Category Geology

   Many windblown drops can be forced together to form what Weather reporters call ‘sheeting rain’, but rain is always born as minuscule drops of condensed  water vapour explains ‘Clouds and Weather’ by John A. Day and Vincent J. Schaefer (Houghton Miffin Company), U.S. The formation of these droplets depends on the right amount of water vapour at the right pressure and temperature, but it also requires the presence of tiny solid particles of matter in the air on which the water vapour can gather and condense.

            These bits of dust and salt are called cloud condensation nuclei. Salt starts collecting vapour at about 80 percent relative humidity, while bits of clay begin to take on water molecules at 100 percent relative humidity.

            

            Rainfall is measured in terms of the level or height to which water is collected or accumulated on a flat surface through rain. It is usually expressed in millimeters to the nearest whole number. Rather than measuring all the rainfall falling over a large areas, which is impractical, rainfall is measured at a number of points over the area. There are many instruments for measuring rainfall; the most commonly used is the rain gauge.

            Rain gauge consists of a funnel (5”-6”in diameter), a measuring tube (usually one tenth of the funnel in diameter to measure accurately even the trace amounts of rainfall) and the outer cylindrical cover with a base. The rainfall falling into the funnel is directed into the measuring tube which is calibrated accordingly.

            The excess water, if any, overflows the tube and is collected within the outer cover. This is measured subsequently. Based on the material by which the parts of a rain gauge are made, it may be fibre glass type or metal type. For continuous recording of rainfall, recording rain gauge is used where the rise of water level is automatically monitored continuously. Recording rain gauge may be with float type recorder or weighing type recorder.

            In float type recorder the vertical movement of the float (with the rise of water level) is recorded by a pen on a chart fixed on a rotating drum; whereas in the weighing type, the weight    of the receiver is recorded by an weight balance. The rain gauge must be placed at horizontally (at about 12” height from the ground) at a distance of twice the height of the nearest objects like trees, buildings etc.

            Rainfall occurring in any place is simply measured as the height of the rainwater on the land in that place provided it is not lost due to run-off, evaporation etc. and the land is flat. Measuring rain this way is however impossible. A rain gauge must be used.

            A simple rain gauge which any one can use to measure rain at his place consists of a funnel (3”to 4” in diameter) fitted into a bottle (about 1 litre capacity) to collect the rain water and a measuring cylinder. (An air-vent is to be provided to prevent accumulation of water in the funnel in case of heavy down pours.)

            The rain gauge is kept on the ground in the open without obstructions from buildings, trees, etc. if the rainfall, over a period of time is 1cm at a place where the rain gauge is kept, then the height of the rainwater collected would also be 1 cm only if the bottle is flat at the bottom and its area cross section is the same as the area of the opening of the funnel.

            Since this specification cannot be followed the volume of water so collected has to be measured (this will be constant for a given size of funnel irrespective of the size or shape of the bottle) to know the amount of rainfall. Suppose the area of the opening of the funnel is 80 cm2then for 1 cm of rainfall the volume of water would be 80 cm2 x 1 cm that is 80 cm3. This amounts to 8 cm3 of water for every mm of rain.

            Thus if the total volume of rain water (in cm3) collected, over a specified period, is divided by 8, we get rainfall in mm in that place over the given period. To get accurate rainfall data quickly by directly observing the water level, a modified form of the above described rain gauge is used in all meteorological observatories. In this, rain water is collected in a narrow graduated tube so that the height of rain water is increased several times for the same amount of rain.

            This facilitates accurate measurement of even low rainfall like 1 mm or less. If the area of fifth of the area of the opening of the funnel then for 1 mm rainfall the height of rain water in the tube would be 5mm. if the graduations and made accordingly, the water level in the narrow tube directly gives the rainfall.

            For measuring continuous rain (which lasts several days on many occasions) automatic rain gauges are in use. In one type, called weighing type, the rainwater as it falls is weighed and translated into a continuous record on a clock-chart. Thus gives not only the total rainfall but also the time of its occurrence and its intensity.

  Precipitation in clouds may be initiated by two different processes. One of those is the coalescence process which is favoured in clouds that are relatively warm with high water content. In this process once the precipitation particles are formed they grow primarily by sweeping out cloud droplets on its trajectory or by combining with one another. This process depends upon various factors such as water content and droplet size.

            The second process is known as ice crystal process. Ice crystals appear to form in clouds when temperature drops down to -15 degrees Centigrade. In general, water in contact with most materials freeze at temperatures warmer than -40 degrees Centigrade, and sublimation will occur on most materials at super cooling’s. So the ice crystals present in clouds serve as ice nuclei (around which a droplet may form). So precipitation may be encouraged by exploiting one of these two mechanisms.

            Meteorological conditions essential for artificial production of rain are similar to those leading to natural rainfall. In natural rain process in convective clouds (warm clouds) one droplet in 10 6 grows to become a raindrop. This is about one droplet in five litres.

            One approach to stimulate rain in warm clouds is to increase this concentration of large droplets by spraying water from airplanes flying at cloud base. These droplets (radii of 20-30 microns) should be large enough to be in a favoured position for growth. Salt particles may also be injected in cloud base to provide centres on which cloud droplets can form. For super cooled (below freezing) clouds, the most effective seeding agent is dry ice.

            Nucleation occurs most readily on surfaces having a lattice structure geometrically similar to that of ice. The material that most closely approximates ice in lattice structure, known so far, is silver iodide (AgI).

            In case of stratiform clouds (clouds with no vertical extent but cover a large area) whose top is super-cooled, the natural precipitation process may proceed slowly due to scarcity of ice nuclei.

            In such cases the introduction of ice crystals near the cloud top by seeding AgI or dry ice may cause precipitation that would not occur otherwise. The introduction of these agents artificially into the clouds in concentration of about one per litre is expected to promote precipitation.

            Some of the clouds which cause rain are the stratus and stratocumulus clouds. Cumulonimbus clouds are the main rain clouds.

            Rainbows are arc shaped due to a simple geometrical principle. When the Sun shines after a shower, we often see an arc of beautiful colours in that part of the sky opposite to the Sun. If the rain has been heavy, the bow may spread all the way across the sky and its two ends seem to rest on the Earth below. The cause of this interesting phenomenon is the reflection and refraction of the Sun’s rays as they fall on drops of rain. As a ray passes into a drop of rain, the water acts as a tiny prism. The ray is bent, or refracted as it enters the drop and is separated into different colours. As it strikes the inner surface of the drop it is further refracted and dispersed. What we see in the heavens is a beautiful natural spectrum, produced by many drops.

            There are seven colours (wavelengths) in each bow – violet, indigo, blue, green, yellow, orange and red. But these colours blend into each other so that the observer rarely sees more than four or five clearly. Each colour is formed by rays that reach the eye at a certain angle, (about 42 degrees for primary bow and 50 degrees for secondary bow) and the angle for a particular colour never changes. The higher the Sun the lower the bow and if the Sun is higher than 40 degrees, no bow can be seen. According to simple geometrical principles, the rain drops which lie at this particular angle and direction opposite to the Sun lie in the form of a full circle of a part of it (arc). Even if there are enough rain drops to form a full circle, to an observer on Earth it will look like an arc as it is limited by the horizon. When the Sun is near the horizon, an observer on a high mountain or in a balloon may see the whole circle of the rainbow.

            The size of a rainbow is fixed by the way the Sun’s rays go through the raindrops. When a light ray strikes a raindrop, part of it is reflected and lost and part is refracted into the drop. When this ray hits the back of the drop, part of it is refracted out and part is reflected back to the front surface. Part of this reflected ray is again reflected and part is refracted back out. If the original ray hits near the centre, it will be deflected by  and return along the same path. This is how casts eyes work, but you will never see sunlight reflected this way because of the shadow cast by your head.

            But what happens if the original ray hits the raindrop off-centre? As the point of contact moves away from the centre it reaches a point where many rays return virtually in the same line, and reinforce each other to make a bright return at  from the sun line – the line from the Sun to the raindrop. These returns happen at all points around the sun- line, and combine to form a bright cone of angle  with its axis on the sun-line (see raindrop B). The light ray is split into its component wavelengths by the raindrop, and different colours are refracted by   different amounts – red less, blue more. So the bright cone shows rainbows colours, with red on the outside.

            If you look at a sunlit sky, full of raindrops, your eye will be on the surface of the bright cone of raindrops  from your antisunline – the line running from your eye to the top of your shadow on the ground. So you will see the rainbow as a circle that is  from the antisun-line, with the red on the outside. The original rays which hit the drop at the wrong place to form the rainbow will produce a very faint return, always less than from the antisun-line, and so inside the rainbow. This makes the sky appear darker above the bow.

            However, a secondary bow can form outside the primary. It is caused by a double reflection of rays striking raindrops. Some of the lost reflected light mentioned in the first paragraph can be reflected twice in the raindrop and therefore still reaches an observer on the ground as it finally exits the drop at an angle from the antisun-line. The fact it is reflected twice means the red will now be on the inside of the cone, and fainter.

            The variation in apparent size of rainbows is due o several factors. If the Sun is higher in the sky then of the rainbow’s arc will be above the horizon (where it is more visible), and hence it will seem smaller-even though it is still  from the antisolar point. The antisolar point is the point where an imaginary ray connecting the Sun and the observer meets the ground, coinciding with the top of the observer’s shadow. If the Sun is above the horizon, the antisolar point is below the horizon, if the sun is below the horizon the antisolar point will be in the sky.

            Similarly, the extent and distance of the water droplets (from the observer) can give rise to partial arcs, which obviously appear smaller than a full bow. Finally a rainbow’s relative size is subject to the same optical illusion that makes the moon appear larger when it is lower down in the sky we can more readily compare its size to the objects on the horizon. So a rainbow behind some houses may appear smaller than a rainbow spanning the open countryside.

  Warm, wet air surges upwards into the sky and cools dramatically forming thunderstorms. Some of the water inside the clouds freezes and strong air currents make the ice and water droplets bump together. This knocks tiny charged particles called electrons from the ice and so there is a build-up of electrical charge. This charge is released by a stroke of lightning. The lightning heats the air around it to an incredible 30,C (54,F), five times hotter than the surface of the Sun. this heat causes the air to expand very fast; i.e., faster than the speed of sound. It is this which causes the crash of thunder.

            If the thunderstorm is overheard, we can hear thunder and see lightning simultaneously. If it is not overheard, we can see lightning first, as light travels much faster than sound.

            By counting the seconds between the lightning and thunder and dividing by three we get the distance to the storm in kilometers and dividing by five we get the distance in Miles. Dark thunderclouds are formed on hot and humid days. A thundercloud is usually about 5 kms (3 Miles) across and 8 kms (5 Miles) high.