Teaching Science-02

by S.B.Asoka Dissanayake visit www,writeclique.net

There is a bizarre (somewhat insane) discussion going on local English Papers. This is something I penned down many moons ago. Reproduced here for any sane person's perusal.

Weather as a Model for Scientific Investigation
As an example the weather can be investigated in a scientific way. This examination is not complete or comprehensive but taken as an instruction model for discussion and refinement. The limitations inherent in any model in illustrating a scientific fact (as a form of analogy) is also discussed briefly.

Whether one studies mathematics, chemistry or physics, science teachers are very comfortable in using equations to explain scientific themes. These equations have well balanced structures and are ideal for explanation (not investigation) of scientific notions but may fall short of that reality. That is something I would like to delve into.

My intention is to highlight the fallacy of using equations and equilibriums to state scientific facts as absolute truths (In a philosophical sense - the meaning of the word meaning?).

This approach of equations and equilibrium fails miserably when discussing the pattern weather and its behaviour.

This was what I have encountered as a child and still do and the recent tsunami was an eye opener to rekindle my latent interests in science of weather reporting. Even though this is not and attempt of in depth to analysis of weather or its reporting, an oblique reference is made to weather as a focal tenants of disagreement with the way the science is taught in our class rooms.

I have started addressing this in the Part 1 of the essay and this should be read in continuity with that and the Part 3 of my own observations that follow.

In weather an equilibrium state is never achieved in a scientific sense. It illustrates the uncertainty principle in general. Instead of an equilibrium state, cyclic phenomena are evident in weather patterns Weather is discussed in some detail below. The order and change (chaos) can be grasped without any difficulty unlike other scientific principles. The cyclical nature is apparent when one talks about monsoons and inter-monsoonal rain. What is evident is constant change but the order only becomes apparent because of repeated sequence of events (change and cyclic change adequately fits into this notion).

The cyclic nature of the phenomena is studied when forecasting of weather and it behaviour.

The model is the cyclic pattern of the water cycle.

Discussion is based on a scientific description of clouds and their behaviour which I copied from a web site with modification to suit the current discussion. To begin with there were over a hundred new scientific and technical words in in a short space of few passages. That is what I consider as the biggest handicap to a student or an average inquirer with open mind.

I would attempt to put that in perspective in simple terms as is possible but there is no guarantee.

It as a bold attempt since the few passages that I cover involve the entire package of scientific domains form physics to chemistry to mathematics to dynamics and logic and logistics.

Building Clouds

Although the formation of clouds can be quite complex in full detail, it can be simplified for a wider nonscientific audience.

There are two basic ingredients to satisfy formation of clouds; water and dust.
On earth naturally occurring clouds are composed of either water in its liquid or solid state. On other planets, where the surface atmosphere is different from that of the planet earth clouds may form from other compounds and that is not under discussion.

Thus, the primary recipe in forming clouds is water.

Collection of a sufficient quantity of water in a given space in its vapour state at a given time when the essential prerequisites (the temperature, the altitude, the pressure and the movement of molecules) are met the water vapour is transformed into clouds in either liquid or solid state.

The water vapour content of the atmosphere varies from near zero to about 100 percent, depending on the moisture on the surface beneath and the air temperature and condensation.

The water vapour content at a certain point of time and space needs to be ideally saturated to form clouds.

Next, recipe one needs is some dust.

Without "dirty air" there would likely be no clouds at all or only high altitude ice clouds. Earth atmosphere is never clean as one would expect it to be for healthy living (man's perspective). Even the "cleanest" air found on Earth contains about 1000 dust particles per cubic meter of air.

Neither a large amount nor large particles nor all dusts would satisfy the primary needs of forming clouds. Dust is needed for condensation (nidus or the nucleus) sites on which water vapour may condense or deposit as a water droplets (liquid) or ice crystals (solid). Certain types and shapes of dust and salt particles, such as sea salts and clay, make the best condensation nuclei. With proper quantities of water vapour and dust in an air parcel, the next step that has to be satisfied is the cooling of that air mass (i.e; cooling of the air parcel having a dust content of particular size and shape ) to a particular temperature conducive for the formation of cloud droplets or ice crystals (suspended in air in as a massive aggregation).

Viola, the clouds are formed.

Just as there are many ways to prepare a recipe, there are many different ways to form clouds. The recipe can be expanded with new ingredients for the precipitation to occur.

Professor John Day, the Cloud Man, has taken the simple cloud recipe, added a few more details and continued it until it makes precipitation(rain).
He calls this The Precipitation Ladder.

As with a simple recipe, he begins the process with the basic ingredients of dirty air and water vapour. As with cooking it is regulated (in real sense there is no regulators but changing states) to achieve the desired effect. He takes the ingredients through the rungs of the ladder in several stages (several processes) to form a cloud.

Ascent and Expansion are two of the main processes that result in the cooling of an air parcel in which clouds will form. We mostly think of moving air as wind flowing horizontally across the surface. (The movement of air is almost chaotic in a scientific sense but not random).

Air moving vertically is extremely important in weather processes, particularly with respect to clouds and precipitation. Ascending air currents takes the process up the Precipitation Ladder. The processes are assumed to be reversible. With descending air currents the process comes down the ladder reversing the effects until finally water vapour and dust are left in the air stream (mass of atmosphere) of movement.

There are four main processes occurring at or near the earth's surface which give rise to convergence, convection, frontal lifting and physical lifting of the ascending air.

Convergence occurs when several surface air currents in the horizontal flow move toward each other to meet in a common front. When they converge, there is only one way to go and it is upwards only. An area of low pressure (cell) build up on the surface of the earth is an example of where the converging air currents result in rising of air at the center of the converging currents. The air at the center rises to accommodate the redistribution of various air pressures (wind) that build up due to variable degree of cooling and warming of the atmospheric air creating low and high pressure points in the atmosphere. Convection occurs when air is heated by contact with a warmer land surface until it becomes less dense than the air above it. The heated parcel of air will rise until it has again cooled to the temperature of the surrounding air.

Frontal lifting occurs when a warmer air mass meets a colder one. Since warm air is less dense than cold, a warm air mass approaching a cold one will ascend over the cold air. This forms a warm front.

When a cold air mass approaches a warm one, it wedges under the warmer air, lifting it above the ground. This forms a cold front. In either case, there is ascending air at the frontal boundary.
Physical lifting, also known as orographic lifting, occurs when horizontal winds are forced to rise in order to cross topographical barriers such as hills and mountains. Whatever the process causing an air parcel (volume or quantity) to ascend, the result is that the rising air parcel must change its pressure to be in equilibrium with the surrounding air. Since atmospheric pressure decreases with altitude, so too must the pressure of the ascending air parcel. As air ascends, it expands. And as it expands, it cools. And the higher the parcel rises, the cooler it becomes.

Now that the cooling has begun the air parcel is almost ready to form a cloud.

The air parcel cools until condensation point is reached.
The next several rungs of the Precipitation Ladder describe the processes through to the condensation of liquid water.
As the air cools, its relative humidity will increase -- a process Prof. Day terms humidification. Although nothing has yet happened to change the water vapour content of the air, the saturation threshold of the air parcel decreases as the air becomes cooler. With decreasing saturation threshold the relative humidity increases proportionately.

Cooling is the most important method for increasing the relative humidity but it is not the only one.

Another is to receive more water vapour through evaporation or mixing with humid air that come in contact (cloud that has already formed) with the result of moving air currents containing more (various degrees) water vapour.
To form a cloud, humidification may eventually bring the air within the parcel to saturation. At saturation the relative humidity is 100 percent. Usually a little more humidification is required to bring the relative humidity above 100 percent, a state known as supersaturation, before a cloud forms. When air becomes supersaturated, its water vapour condenses out.

If the quantity and composition of the dust content is ideal, condensation may begin at a relative humidity below 100 percent. If the air is very clean, it may take high levels of supersaturation to produce cloud droplets. But typically condensation begins at relative humidity a few tenths of a percent above saturation.

Condensation of water into condensation nuclei (or deposition of water vapour as ice on freezing nuclei) begins at a particular altitude known as the cloud base or lifting condensation level.

Water molecules attach to the particles form cloud droplets which have a radius of about 20 micro meters (0.02 mm) or less. The droplet volume is generally a million times greater than the typical condensation nuclei.
Clouds are composed of large numbers of cloud droplets or ice crystals or both. Because of their small size and relatively high air resistance, they can remain suspended in the air for a long time, particularly if they remain in ascending air currents. The average cloud droplet has a terminal fall velocity of 1.3 cm per second in relatively still air. To put this into perspective, the average cloud droplet falling from a typical low cloud base of 500 meters would take more than 10 hours to reach the ground.

Forming Precipitation

We know that all clouds do not produce rain that strikes the ground. Some may produce rain or snow that evaporates before reaching the ground, and most clouds produce no precipitation at all. When rain falls, we know from measurements that the drops are larger than one milli meter. A raindrop of diameter 2 mm contains the water equivalent of a million cloud droplets (0.02 mm diameter). To get some precipitation from a cloud, there must be additional process within the cloud to form raindrops from cloud droplets.

The next rung of the Precipitation Ladder is Buoyancy or Cloudiness which signifies that the cloud water content must increase before any precipitation occurs. This requires a continuation of the lifting process. It is assisted by the property of water of giving off heat when changing from vapour to liquid and solid states, the latent heats of condensation and of deposition, respectively. If the vapour first changes to a liquid before freezing, then there is the latent heat of condensation released and followed by the release of the latent heat of freezing. This additional heat release warms the air parcel and adds to the lifting effect. In doing so, the buoyancy of the parcel relative to the surrounding air increases, and this contributes to the air to rise further rise.
Now in the cloud, there must be a Growth of cloud droplets to sizes that can fall to the ground as rain without evaporating. Cloud droplets can grow to a larger size in three ways.

The first is by the continued condensation of water vapour into cloud droplets and thus increasing their size until they become droplets. While the first condensation of water onto condensation nuclei to form cloud droplets occurs rather quickly, continued growth of cloud droplets in this manner will proceed very slowly.

Second, growth by collision and coalescence of cloud droplets (and then the collision of rain drops with cloud droplets and other drops) is a much quicker process. Turbulent currents in the clouds provide the first collisions between droplets. The combination forms a larger drop which can further collide with other droplets, thus growing rapidly in size. As the drops grow, their fall velocity also increases, and thus they can collide with slower falling droplets.
A 0.5 mm-radius drop falling at a rate of 4 m/s can quickly overtake a 0.05 mm (50 micro meter) drop falling at 0.27 m/s. When drops are too large, however, their collection efficiency for the smallest drops and droplets is not as great as when the drops are smaller in size. Small droplets may bounce off or flow around much larger drops and therefore do not coalesce. A drop about 60% smaller in diameter is most likely to be collected by a large drop.
Clouds with strong updraft areas have the best drop growth because the drops and droplets stay in the cloud longer and thus have many more collision opportunities.

Finally, it may seem odd, but the best conditions for drop growth occur when ice crystals are present in a cloud. When small droplets form, liquid water should be cooled well below 0º C (32º F) the freezing point. In fact, under optimal conditions, a pure droplet may reach - 40º C (or 104º F) before freezing. Therefore, there are areas within a cloud were ice crystals and water droplets co-exist. The ideal condition necessary for precipitation, in other words, rain has been duly satisfied.

The technical terms that were associated with the passage I obtained from a web page (which I have changed to make the flow as I would have wished) are enormous and even with a mastery in English language and grammar one may find it difficult to understand the scientific concepts in its entirety. Put that into simple English is difficult enough but I would make an attempt to simplify the contents.

Summary

For the synthesis of the above discussion in simple terms simple enough for younger age group (instead of 7 stages) person to understand I would break it down the concept of cloud formation into 3 or 4 essential stages without upsetting the authors description of the events in a hypothetical environment.

1.Formation of water vapour in a focus of dust particle of a particular shape and size.

2.Ascent of that tiny water vapour mass enclosed around the dust particles with the change in wind currents

3.Cooling and Condensation at high altitude

4.Acquisition of a particular size when gravitational pull brings it down (down to earth attitude) to the earth surface.

Next part of the discussion I would make my observations on the basic scientific tenants of uncertainty principle, change, chaos, gravity, entropy and thermodynamics in a superficial and philosophical point of view.