reach Certain Temperatures (Source: CBE Clima Tool 2021)

Figure 3.4 Diurnal Temperature Swings for Sharjah (Source: CBE Clima Tool 2021)

Figure 3.5 Wet Bulb Depression for Mild, Warm and Hot Period for Sharjah (Source: CBE Clima Tool 2021)

Table 3.1 Specific Humidity and Vapor Pressure for Sharjah (Source: Balakrishnan 2012) building structures. In addition, outdoor spaces such as roofs and courtyards could be used during those hours for resting and other activities.

Humidity

Since Sharjah is located between the coast of the Persian Gulf and the Gulf of Oman, the humidity levels are high during specific periods. The average relative humidity of 38-65% conceals reasonably high levels of absolute humidity in the hot period that rises to 14-17 g/kg from June to September (Figure 3.2). However, with the exception of these four months when the wetbulb temperature is too high, the temperature difference between the drybulb and wet-bulb (wet-bulb depression) reaches regular peaks of 10-15K during the daytime, suggesting potential for the use of evaporative cooling as a passive cooling strategy (Figure 3.5).

From a physiological view, the potential for evaporative cooling can also be determined by the vapor pressure difference between the skin surface and ambient air during the three periods; the rate of evaporation from the body is directly proportional to the vapor pressure difference. Under comfortable conditions, when the skin temperature is 33°C, the vapor pressure of the skin is 37mmHg (Givoni 1976).

Comparing the actual vapor pressure in Table 3.1 to the skin vapor pressure at 33°C, evaporative cooling has a high potential during the warm and mild period (November to April) and a medium potential during May and Oct. However, it has a bleak potential from June to September as the vapor pressure of the saturated air exceeds that of skin in comfortable conditions (37mmHg (Balakrishnan 2012, 23).

Solar Radiation

Solar radiation (both direct and diffused) that strikes an unobstructed horizontal surface is high throughout the year in Sharjah (Figure 3.2), varying in the range of 3.5 to 7.2 kWh/m2. Simultaneously, the annual diffused solar radiation on an unobstructed horizontal surface composes around 41% of this global radiation, which is also high. Heat gains caused by solar radiation from the envelope have the highest impact on indoor temperatures, as discussed in Section 5.1. Therefore, it is essential to control and minimize the impact of solar radiation through form and other microclimatic designs to create the most suitable indoor setting.

Figure 3.6 illustrates the total average monthly global horizontal solar radiation on unobstructed vertical planes in different orientations in Sharjah. While the east and west orientations receive almost twice the amount of solar radiation than the south orientation during the hot period (May to August), vertical planes facing North receive almost the same amount of global solar radiation as the south orientation in this period. This is because a north-facing plane receives direct solar radiation in the early hours of the morning and the late hours of the afternoon during this period. However, during the mild and warm periods, the south-facing planes receive the maximum radiation due to the sun’s altitude.

Figure 3.6 Monthly Global Radiation Values on the Vertical Planes, Sharjah (Source: CBE Clima Tool 2021)

Figure 3.7 Cartesian Sun Chart for Sharjah overlaid with Global Horizontal Radiation (Source: CBE Clima Tool 2021)

Figure 3.8 Global and Diffuse Horizontal Illuminance for Sharjah (Source: CBE Clima Tool 2021)

Figure 3.9 Wind Rose for Mild, Warm and Hot Period for Sharjah (Source: CBE Clima Tool 2021) The annual average of sunshine hours per day in Sharjah is approximately 9 hours during the mild and warm periods. However, in hot periods it can rise to 11 hours per day. The high solar intensity is due to the sky’s clearness and the high sun’s high altitude, which is almost perpendicular during the hot period (Figure 3.7).

Sky Illuminance

The sky luminance in Sharjah is high throughout the year, with the global luminance value ranging from 15 to 70 Klux in the mild period and 50 to 100 Klux in the hot period (Figure 3.8). About half of it results from diffuse illuminance from the sky vault. Therefore, according to Yannas (2008), under these circumstances, 1 to 2% of outdoor illuminance is more than enough to meet the daylight requirements for any indoor activities. Given the design brief and occupant definition, since the bedrooms will be used during the day by the occupants to rest and sleep, it is paramount to control the high luminance levels during the daytime.

Additionally, sky cover conditions in Sharjah vary slightly from one period to another. Although light clouds can appear during the mild period, the sky is mostly clear throughout the year. This can help to define the effectual glazing ratio and floor plan depth for the proposed typology.

Prevailing Winds

Figure 3.9 shows the frequency, wind direction, and wind speed for Sharjah. It indicates an average wind speed of 3.3 to 4.5m/s throughout the year, with the strongest winds coming from the North-West, in the direction of the Gulf, for most of the months. However, from July to September (the hot period), the predominant wind direction is recorded as North. These high wind velocities can relieve outdoor spaces and streets during the hot period and should be considered while determining the placement of openings to maximize natural ventilation.