Raindrop Elevation & Colour
Raindrop Elevation & Colour
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About the diagram
About the diagram
- This diagram deals with how a single raindrop can contribute to the formation of a primary rainbow.
- The diagram shows that a single falling raindrop appears red then orange, yellow, green, blue and finally violet as it falls towards the ground.
- It is the elevation of a droplet relative to the rainbow axis and observer that determines its colour at any moment as it falls.
- Notice that the diagram shows a ray of white light entering the top half of the raindrop and reflecting once off the interior surface before exiting towards the observer. This is always the case if a raindrop is part of a primary bow.
- So this is the sequence:
- At its largest angular distance from the axis, a raindrop appears red to an observer as it enters into the outer edge of the primary bow.
- Moments later, as that same raindrop falls, and its angular distance decreases, it changes colour, first from red and finally to violet. The diagram shows the moments at which it appears red, yellow and blue.
- As it falls further and its angular distance reduces below 40.70 it exits the inside edge of the bow.
- The droplet is now almost invisible but continues to contribute a little to a scattering of white light that fills this area within the arcs with a light glow.
Overview of raindrops
An idealized raindrop forms a sphere. These are the ones that are favoured when drawing diagrams of both raindrops and rainbows because they suggest that when light, air and water droplets interact they produce predictable and replicable outcomes.
- In real-life, full-size raindrops don’t form perfect spheres because they are composed of water which is fluid and held together solely by surface tension.
- In normal atmospheric conditions, the air a raindrop moves through is itself in constant motion, and, even at a cubic metre scale or smaller, is composed of areas at slightly different temperatures and pressure.
- As a result of turbulence, a raindrop is rarely in free-fall because it is buffeted by the air around it, accelerating or slowing as conditions change from moment to moment.
- The more spherical raindrops are, the better defined is the rainbow they produce because each droplet affects incoming sunlight in a consistent way. The result is stronger colours and more defined arcs.
- Raindrops start to form high in the atmosphere around tiny particles called condensation nuclei — these can be composed of particles of dust and smoke or fragments of airborne salt left over when seawater evaporates.
- Raindrops form around condensation nuclei as water vapour cools producing clouds of microscopic droplets each of which is held together by surface tension and starts off roughly spherical.
- Surface tension is the tendency of liquids to shrink to the minimum surface area possible as their molecules cohere to one another.
- At water-air interfaces, the surface tension that holds water molecules together results from the fact that they are attracted to one another rather than to the nitrogen, oxygen, argon or carbon dioxide molecules also present in the atmosphere.
- As clouds of water droplets begin to form, they are between 0.0001 and 0.005 centimetres in diameter.
- As soon as droplets form they start to collide with one another. As larger droplets bump into other smaller droplets they increase in size — this is called coalescence.
- Once droplets are big and heavy enough they begin to fall and continue to grow. Droplets can be thought to be raindrops once they reach 0.5mm in diameter.
- Sometimes, gusts of wind (updraughts) force raindrops back into the clouds and coalescence starts over.
- As full-size raindrops fall they lose some of their roundness, the bottom flattens out because of wind resistance whilst the top remains rounded.
- Large raindrops are the least stable, so once a raindrop is over 4 millimetres it may break apart to form smaller more regularly shaped drops.
- In general terms, raindrops are different sizes for two primary reasons, initial differences in particle (condensation nuclei) size and different rates of coalescence.
- As raindrops near the ground, the biggest are the ones that bump into and coalesce with the most neighbours.
Some key terms
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