1. Guided Notes about Severe Weather
Chapter 13, Section 2

2. A cold front that moves into warmer territory can lift and condense a continuous supply of warm air. When this happens, a line of thunderstorms can last for hours or days as they continually regenerate themselves with the new, warm air introduced into the updrafts.

3. 2. As the instability of the air increases, the strength of the storm’s updrafts and downdrafts intensifies. The storm may develop into a self-sustaining, extremely powerful supercell, characterized by intense, rotating updrafts.

4. 3. Of the 100,000 thunderstorms that occur each year in the U
3. Of the 100,000 thunderstorms that occur each year in the U.S, only about 10 percent are considered to be severe.

5. 4. Lightning is electricity caused by the rapid rush of air in a cumulonimbus cloud. A lightning bolt forms when friction between updrafts and downdrafts separates electrons from some of their atoms either in the cloud or near the ground. To relieve the electrical imbalance, an invisible channel of negatively charged air moves from the cloud to the ground.

6. 5. When the stepped leader near the ground, a channel of positively charged ions, called the return stroke, rushes up to meet it. The return stroke surges from the ground to the cloud, illuminating the channel with about 100 million volts of electricity.

7. 6. A lightning bolt heats the surrounding air to about 30,000 degrees Celcius, which is five times hotter than the surface of the Sun. Thunder is the sound produced as the superheated air rapidly expands and contracts.

8. 7. Violent downdrafts that are concentrated in a local area are called downbursts. Based on the size of the area they affect, downbursts are classified as either macrobursts or microbursts.

9. Sequence how hail forms in a thunderstorm:
Supercooled water droplets encounter ice pellets in a cloud and freeze on contact, causing the ice pellets to grow larger.
Updrafts and downdrafts in a cloud cause more supercooled water droplets to encounter the ice pellets, making them grow even larger.
Eventually, the ice pellets are too heavy to be held aloft by the updrafts, and they fall to Earth as hail

10. 9. If rain falls faster than the ground can absorb it, flooding can occur. Floods are the main cause of thunderstorm-related deaths in the U.S. each year.

11. 10.A tornado is a violent, whirling column of air in contact with the ground. A tornado forms when wind speed and direction change suddenly with height, a phenomenon known as wind shear.

12. 11. Under the right conditions, wind shear can produce a horizontal rotation near the earth’s surface. If this rotation takes place close enough to the thunderstorm’s updrafts, the column of wind can be tilted from a horizontal to a vertical position.

13. 12.The Fujita tornado intensity scale ranks tornadoes according to their path of destruction, wind speed, and duration.

14. Describe where and at what time of day and year tornadoes are most likely to occur:
Most tornadoes form in the spring during the late afternoon and evening.
In the central United States, a region called “Tornado Alley” extends from northern Texas through Oklahoma, Kansas and Missouri