Gambian Climate and House Design

Gambia Climate and House Design

House design mainly revolves around 2 factors – the people who are going to use the house, and the climate you have to design the house in.
By understanding the climate, we can start to put into place design patterns that help the occupants feel comfortable at all times of the day and year.
We have put together this essay on the climate of The Gambia and its effect on house design.
If you have any comments please contact add them below.

If you travel from Europe to Gambia and keep an eye out of the plane window you will see the dry desert of Mauritania and then Senegal change dramatically to the subtropical vegetation of the river Gambia.

Being between the equator and the tropic of Cancer, Gambia has a definite subtropical feel and agreeable climate with a fairly regular maximum temperature all year round hovering around the 30C mark.

The principal climatic feature is the distinct variation between its short, but very rainy wet season, and it’s long and hot dry season.

From the point of view of the house design, it would seem that the distinct wet and dry seasons would create the need to accommodate wide design requirements.
This is certainly true to a degree, but this is not as big a problem as in European/USA because the maximum temperature and sunshine hours are still regular year round despite the wide variation in rainfall and humidity.

So our overall house design is to reduce the intense heat creating sunlight during the long dry season, then to make the humid rainy season comfortable with natural and created air movements.

We also want to utilise the lower sun angles in the early months of the year to heat low sections of solid walls to carry some heat into the lower temperature nights. And we need to shade the high sun angles during the wet and humid summer.

Let’s look at each climatic factor and its affect on house design.


TEMPERATURE
The external temperature is important of course for general comfort. While we cannot change the outside temperature, we can reduce the affect is has on the internal temperature of the house by design.

Average Temperature
The average daytime temperature year round at Banjul is 24C (75F). This shows the subtropical nature of the country.
This is within the comfort range of most people, especially those who have acclimatised. Further inland the average temperature will be higher.

What is most important though, is the maximum and minimum temperatures and day time variations.

Maximum Temperature
The average maximum daytime temperature on a monthly basis ranges very close to the 30C mark with it slightly higher (to 34C) just prior to the end of the dry season in March / April and going down to 29C in August in the middle of the wet season.

This is a very small variation in range and means we do not have to do any heating during the daytime to keep comfortable in the houses.

From the point of view of house design, as long as we can keep the heat away from the house during the day, and maintain breezes (natural or fan) then living conditions can be comfortable.

Also, these subtropical temperature figures might divert house designers away from the fact that the temperature can go mild at night as can be seen in the following;

Minimum Temperature
Of interest to house design is the fact that the average daily temperatures are much lower during the “dry”, (down to 16C during Jan (European winter)) and then slowly rise and remain consistently at 24C during the 5 months of the wet season from June to October.

So the daily range is much greater during the dry and the nights do feel cold as the day temperatures keep the human body acclimated to higher temperatures. So while 16C is high by European standards, in Gambia it does feel cool and cold at times in the night and early morning.

Using some form of heat sink in the building that will hold the high daytimer temperature and let it out during the night is a good strategy as long as it does not overheat during the wet season where the heat would bring the already high temperatures too high for night time comfort.


HUMIDITY
In general the humidity levels are high. With average maximum year round humidity figures of around the 80%. It does of course go up to 96/7 in September/October, the height of the wet.

What is of interest is that the minimum average humidity does not drop below 60% in the wet, whereas in the dry it goes right down to 22% minimum in February.

This is important for house design as the lower humidity in the dry allows for evaporative cooling using simple evaporative devises and water features and pools.

However the problems come in the wet with the high humidity greatly reducing the cooling effect of any evaporative cooling design. There is only two solutions here.

The first is to utilise air conditioning (high energy) and it is important to design in a air conditioned internal room or sunken area to reduce the energy needs. The second option being to move have high velocity air flow over the occupants (exhaust fans or downward blowing fans). Because of the high minimum humidity, this is also needed during the night as well as day.


SUNSHINE HOURS
The average daily sunshine hours are impressive, hovering around the 9 – 10 almost every month except during the July to October wet season.

This is of course important for allowing year round solar electrical and solar water heating. It also shows the huge amount of solar radiation coming down to the earth and the need to intercept it with adequate vegetation or roof lines.

The radiation also can be reflective, so wide roof overhangs must also be combined with methods for reducing reflective sunlight coming down outside the roof line and reflecting up of concrete or hard ground. Methods can include grassed areas, semi shade vegetation and pergolas etc.


SUNSHINE ANGLE
It is vital to understand the path of the sun in order to reduce its powerful tropical heating effect and also make best use of it in heating mass walls to transfer night time heat during the colder night dry.

Being close to the equator (latitude 13 degrees North), the sunshine angle does not vary greatly, but it does vary, and because of the long hours of intense sunshine, this variation can be important for affecting comfort in houses.

The Republic of Gambia is located in West Africa with a Latitude and Longitude of 13 degrees 28’N and 16 degrees 34’W respectively.

Sun angle calculations show that summer (June / July) angles is directly overhead (90 degrees), but during winter (December/January) the sun angle goes down to 70 degrees. and while not a lot by European standards, this is significant to get under small roof and window overhangs and heat the wall. This can be a problem if it heats too much, or useful if you want to heat the lower part of a wall to carry some heat into the night.

The sun track is also important for design as the easterly morning sun is suitable for kitchen / breakfast area, moving towards shaded bedrooms for the intense heat of the day and then finally to outside eating areas at night on the westerly side where the carry over heat from the day will allow comfortable conditions into the night.

Building orientation is also important with intense tropical sunshine. A square building will catch more direct and reflected heat on the walls than a longer thinner one if you orient the longer one on the east west line. The sun first hits the small easter end gable, then spends much of the day heating the roof rather than the walls, finally to heat the western gable at nightfall.
With adequate roof insulation (artificial or thatch/hay roof) this reduces overall heat gain.


RAINFALL
Although the months of heaviest rainfall are August and September, we have found that most rain occurs at night and has not detracted from our clients’ enjoyment of their holiday.
The annual rainfall in most parts of the country is around 51 inches (1,296 Millimetres). And with it coming in mainly the 3 months of July /august / September you can imagine the intense rain during the wet.

This is of interest to house designers for 2 reasons.
1/ – Intensity. This is no gentle European rain. This rainfall is short and intense and roof angles and ground landscaping must be able to accommodate this intense rain.
2/ – Water Supply. Of more importance for survival here is the difference between the long dry period of no rain, and the short wet period when water comes. For those contemplating rainwater harvesting, this means a long period of storage, and storage also in case of late rains after dry.

Because up to 80% of the aquifer recharge comes from the rainfall (the rest running underground from inland), the rain is important both for recharging water storages, and also for recharging the aquifer for pumping. Without a successful rainy season, the aquifers would be low.


WIND
Wind figures compiled and published on the web by GAmbia Civil Aviation Authority show a general year round wind direction coming from mainly north, but also west and east. No consistent southerly winds occur.

The dry season is from mid October to Mid June with the Harmattan wind (north east trade winds) keeping the humidity low bringing hot winds down from the Sahara and Sahel. So from November through to June the winds are northerly or north easterly / easterly. These winds are consistently the strongest (over 7m/second average) and have the greatest maximum wind speeds (up to 10m/second).
Once the wet season hits in June, the wind swings around to the west for the wet season and continues westerly through to October at the end of the wet. This wind brings the rain and humidity from the Atlantic and the wind speed drops down several m/second.

From the point of view of house design. The overall northern winds mean we should orient our houses with the long axis east to west to make it easier for the winds to blow through the house. This fits in nicely with putting the long axis east / west to reduce overall heating from the sun.

During the dry season, there are good winds and we should design to catch these to help cool during the day with large windows on the northerly and southerly side of the building to create air flow.

The drop in speed and change in direction during the very humid and hot wet season makes it hard to rely on passive wind cooling to reduce the heat stress, and we have to look at this period to have mechanical fans and perhaps a air conditioned room or area to be able to be comfortable.

From the point of view of wind electrical generation, good wind is available during the dry, but during the wet (when you may need greater energy reserves for air conditioning and electrical fans, there is less wind speed (and less solar radiation) and batteries need to be able to accommodate, or back up generator might need to be used.

With respect to large scale electrical generation from wind, a wind map of Africa shows that Gambia does not have the wind speeds needed for large scale turbine wind generation with an average speed of less than 5.9m/second. But if wind generation costs come down, then there is scope for village based generation.


MICROCLIMATE
Most of the preceding observations are for the coastal strip of Gambia where most of the house building is going on at the moment. But if you are planning to build further inland, there are some climatic variations that must be taken into consideration for passive house design.

Inland, in general, it is hotter. With wider minimum and maximum temperatures and less humid overall.
It is more like the Sahel regions to the north and the architecture of these regions is where we can learn some design adaptions. The houses of this region are moorish in design and have solid walls that act to stop daytime heat from getting into the houses, but also absorb enough to carry over so that the heat starts to enter the houses at night.
Inland the cool season is shorter and daytime temperatures are very high between March and June.
Other house design factors that should be taken into account is the cultural effects of climate and house design and the aclimatisation of the house users.


CULTURE, CLIMATE AND HOUSE DESIGN
In design, the relative closeness of Gambia to the desert like Sahel of Senegal, Niger, Mali etc and it’s similar islamic religion and culture has drawn many house designs to be that of the dryer regions. And these more solid, open mass houses were created to attract and store day heat for release during the colder night temperatures. A problem not faced in Gambia with its closer day and night temperatures.
Gambia is not part of the “Sahel” or marginal desert country. It is the start of the subtropics of West Africa and its architecture should reflect this.


ACCLIMATISATION
Because the dry season coincides with the European and USA winters and early spring, most persons who wish to establish a holiday house will be wanting if for the dry season mainly.
Acclimatisation is thus a problem if they are coming from cold weather to hot, and may only want to stay 2 – 4 weeks. May need to have cool areas or air con.

It is important to note that regardless of the actual temperature, it is the affect it has on the inhabitants that is important. If the persons living in the house are year round occupants, they will have acclimatised to the subtropical and they will be able to tolerate temperatures of up to 10C higher than if someone comes out from a cooler Europe. It will take 1 – 2 weeks for them to acclimatise.

So, of more importance to design is “Who” will be using it and when.

If the use is short trips by unacclimatised persons, then the use of a “cool area” (either sealed air conditioned, or “cool sunken area”) is important.
For year round inhabitants, optimising and creating breezes is very important for maintaining comfort.

Kevin Ison – Eekos


REFERENCES

– https://www.worldtravelguide.net/guides/africa/gambia/weather-climate-geography/

http://www.gambia.co.uk/docs/about_the_gambia/know_before_you_go/climate.aspx
Quick guide for tourists, graphs here give a good overview of the subtropical nature of Gambia and also compare it to London (successfully asking the question – why would you want to live in UK?!!)

– http://susdesign.com/tools.php
Excellent range of online tools for calculating sun angles and effects on buildings.

https://www.gambia.co.uk/weather
– Gambia average monthly weather climate – very good to give overall picture of all aspects of climate.

http://www.geni.org/globalenergy/library/renewable-energy-resources/world/africa/wind-africa/index.shtml
– shows extrapolated wind speeds at 80m (large turbine height) and potential for low cost wind power generation