The Power of The Rain in December, 2021 over Selangor
It has been raining, and raining all day and night towards the 2nd week of December, 2021 last year, and I was unable to sleep entire nights over the months due to a leg pain from a venous stasis ulcer. It has been like that for nearly 2 years now since the Covid pandemic
Continuous heavy rains have been falling over Malaysia due to the North-East Monsoon with flood water submerging several parts of this country last December. Being unable to sleep, I thought I should write something wondering where all these rains and water came from?
In this country during the NE monsoon seasons most of the rain falls over the eastern part of Peninsular Malaysia.
Peninsular Malaysia contains numerous mountain ranges running parallel from north to south along the peninsula. The main mountain range dividing the east and west coast of Peninsular Malaysia is the Titiwangsa Mountains with Mount Tahan or Gunung Tahan being the highest point in Peninsular Malaysia with an elevation of 7,175 ft (2,187 m) above sea level. It is this main range of mountains that blocks the blunt of the north-east monsoon rains from crossing over to lash the western coast of Peninsular Malaysia.
Most rain clouds are below 2,500 metres, and the main range of mountains effectively act as a barrier for the monsoon rain clouds to cross over towards the west coast.
But in December of last year 2021 it was quite a different scenario. Heavy rains were experienced over the western part of Peninsular Malaysia.
The question I ask myself how do we actually measure the amounts of rainfall that fell, not just in term of inches or millilitres as is usually done with little understanding by most people, but in terms of actual volumes and mass over a certain area such as in the State of Selangor where I stay, and where did the power of the rains come from?
So, without much sleep in my life these days, I decided to deal with this question entirely in a different way rather than merely expressing precipitation or rainfall in inches or in millilitres. Let’s have a look at what I have in mind, just to share with you.
The Measurement of Rain:
The most common rainfall measurement is the total rainfall depth during a given period and is expressed in millimeters (mm). For instance, we might want to know how many millimeters of rain fell over the course of 1 h, 1 day, 1 month, or 1 year. But what does this actually mean in terms of cubic metres or the mass of the water that fell?
This is done by using rain gauges to measure the precipitation in millimetres in height collected during a certain period.
The standard instrument for the measurement of rainfall is the 203mm (8 inch) rain gauge. This is essentially a circular funnel with a diameter of 203mm which is kept in an open area, so that it collects the rain into a graduated and calibrated cylinder. The measuring cylinder can record up to 25mm of precipitation but with an overflow for extra measurements
The precipitation value in mm refers to the amount of rain per square meter in one hour. This means that one millimeter of rainfall is equivalent of one liter of water per square meter.
Now, let us see where the rains come from?
Categories of Clouds:
There are ten main cloud types, which are further divided into 27 sub-types according to their height shape, colour and associated weather, Clouds are categorised as low as 2.5 km from the earth’s surface, to the middle (2.5 to 6 km), or high (above 6 km).
The major clouds are: Altocumulus, Altostratus, Cirrocumulus, Cirrostratus, Cirrus, Cumulonimbus, Cumulus, and the Nimbostratus.
Cumulonimbus clouds, sometimes called "thunderheads," are associated with thunderstorms, lightning and intense, heavy rains as well as hail. Cumulonimbus clouds grow vertically and commonly adopt an anvil shape, with a low, dark base often only 305 metres (1,000 feet) above ground and tops reaching up to 15,200 metres (50,000 feet) into the atmosphere.
Cumulonimbus clouds are the kings of all clouds, rising from low altitudes to more than 60,000 feet (20,000 meters) above ground level. They grow due to rising air currents called updrafts, with their tops flattening out into an anvil shape.
Using radiosonde observations, it was found that clouds during the SW monsoon season mainly concentrated below 2,500 m. A layer with relatively void clouds was present between 2500 and 4000 m. Meteorologists call this region a cloud-free zone. Probably this observation also applies to the NE monsoon
Types of Rains:
Let’s now look at the types of rain.
According to the Department of Irrigation and Drainage, Ministry of Environment and Water, Malysia, rainfall intensity in an hour may be light (1-10 mm), moderate (11 – 30 mm), heavy (31- 60 mm). very heavy (>60 mm).
Convective rain more than 60 mm in 2 to 4 hours duration (typical) may cause flash floods. However, monsoon rains are typically of long duration with intermittent heavy bursts and the intensity can occasionally exceed several hundred mm in 24 hours. This was what was experienced in December last year.
According to the May 2020 Journal of Physics Conference Series: DOI:10.1088/1742-6596/1529/5/052014 the annual average rainfall in Peninsular Malaysia is 2,420 mm, 2,630 mm for Sabah, and 3,830 mm for Sarawak
However, there was a record rainfall in Selangor that passed the 380 mm mark in the State of Selangor towards Christmas last year (2021)
Nevertheless, the report did not state the duration of the rainfall, the exact location or if the precipitation was uniform throughout the State of Selangor, Malaysia. We presume the data was collected randomly from several locations throughout Selangor and not just over a limited area in Selangor. We may presume the rain was wide-spread since where I stay in Gombak in Selangor we too experienced heavy rains as it was in Shah Alam, the capital of Selangor, over 30 km away. Parts of Shah Alam were inundated by severe floods down-stream from where I stay near the Gombak River that then joins another river toward that capital. All the water collected upstream flowed downstream to flood Shah Alam Whatever it was, it was still the amount of rain that passed the 380 mm mark, probably over the entire State of Selangor.
Since precipitation in millimetres in height collected is equivalent to litres per square metre as already mentioned, and since the area of Selangor is 8,104 square km, we may presume whether or not the rain was uniformly distributed over the State, the total amount of rain water over Selangor during the period would be 380 mm x 8,104 x 1,000,000 sq. metre = 3.07952 x 10^12 litres (3,079,520,000 cubic metres). This is also the mass of rain water in kilogram since the density of water is one kilogram per cubic metre. That was a tremendous amount of water that flooded several parts of the State of Selangor up to the top of houses and buildings.
Quantities of Water:
Let us imagine if this amount of rainfall in Selangor alone were to be all drained into the various water catchment areas (dams and lakes) in the State, what would the filling capacity like?
For instance, the Sungai Tua Batu dam where I get my water supply, covers an area of 2.50 square km. a height of 44 meters, and a surrounding catchment width of 50 square km, has a maximum capacity of 36.6 million cubic metres, but a holding capacity of 30,199 million litres (30,199,000 cubic metres). This means the collective volumes of rainfall elsewhere in Selangor, if channelled into the holding capacity of this dam, would be able to fill it 102 times over in one sitting. That’s a tremendous amount of water from this rainfall in Selangor alone that caused severe floods up to houses and buildings in several areas
There are 7 major dams in Selangor that supplies water to the entire Klang Valley including Kuala Selangor, Hulu Selangor, Putra Jaya, Kuala Lumpur, Pedalling Jaya, Klang and elsewhere. These 7 dams are the Sg. Selangor Dam, Sg. Tinggi Dam, Tasik Subang Dam, Klang Gate Dam, Semenyih Dam, Sg. Langat Dam & Kelau Dam
For instance, the Klang Gates Dam in Ulu Klang, Gombak District, Selangor, Malaysia is one of the major sources of drinking water for residents of the Klang Valley, where the national capital, Kuala Lumpur, is located. The dam capacity is 25,104 million liters (25,104,000 cubic metres).
According to statistics from the Selangor Water Management Authority (Luas) website, the Langat Dam recorded a capacity of 69.37 per cent or 23.65 million cubic litres, while the Sungai Selangor Dam was at 78.17 per cent (179.8 million cubic litres). That was the record on 1 Apr 2016
This also means if the rainfall over Selangor in the days just before Christmas were to be completely challenged into the Sungai Selangor Dam or the Langat Dam, two of the biggest dams in the State of Selangor, it would be able to fill the huge lake and dam of Sungai (River) Selangor up to 17,130 times and 130,200 times for the Langat Dam respectively. That was the amount of water that fell out of the sky over Selangor alone, let alone throughout East and West Malaysia and elsewhere across vast stretches of the South China Sea from where the NE monsoon blows across
The rainfall over Selangor was so massive that it inundated several areas with flood waters covering several housing estates in Selangor alone, not counting the tragedy experienced in several States in Malaysia. I am lucky to stay in a place in Gombak where the only place that was flooded were the drains outside my house.
In a study on the conditional probability structure and statistical dependence, the amount of rainfall depends on several gauging stations located around Peninsular Malaysia, namely Subang, Senai and Kota Bharu. Daily rainfall measurements for all stations were collected from the Department of Meteorology, Malaysia are long and reliable, with at least 40 years of data. The average annual rainfall estimated for Kota Bharu, Subang and Senai are 2,627 ± 574 mm, 2581 ± 399 mm and 2499 ± 340 mm, respectively.
The Power Behind:
Our next question is, where did the power that drives the rain, not even accounting the strong winds that normally accompany the rains come from? Obviously, it must come from the ocean waters of the South China Sea during the NE monsoon that lifted the water up as rain clouds by the energy of the Sun
The energy required to evaporate the ocean waters by the Sun into rain clouds 2,500 metres up into the air as potential energy is equivalent to the kinetic energy or the power of the rain that fell.
This means the potential energy given by mass (m) x acceleration of gravity (g) x the presumed height of rain clouds for the area of Selangor then was:
3.08 x 10^12 kg of rain clouds x 9.8 m per second per second x 2,500 m in altitude = 7.5 x 10^16 (75,000 trillion) Joules.
This mean that the Sun spent 1.9 x 10^ - 8 % (0.000,000,019 %) of its energy output per second to lift up the ocean waters and blowing them across the South China Sea in the NE Monsoon and dropping them off the State of Selangor alone just before Christmas last year (2021), not counting the entire Malaysia and vast stretches of other land masses and the South China Sea where heavy rains also fell during this NE Monsoon. We shall go into the calculation for this energy expenditure shortly.
The amount of energy we receive from the Sun is measured by the solar constant
The solar constant embraces all types of solar radiation, not just the visible light. It is measured by satellite to be roughly 1.366 kilowatts per square meter (kW/m²).1 Watt = 1 Joule per second (1W = 1 J/s) which means that 1 kW = 1000 J/s. A Watt is the amount of energy in Joules per second
The direct solar irradiance at the top of the atmosphere fluctuates by about 6.9% during a year. This variation is between 1.412 kW/m² in early January to 1.321 kW/m² in early July due to the Earth's varying distance from the Sun. The total amount of energy the Earth receives from the Sun is about 1.740×1017 W, plus or minus 3.5%.
This is measured by the solar constant that varies over long periods of time, but it is approximately 1.366 kW/m². This is equivalent to 1.96 calories per minute per square centimeter,
If we use this average of 1.366 kW/m², without going into complicated astrophysics or nuclear physics, it is possible for us to calculate approximately how much energy is generated by the Sun by determining the total amount of energy radiated over the surface area of the sphere described by the average distance of the Sun to Earth which is about 1.5 x e11 metres (150 million kilometres), assuming there is no loss of energy on the way from the Sun to this sphere
This works out to be A = 4 π r^2 x 1.366 kilowatts / m² = 3.86 x 10^23 kilowatts (3.86 x 10^26 watts), or 3.86 x 10^26 Joules per second, where r is the Sun-Earth distance at 1.5 x e11 metres (150 billion metres).
More accurately using more advanced methods and advanced calculations done by scientists in nuclear physics and astrophysics whose calculations is beyond me as I am not an astrophysicist or a solar nuclear physicist, this works out to be 3.846×1026 watts or 384.6 septillion watts which is not bad using our simple method in the physics of radiation against far more advanced methods done by astrophysicists. The Sun energy output per second is equivalent to about 9.192×1010 megatons of TNT
The Sun produces this energy by fusing its core of about 600 million tons of hydrogen into helium every second, converting 4.26 million metric tons of matter into energy every second as a result.
Thus, we can see the power of the rains, storms, monsoons, hurricanes, typhoons, the floods everywhere, whatever they are, must come entirely from the Sun. without which nothing moves on this Planet Earth
This Earth is ravaged by violent forces of Nature everywhere, not just in Malaysia, let alone Selangor that experienced heavy rains and floods. It was given just as an example the energy of rains over Selangor only
Thank You for reading just to share.
The above calculations were based on the following equivalents:
cubic metre = 1000 litres
1 cubic meter of pure water = 1000 kg
1 sq. km = 1,000,000 square metres
1 meter = 1,000 mm
1 square metre = 1,000,000 square millimetre
1 litre = 1,000,000 cubic millilitres
1 mm of rain = 1 litre of rain per square meter
1 kilowatt = 1,000 watts = 1,000 Joules per second
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