Friday, September 25

Shapes in the sky – weather watching

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Low level stratus and altostratus clouds above fields.

IN a previous article, I referred to my lack of boredom for months whilst in Kuching during the various stages of Movement Control Order. I well remember the words of the mid-19th century French novelist Gustave Flaubert, “Nothing is boring if you look at it for long enough. There must always be a secret truth to such.”

Staring at the skies and watching clouds form and disappear together with the build-up of humidity allowed me to predict the weather for the next few hours or even the next few days. How many of us, in the past, have sat or lain on a beach or in our garden and looked up at cloud patterns and their movements in the sky? Did we see familiar faces or land shapes that we recognised in those shapes? Seeing such shapes in clouds is the phenomenon pareidolia. This is a normal human tendency in one’s mind’s eye to relate patterns of shapes that we see to our memories.

I have seen the coastal outline of northern Sabah with its dog-like pricked ears and gaping mouth in cloud formations and castle like fortifications in the air as the cumulus congestus clouds tower upwards to 2,000 to 3,000 metres above my head in Borneo! Readers of my vintage will well remember a meaningful song of Jodi Foster (1969) entitled, ‘Both Sides Now’, and one still relevant in our confused world of today.

The first verse of the lyrics reads:

“Rows and rows of angel hair

And ice cream castles in the air

And feather canyons everywhere

I’ve looked at clouds that way.”

What are clouds?

They are but masses of water droplets in which there is a continuous accretion by condensation and subsequent loss by evaporation. A rising body of air, a bubble, is cooled to dew point at which condensation then takes place, much like the steam from a boiling kettle or a pot on an oven’s hot plate. Condensation level demarcates the cloud’s base. As the margins of such heated bubbles of air mix with drier air surrounding them then the relative humidity of the air is reduced below 100 per cent thus leading to the evaporation of the cloud’s water-droplets.

Alternatively, rain may fall from the cloud thereby removing a proportion of cloud water. Latent heat is produced from condensation thereby giving a cloud the impetus to rise even higher in the atmosphere.

The very first classification of cloud types is attributed to a French naturalist, Lamark in 1802. In the following year, Luke Howard, the so-called Father of Modern Meteorology, produced a new classification using Latin names which had international significance such as cumulus, stratus, and cirrus, respectively meaning, piled up, layer or sheet, and hair-like!

There are many varieties of each these three cloud types, most of which are best described in tabular form, outlining only 10 main types. Here, in Borneo, we may wonder how much rain is likely to fall from each type of cloud that we can observe. We can bet our bottom dollar that, in our tropical climate, the warmer the air is to start its uplift, the more water vapour a cloud can hold so the more sudden and with larger droplets the rain will fall than in more northerly latitudes. Such heavy and intense down pours often lead to flash flooding.

Unfortunately, the human brain does not have the capacity to accurately estimate slope angles on hill sides nor is it able to measure the actual vertical height of cloud base levels or their altitude in the sky. Instruments are used for these purposes.

Rising cumulus clouds during summertime in Somerset.

Cloud types

Cumulus clouds, those cotton wool puffs are symbolised in our children’s early paintings. Actually, they only have a lifetime of between five and 30 minutes. Cumulonimbus clouds are a different matter for they are accompanied by thunder and lightning. A major factor in their formation is the upward extension of the cloud where water droplets turn to ice, usually at a height of 18km over Borneo and at only 12km over Western Europe. It is within such clouds that vertical currents of hot air can reach up to speeds of over 50km per hour. Hence, when an announcement occurs on an aircraft flight to fasten seatbelts and that food will no longer be served and toilets are closed, we know that we are in for a turbulent ride! When the upper limit of convection occurs is reached, these clouds flatten out into an anvil shape.

Stratus clouds are associated with a stable atmosphere with widespread cooling over a large area. They have a wide horizontal extent and less vertical depth for they are associated with weak vertical air currents. In Borneo, this layer cloud may lead to early morning fog and poor visibility but within a couple of hours the above sun soon ‘burns off’ or evaporates these low lying clouds.

Cirrus clouds are at a very high level and formed entirely of ice in the top 2km of the troposphere where the air is well below freezing point. With hardly any moisture in the air at this level there is no rain and thus any condensation occurs in the form of ice crystals. Watching such wisps of these clouds for long enough, they change rapidly and often with two lines of cirrus going in different directions. This implies that there are strong and variable winds at such high altitudes.

How much do clouds weigh?

Clouds are much heavier than we may perceive, at ground level, in our views of only two dimensions. It is only on an aircraft fight that we can see their density and extent from above. On a clear day with individual cumulus clouds overhead, at a couple of kilometres high, each cloud holds over 500 tonnes of water in minute droplets. These are so light that they stay floating in the sky buoyed up by gentle air currents from below. However, a cumulonimbus cloud holds roughly two million tonnes of water thus explaining why they rain so hard pounding down on our rooftops for 30 minutes or a few hours. Most of this rainfall will soon evaporate in our tropical climate with a subsequent uplift from hot land surfaces back into the atmosphere. What triggers rain to fall? That is another story.

Urban area weather patterns

Whatever large urban area we may live in, we see variations in rainfall in different parts. This is well exemplified in both Kuching and Kota Kinabalu, with both cities having scattered commercial hubs. In Kuching, I have noticed this with rain pouring down in Tabuan Jaya, yet in Tabuan Laru and Muara Tabuan, only a few kilometres away, the roads are bone dry. This you might say is due to high winds in the atmosphere pushing storm clouds in a particular direction.

It is more likely attributed to the heat generated by commercial buildings and the heat reflected back into space from the density of their concrete structures leading to rising thermals pushing cumulonimbus clouds up to greater altitudes. If you live in a high rise apartment, look out from your balcony and observe the bottom literally fall out of a dense cloud, in the form of torrential rain, some distance away from your location.

You may well ask yourself as to when it will rain over you when you see flashes of lightning in the distant skies. In such ‘green cities’ as Kuching and KK, watch for the rustling foliage on trees, then the top branches swaying, and the temperatures falling slightly for winds are rushing in to create thermal uplifts giving the approaching cumulonimbus clouds greater uplift. Heavy rain will soon follow!

In my rural environment in the UK, much the same happens to vegetation in an approaching summer convectional thunderstorm. In fact, only a few weeks ago, in British Summer Time, hailstones landed on my house and covered the country lanes.

Besides looking at clouds as weather signals, I look for bird movements and their heights in the air above. In summertime, hawks fly very high using rising heat thermals to good effect but if the swifts and swallows fly low in search of flying insects I know that impending rain is just an hour or so away. Nature tells us much about our present and expected weather.