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Severe Weather NOAA • Thunderstorms experience a lifecycle that features three stages: • Cumulus stage • Mature stage (classic thunderstorm we see) • Dissipating stage Flyngo.com Severe Weather • Warm air near surface rises into cool, dry air aloft. • When T = TD cumulus clouds form. • Entrainment - mixing of cloud/outside air. • Entrainment of dry air evaporates morning cumulus clouds. • As entrainment continues into p.m., atmosphere becomes more moist, thus less cloud evaporation. • As warm air rises in cloud (updraft), condensation releases heat into cloud, thus warming the air and causing it to rise further. • The cumulus cloud is now a towering cumulus cloud. Flyngo.com Severe Weather Flyngo.com Severe Weather • As the updrafts in the towering cumulus reach higher into the atmosphere (occasionally over 60,000 ft.), the collection of condensed cloud drops turns into snow/rain and falls toward the ground causing a downdraft to form. • The rain/snow drags down colder air (frictional drag) thus enhancing the downdraft velocity. • Some of the falling precipitation evaporates below cloudbase, thus cooling the downdraft further which enhances the downdraft velocity. • Often, cooler and drier air at mid-levels of the atmosphere (around 700 mb) advects into the cloud and enhances evaporational cooling of the downdraft air. Again, this will enhance downdraft velocity. Flyngo.com Severe Weather • When the cold downdraft air reaches the ground, it spreads out in all directions, but primarily directly ahead of the moving thunderstorm. This cold blast of air is called a gust front and is similar to a cold front. • The gust front lifts warm, moist air ahead of the storm, thus creating new cumulus clouds. • The mature-stage t-storm will appear as a large cumulonimbus cloud with an anvil-shaped cloud top extending ahead of the t- storm. The anvil-head is caused by strong upper-level horizontal winds and by updrafts reaching the tropopause. • In severe t-storms, cloud tops may overshoot the tropopause so there will be some cloud extending into the stratosphere. This is known as an overshooting top. • The mature-stage occurs as long as warm, moist air is lifted. Flyngo.com Severe Weather Flyngo.com Severe Weather • Large downdrafts remain with very weak or no updrafts. (Note: a dissipating t-storm will still consist of heavy rain, frequent lightning, and strong winds.) • Reasons for t-storm demise: a. moist air no longer lifted b. downdrafts choke off updrafts c. gust front diminishes or outruns t-storm d. strong upper-level wind shear e. too much dry air enters storm f. friction of mountains g. large-scale sinking air suppresses updraft Flyngo.com Severe Weather Flyngo.com Severe Weather Outdoored.com • Caused by a difference in charge between cloud and object being struck. • Normally, the cloud base is negatively charged while the cloud top and the ground collect positive charges. • When opposite charges flow toward each other there is said to be electrical current. A substance that allows current to flow is said to be a conductor (metals, water). A substance that inhibits current is said to be an insulator (rubber, air, glass, wood, etc.) When opposite charges are separated by an insulator a charge potential is created. This can only occur in a cumulonimbus cloud. Severe Weather Outdoored.com • The process of cloud electrification is still not completely understood. The latest research indicate that the freezing of cloud/rain drops combined with strong updrafts/downdrafts creates a large charge separation between the top and bottom of the cloud. • Lightning is the result of opposite charges flowing toward each other. This current may be between two clouds or between cloud and ground. Cloud to cloud lightning is known as sheet lightning and is not dangerous. • Tall objects tend to be struck often because they are closer to the cloud thus there will be less air to travel through. Severe Weather Outdoored.com • Negative charges from cloud base move downward toward positively charged object at surface. This occurs in steps known as stepped leaders. Each stepped leader travels between 50 and 100 meters, stops for approx. .00005 seconds, then continues. These stepped leaders are INVISIBLE TO THE NAKED EYE! • As the tip of the stepped leader reaches the object, positive charges flow upward from the object to meet it. As they meet, there will be a massive flow of charges known as the return stroke. • The return stroke is the familiar visible lightning bolt. The return stroke lasts for about .0001 seconds! Severe Weather Outdoored.com • There may be several return strokes in an instant. This will cause the bolt to "flicker.“ • Sometimes "branches" are seen on the main bolt. These are known as dart leaders. • Often the positive charges flowing toward the stepped leader will cause the air to have a greenish glow. This is known as St. Elmo's Fire. Severe Weather Examiner.com COMMON MYTHS: 1. Lightning never strikes the same place twice. FALSE 2. Tires keep you safe in your car. FALSE 3. Heat lightning is caused by hot air at night. FALSE See: http://profmandia.wordpress.com/2010/07/12/no-this-weather-really-blows/ Severe Weather clap of thunder rumble of thunder thunder combo Environment Canada • Lightning heats the surrounding air to over 54,000oF. As the air expands it generates a sound wave. • Sound travels approx. 0.2 miles per second. Thus, the time between the flash of lightning and the thunder is five times the distance in miles. For example, if there are 10 seconds between the lightning flash and thunder, the lightning must be 2 miles away. • Thunder will be a loud crack when the bolt is close to you or perpendicular to you, while a rumble is heard when the bolt is farther away or more parallel to you. Severe Weather NWS JetStream • Hailstones are pieces of ice ranging from pea-sized to softball-sized. • It takes 10 billion cloud droplets to form a golf ball-sized hailstone. • Hail is formed when droplets are carried above the freezing-level by strong updrafts. The droplets freeze and start to fall. As they fall, rain drops freeze onto them, causing larger hail. As long as the updrafts are strong, the hailstone will not fall. The longer it stays in the cloud, the larger it can get. • Hail only occurs in strong to severe t-storms. Therefore, hail can be a good indicator that tornadoes may form. Severe Weather ContinentalCollision.com Severe Weather Severe Weather NWS JetStream • Most tornadoes form in rotating t-storms called Super-Cell thunderstorms or Mesocyclones. • These storms may survive for many hours and move across several states during their lifetime. • Tornadoes usually form in the SW corner of the t-storm out of the wall cloud. The wall cloud is an isolated lowering of the cumulonimbus cloud base. The wall cloud is usually located behind the rain and hail. Rotating wall clouds are usually a good indicator of tornado formation. Severe Weather Walter at Cuny.edu NWS JetStream • Tornadoes form when a horizontally rotating air column is tilted into the vertical by intense t-storm updrafts. Due to conservation of angular momentum, the vertical vortex (air column) will begin to spin very fast. •Some tornadoes have been observed to have smaller vortices inside the main funnel called suction vortices. These tornadoes can be extremely violent. Severe Weather Minnesota Climatology Working Group • The average width of a tornado is between 100 and 600 meters, but have been observed to be over one mile wide. • Tornadoes usually last between 15 to 30 minutes but have been observed to last over hours. • Normally, about 100 people are killed each year by tornadoes. • A waterspout is similar to a tornado, but forms over water and is much weaker. • A funnel cloud is NOT a tornado. It is a rotating funnel that never touches ground, so it causes no damage. Severe Weather NOAA • Tornado Alley is a region in the US that features more tornadoes than any place on earth. Severe Weather Wiki • Tornado Alley is a region in the US that features more tornadoes than any place on earth. Severe Weather MissBzScience Wikispace • The Enhanced Fujita-Scale measures tornado intensity. • Weak (EF0-EF1) occur about 60% of time and cause 2% of fatalities. • Strong (EF2-EF3) occur about 38% of time and cause 30% of fatalities. • Violent (EF4-EF5) occur about 2% of time and cause 68% of fatalities.
