Lightning and Aurora use links
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www.webexhibits.org/causesofcolor/4.htm
http://en.wikipedia.org/wiki/Lightning

Lightning


How Lightning is made


Lightning is normally viewed as cloud-to-ground. This is caused during a storm when particles of ice and rain collide. This causes the lower regions of the clouds to be negatively charged. Objects on the ground (houses, trees, the ground); however, become positively charged, which makes an unbalance. After this occurs, negative chalightningsunset.jpgrges called "step ladders" move from the bottom of the cloud to the earth. The first one moving is called the leader. As it moves to the ground, it makes a sort of "track" to the ground. It also changes air molecules into charged particles. At the moment it reaches a tree, house or the earth (remember that they are positively charged) electrostatic energy flows to the earth in a discharge. This is what causes the cloud-to-ground strikes that are so common in thunder storms.
Lightning can also form when volcanic eruptions and forest fires make enough dust for static.

Another aspect of lightning is the re-strike.In each lightning strike, there are more than one "stroke" (there could be 3 or 4). These strokes cause that flashing, strobe-light effect ( I'm sure we've all experienced it. Kinda similar to Mr. Schmidt flickering the lights, only it's lightning).

Types of lightning


There are many types of lightning. these types include
  1. Cloud to ground: The most well known type of lightning, and the second most common type of lightning. It occurs when lightning strikes the ground from a
cloud (hence the name). This type of lightning is the most lethal type of lightning because it strikes the ground.

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  1. Bead lightning: A form of cloud to ground lightning in which breaks up into separated sections. Many theories arise as to why this occurs, and two examples are that A) Viewers see parts of the lightning end-on, which look brighter, or B) The width of the lightning channel changes, and the wider portions look like a string of beads.
  2. Ribbon lightning:This phenomenon is seen in lightning storms with high winds. As explained earlier, strikes have multiple "strokes" in ribbon lightning, the wind blows the strokes farther to the side than the strike before it, so it looks like ribbons
  3. Staccato lightning: This type of lightning has one very short stroke, and looks like one big flash.
  4. Forked lightning: This type of lightning has forked branches.
  5. Ground-to-cloud lightning: This type of lightning is caused when the step leaders come out of the ground instead of the clouds. This type of lightning is much more rare than cloud to ground lightning.
  6. Cloud to cloud lightning:This type of lightning, like the name suggests, goes from cloud to cloud. There are two different ways this lightning is classified. One is called inter-cloud lightning, and this is when it strikes between two different clouds. The other is called intra cloud lightning, and this is when it strikes within the same cloud. Of the types of lightning, intra cloud lightning is the most common. Intra cloud lightning occurs between the anvil (topmost part and often anvil shaped) and lowermost portions of the cloud. Another name for Intra cloud lightning is "anvil crawler" because it starts below or within the anvil and "crawls" In the upper layers, with a lot of branching.
  7. Sheet lightning: This type of cloud to cloud lightning that brightens the surface of the clouds. If you've ever been in your house and all of a sudden, it appears as though someone set twenty lights outside, you've probably experienced this lightning.
  8. Heat lightning: A type of lightning where lightning is to far away to be heard. This is because the sound dissipates before it reaches the viewer.
  9. Dry lightning: This type of lightning has no precipitation, and is the leading cause of forest fires. Related to this are pyrocumulus clouds. These clouds cause lightning when the air far above ground is cool, and the ground is extremely hot (such as when a forest fire or a volcano is present). This causes convection which in turn causes lightning. So, forest fires cause dry lightning which causes more forest fires, that again make dry lightning, etc.
  10. Rocket lightning: cloud discharge, usually horizontal at cloud base, and advances through the air.
  11. Positive lightning: This type of lightning only happens in clear/partly cloudy skies, hence the name "bolt from the blue" a nickname for this lightning. Unlike most lightning, the positive charge is in the clouds instead of the ground. This lightning travels horizontally for miles before going down to meet the negative charge. Due to its long trip, it has a stronger charge. Because these strikes are so strong, and the complete randomness of where they hit, these strikes are extremely dangerous. Positive lightning is more common during winter storms and thunder snowstorms (a thunder storm where there is snow instead of rain) or at the end of a thunderstorm.
  12. Ball lightning: The causes of this lightning are still unknown. Ball lightning describes bright, ball shaped lightning that could be the size of a pea to several meters in diameter. While this type of lightning has been observed by bystanders, but has not been properly recorded by meteorologists.
  13. Upper-atmospheric lightning: Reports of this strange lightning have been around for more than a century, but only recently have we looked in to it.
  14. sprites: These huge electric discharges happen high above the storm clouds. Named after the sprite Puck from Shakespeare's A Midsummer's Night Dream Dream, they are triggered by positive cloud to ground lightning. They are reddish or blue-green in color, and are often seen in groups. (example in picture above)
  15. Blue jets: Jets generally come from the top of the cumulonimbus (thunder) cloud. Jets are brighter than sprites, and are blue (duh). Their shape is cone-like.
  16. Elves: these flattened glows are a couple hundred miles in diameter. Unlike sprites, elves are not just named so after a fantasy character. Elves stands for "Emissions of Light and Very Low Frequency Perturbations from Electromagnetic Pulse Sources"

Triggered Lightning


Lightning can be triggered by:
  • rocket launches
  • volcanoes
  • lasers


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Auroras


How it's formed


Auroras are caused when charged particles in the solar wind ionize the oxygen and nitrogen gases in the atmosphere. This process occurs in the ionosphere (we learned about this in the atmosphere unit) The color of the light depends on two things: the structure of the atom, and the energy of the particle that hit it originally.

...Solar Wind?


In the explanation above of how auroras are formed, it stated that the solar wind's particles add energy by colliding with other atoms. But what is the solar wind? The solar wind is created when atoms collide in the heat surrounding the sun. The newly formed particles are what make up the solar wind, and it also has its own magnetic field. Think back to previous science lessons. You might recall that the earth itself has a magnetic field around it, with a north and south pole (remember that compasses always point north? It's because of the magnetic field). This blocks a lot of the solar wind, however some does manage to get in at the poles.

Types of aurora


Two classifications of aurora are the aurora borealis and the aurora australis. The difference between the two is that the aurora borealis occurs in the north, and the aurora australis forms in the south.

What's the best time to look at auroras?


A dark sky is perfect for viewing the aurora- so try to get far away from cities with many lights. In the Arctic and Antarctic, the best time is during the winter of the hemisphere you're in ( Northern hemisphere for Arctic, Southern for Antarctic). This is not because auroras form in seasons, but because of the dark skies. Another great time for viewing auroras would be during the spring and summer equinoxes.



References and Resources