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Thunderstorms

Thunderstorms

The Chemistry of Thunderstorm

A storm is coming and there is nothing better than staying indoors and putting on a horror movie as flashes of lightning followed by the rumbles from thunder accumulate in the background. How are lightning bolts created? To begin, one must first start by looking at the clouds. There are many different types of clouds that can be separated into three categories depending on their height from ground level: high, mid, and low clouds. Cumulonimbus clouds, the “thunderstorm clouds”, are usually considered as low-level clouds1. However, they can grow to higher altitudes of more than 39,000 feet, putting them into a separate category of clouds called vertical clouds. These storm clouds begin to form as warm air, carrying water vapor, cools and condenses into droplets. The formation of water molecules in these clouds collide with one another, chipping off electrons and creating an electric charge2. Fission occurs due to the separation between the positively and negatively charged particles in the upper and bottom portions of the clouds, respectively5. At the same time, the negatively charged particles, or electrons, migrate to the bottom of the clouds. These electrons create a repulsion towards the electrons on earth’s surface that causes positively charged particles or protons to move up to the ground’s surface. Electrons from the clouds want to reach equilibrium by attracting these protons from the ground. With this, a conductive path is produced in which lightning follows as the attraction between opposite charges are neutralized2.

So what about the thunder that catches one off guard, especially during a horror movie? Well, air around the lightning bolt is heated to about 54,000°F, increasing the air pressure around the lightning. The increasing temperature of air then causes it to expand outward, faster than the speed of sound. This pushes the air particles apart, forming a shockwave or what is distinctly heard as thunder5. There is a saying that for every lightning strike, count the number of seconds until the first sound of thunder to determine the distance between the current location and the storm. The period between the lightning flash and thunder happens because light travels much faster than sound. Light travels at approximately 299,800,000 m/s whereas sound travels at about 340 m/s. This means that while one sees the lightning almost instantaneously, the sound only travels about one mile every five seconds3. To calculate how far the lightning struck, in miles, divide the counted seconds by five. For example, if one counted 10 seconds and divided by five, the lightning strike was about two miles away.

What about the aroma left behind as the storm passes? Part of the scent is from the production of ozone in the atmosphere and again, there is lightning to thank for providing the energy for the chemical reaction. Lightning strikes and splits diatomic oxygen molecules (O2) that are naturally present in the earth’s atmosphere into two separate atomic oxygens (O). Due to unpaired valence electrons, these unstable oxygen atoms, or free radicals, are highly reactive. As a result, they quickly bind to nearby O2 molecules from the atmosphere to form ozone molecules (O3). However, these ozone molecules do not last long. One O3 molecule has a half-life of about 30 minutes due to its chemical instability and oxidizing abilities4. O3 reacts to reform molecular oxygen via oxidation reactions in which ozone is consumed with bacteria and organic materials such as smoke4.

During this process, the ozone molecules are spread into lower altitudes of the atmosphere, which gives off the familiar aroma of the fresh spring rain5.

References:

  1. Keating, C. Lightning. Salem Press Encyclopedia of Science. (2016).
  2. McCarthy, J. Tropic Lightning: Myth or Menace. Hawaii J. Med. Public Health. [Online] 2014, 73, 44.
  3. Moerbius, E. Speed of Light Versus Speed of Sound. http://helios.gsfc.nasa.gov/qa_gp_ls.html (accessed October 12, 2016).
  4. Ozone Solutions. http://www.ozonesolutions.com/info/ozone-faq (accessed October 27, 2016).
  5. Zavisa, J. How Lightning Works. http://science.howstuffworks.com/nature/natural-disasters/lightning.htm (accessed October 4, 2016)