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I.) FORMATION AND MAINTENANCE OF THUNDERSTORM

A.) CUMULUS-STAGE

1. Warm air near surface rises into cool, dry air aloft.

2. When T = TD cumulus clouds form.

3. Entrainment of dry air evaporates morning cumulus clouds.

Entrainment - mixing of cloud/outside air.

4. As entrainment continues into p.m., atmosphere becomes more moist, thus less cloud evaporation.

5. As warm air rises in cloud (updraft), condensation releases heat into cloud, thus warming the air and causing it to rise further.

6. The cumulus cloud is now a towering cumulus cloud.

B.) MATURE-STAGE THUNDERSTORM

1. 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.

2. The rain/snow drags down colder air (frictional drag) thus enhancing the downdraft velocity.

3. Some of the falling precipitation evaporates below cloudbase, thus cooling the downdraft further. This enhances the downdraft velocity also.

4. 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.

5. 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.

6. The gust front lifts warm, moist air ahead of the storm, thus creating new cumulus clouds.

7. 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.

8. 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.

9. The mature-stage occurs as long as warm, moist air is lifted.

C.) DISSIPATION-STAGE

1. Large downdrafts remain with little or no updraft. (Note: a dissipating t-storm will still consist of heavy rain, frequent lightning, and strong winds.)

2. 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

II.) THUNDERSTORM PHENOMENA

A.) LIGHTNING

1. Caused by a difference in charge between cloud and object being struck. Normally, the cloudbase 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.

2. 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.

3. 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.

4. Tall objects tend to be struck often because they are closer to the cloud thus there will be less air to travel through.

5. Process: negative charges from cloudbase move downward toward positively charged object at surface. This occur 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!

6. 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!

7. There may be several return strokes in an instant. This will cause the bolt to "flicker."

8. Sometimes "branches" are seen on the main bolt. These are known as dart leaders.

9. 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.

10. COMMON MISCONCEPTIONS:

1. lightning never strikes the same place twice.

2. tires keep you safe in your car.

3. heat lightning is caused by hot air at night.

B.) THUNDER

1. Lightning heats the surrounding air to over 54,000oF. As the air expands it generates a sound wave.

2. 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, there are 10 seconds between the lightning flash and thunder. The lightning must be 2 miles away.

3. Thunder will be a loud crack when it is close to you, while a rumble when it is further away.

C.) HAIL

1. Hailstones are pieces of ice ranging from pea-sized to softball- sized.

2. It takes 10 billion cloud droplets to form a golf ball-sized hailstone.

3. 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.

4. Hail only occurs in strong to severe t-storms. Therefore, hail can be a good indicator that tornadoes may form.

D.) TORNADOES

1. The average width of a tornado is between 100 and 600 meters, but have been observed to be over 1 mile wide.

2. Tornadoes usually last between 15 to 30 minutes but have been observed to last over hours.

3. Normally, about 100 people are killed each year by tornadoes.

4. 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.

5. 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.

6. Some tornadoes have been observed to have smaller vortices inside the main funnel called suction vortices. These tornadoes can be extremely violent.

7. 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.

8. The Fujita-Scale measures tornado intensity:

F0: 40-72 mph tree branches broken, signs damaged.

F1: 73-112 mph trees snapped, windows broken.

F2: 113-157 mph trees uprooted, structures destroyed.

F3: 158-206 mph vehicles overturned, walls down.

F4: 207-260 mph houses demolished, cars destroyed.

F5: 261-318 mph houses lifted off foundation, carried, and destroyed. Concrete/steel reinforced structures destroyed.

9. Weak (F0-F1) occur about 60% of time and cause 2% of fatalities. Strong (F2-F3) occur about 38% of time and cause 30% of fatalities. Violent (F4-F5) occur about 2% of time and cause 68% of fatalities.

10. 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 cloudbase. The wall cloud is usually located behind the rain and hail. Rotating wall clouds are usually a good indicator of tornado formation.

11. Warning signs of severe weather and/or tornadoes:

i. A rotating wall cloud

ii. Heavy hailstorm

iii. Mammatus clouds

iv. Rotating t-storm

v. A roaring sound like a freight train

III.) FORECASTING SEVERE WEATHER

A.) DEFINITION OF SEVERE WEATHER

1. A thunderstorm will be termed severe if it meets ONE OR BOTH of the following:

i. Winds exceeding 57 mph

ii. Hail 2 cm in diameter

2. If severe t-storms (tornadoes) are LIKELY TO FORM, a severe thunderstorm (tornado) WATCH will be issued.

3. If a severe t-storm (tornado) IS OBSERVED, a severe thunderstorm (tornado) WARNING will be issued.

B.) STABILITY INDICES

1. Since t-storms form in atmospheres that are unstable, stability must be determined.

2. An atmosphere that is unstable will have very warm, moist air at low levels, while cold air at upper levels. This enhances vertical motion of warm air parcels, which leads to cumulonimbus cloud development.

3. The LIFTED INDEX (LI) determines stability by comparing the temperature at mid-levels (500 mb) with that of rising air at the same level. Negative Lifted Indices mean unstable conditions:

LI £ 0 means thunderstorms MAY form.

LI £ -4 means severe t-storms MAY form.

4. The K-INDEX (KI) determines stability by comparing temps and dewpoints of air at several levels of the atmosphere. Positive KI values indicate unstable conditions:

KI ³ 22 means t-storms are EXPECTED.

KI ³ 35 means strong (but NOT severe) t-storms are EXPECTED.

5. Severe weather usually occurs where LI £ -4 and KI is slightly positive. A very high KI indicates a very moist atmosphere. Severe t-storms need dry air at mid-levels to enhance downdrafts, so high KI values actually are not a good indicator of severe weather.

6. SPRING is the season when most severe weather occurs.

C.) FRONTAL LIFTING

1. Since lifting along fronts leads to clouds and precipitation, most severe weather will occur along a frontal boundary such as cold fronts, warm fronts, dry-line fronts, etc. A cold front is the most

common triggering mechanism for severe weather. Most severe t-

storms form ahead of the cold front in the warm, humid air of the

warm sector.

D.) DEWPOINT TEMPERATURES

1. Since warm, moist air rises readily, high temps and high dewpoints may lead to severe weather. Dewpoints ³ 65oF are usually needed for severe weather to occur.

E.) WIND SHEAR AND THE JET STREAM

1. Severe weather, esp. tornadoes, occur when windshear exists between the surface and upper-levels of the atmosphere. If the winds are shifting direction in a clock-wise fashion with height and increasing in speed, t-storms may become severe. For example, surface winds from the south at 15kt, mid-levels winds from the southwest at 50 kt, and a westerly jet stream at 100 kt at high levels.

2. Wind shear will cause t-storms to rotate which can aid in the development of super-cell t-storms and tornadoes.

Helpful Links:

Thunderstorm - Wikipedia
NOAA Severe Storms Spotters Guide
National Severe Storms Laboratory (NSSL)
Thunderstorm Overview

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