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Terms: Water Cycle Precipitation Seasons Rainbows

Water Cycle Water on our planet moves in a continuous cycle. The water evaporates; vapours rise, cool and condense into clouds. The clouds move over the land, and precipitation falls. The water fills lakes, streams and rivers, and eventually flows back into the oceans where evaporation starts the process anew. Water can also penetrate into the soil (11% of the water). Another process is also important in the water cycle, which is transpiration by plant leaves: as plants absorb water from the soil, the water moves from the roots through the stems to the leaves, where it can evaporate.

Precipitation Is the name of water that falls out of clouds: it can be rain, or snow, or hail... In some clouds the tiny water droplets come together by collision to make bigger drops. As the drops become bigger and bigger (volume increases about a million times) they become too heavy for the air to support and they fall as rain.

Seasons The earth is slightly tilted on its axis. As the sun shines on the earth, it shines more directly on the northern hemisphere in June, and more directly on the southern hemisphere in December. That's why the seasons are different in each hemisphere. In the spring and fall, the sun shines fairly straight on the equator, giving both hemispheres equal warming. The theory about the seasons was discovered in the Renaissance by Copernicus

Rainbows The traditional rainbow is sunlight spread out into its spectrum of coloors and diverted to the eye of the observer by water droplets. The "bow" part of the word describes the fact that the rainbow is a group of nearly circular arcs of coloor all having a common centre. This is a good question to start thinking about the physical process that gives rise to a rainbow. Most people have never noticed that the sun is always behind you when you face a rainbow, and that the centre of the circular arc of the rainbow is in the direction opposite to that of the sun. The rain, of course, is in the direction of the rainbow. When light and water meet in the sky on a summer's day, for a few moments, a rainbow will appear. This phenomenon of the atmosphere appears during or immediately following local showers, when the sun is shining and the air contains raindrops. A rainbow can best be seen with polarized sunglasses. We cannot follow the arc of a rainbow down below the horizon, because we cannot see those droplets in the air below the horizon. But the higher we are above the ground, the more of the rainbow circle we would see.

That is why, from an airplane in flight, a rainbow will appear as a complete circle with the shadow of the airplane in the centre. The bow is divided into bands displaying the different colours of the spectrum and is formed by the refraction and reflection of the sun's rays in drops of rain. Reflection is simply the return of light waves from the raindrop's surface. Light which appears to be white, is really made up of a mixture of red, orange, yellow, green, blue, and violet light. When a shaft of sunlight enters a drop of water, a part of it does not pass directly through but is reflected from the inner surface and emerges from the side from which it entered. Moreover, it is refracted both on entering and leaving the water drop. This process, repeated in the same manner for an immense number of drops, produces the primary rainbow, which appears in front of the observer, who has his back to the sun. It has the red band on the outer edge which are long light waves and the blue-to-violet on the inner edge which are short light waves. Another larger bow is often seem outside the primary rainbow and parallel to it. This secondary rainbow is produced in a similar way, but the sun's light is reflected twice before emerging from the raindrop. For this reason, the colour sequence is reversed; red is on the inside edge. And because there is a loss of light with each reflection, it is not as bright as the primary rainbow. The region between the two bows is comparatively dark, for it lacks entirely both the once and the twice reflected rays.