Weighing 'Wind Distribution Pattern' for Building Design in Dhaka City

How ‘Wind Speed’ and ‘Wind Flow Direction’ may shed the way the South and Southeast are shaded

I wish it were windier in Dhaka city!  (Photo by Keenan Constance on Unsplash)

Neither Chicago nor it is Seoul. Dhaka rarely sees breezes of that kind of windy cities. However, superimposing wind data on Windrose Daigram illustrates that not all quiet on the Southern and the Eastern Front. At least 68% of the yearly breeze are from the South, East and Southeast (See Fig. 1). Here, yellow coloured area in the windrose diagram showing that combined percentage of wind flow in Dhaka city. Keeping openings in the South and Southeast would surely aid in natural ventilation of buildings. However, due to equatorial locational aspect, shading design is necessary to modulate solar gains inside the buildings during summer seasons.

Fig. 1: Windrose of Dhaka city coupled with ‘percentage of occurrences’ based on the flow direction.

The Second figure is more interesting, as 0 – 10 km/h (2.78m/sec) of wind speed occurred around 50% of the time in a year (see Fig. 2). It is considered 2 to 3m/sec of wind velocity in urban settings, as 'Refreshing'! Not only that 10 – 15 km/h (2.8 to 4.17 m/sec) occurred nearly 35% of the time in a year. Wind measurement point was 30 feet above the ground level. A further analysis revealed that 5m/sec of wind speed occurred around 43% of the time in a typical meteorological year. Similarly, 1m/sec and 12 m/sec of wind speed is seen 26% and 22% of a typical year respectively. It is to be noted, according to Beaufort’s wind scale 1m/see of wind speed is weighed as light air and hard to feel in outdoor settings. In 5m/sec of wind speed hair blown, dust & loose paper is raised. 12 m/sec of wind speed is treated as a strong breeze and hard to hold umbrellas easily. Learn more about wind speed and the Beaufort's wind scale, by clicking here. 

Fig. 2: Windrose of Dhaka city coupled with ‘wind speed’ on a percentage of occurrence pattern.

April and July are the windiest months. As can be observed in the following two figures, the time between 10 AM to 7 PM is the windiest time period in a given day. Nighttime wind speed is calm than daytime air velocity & occasional gale force wind. Above 19m/sec wind speed is not exceptional, happens more than 7% of time in a typical meteorological year.

Fig. 3: Wind speed distribution of Dhaka city is represented in the above 2(two) plots.

Since air movement in the summer months is considered as the key in providing successful natural ventilation, the provision of wind from the South and Southeast direction should nudge designers thought process. I would share more studies on wind pattern of Dhaka city in later posts. If you have anything to share on wind pattern or analysis, please a comment below.

This study was conducted in the late 2016 and in the early 2017 with Autodesk Ecotect Analysis 2011 and Microsoft Excel 2016. Meteorological data was from Prof. Dr. Monjur Mourshed of Cardiff Univeristy, UK; for the year 2000 to 2009. Besides weather data from www.meteoblue.com aided this study.