Guided Notes about Guided Notes about Severe WeatherSevere Weather

Chapter 13, Section 2Chapter 13, Section 2

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

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

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

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

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

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

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

Sequence how hail forms in a Sequence how hail forms in a thunderstorm: thunderstorm:

1.1. Supercooled water droplets encounter ice Supercooled water droplets encounter ice pellets in a cloud and freeze on contact, pellets in a cloud and freeze on contact, causing the ice pellets to grow larger.causing the ice pellets to grow larger.

2.2. Updrafts and downdrafts in a cloud cause Updrafts and downdrafts in a cloud cause more supercooled water droplets to encounter more supercooled water droplets to encounter the ice pellets, making them grow even larger.the ice pellets, making them grow even larger.

3.3. Eventually, the ice pellets are too heavy to be Eventually, the ice pellets are too heavy to be held aloft by the updrafts, and they fall to Earth held aloft by the updrafts, and they fall to Earth as hailas hail

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

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

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

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

Describe where and at what time of day and Describe where and at what time of day and year tornadoes are most likely to occur:year tornadoes are most likely to occur:

Most tornadoes form in the spring during Most tornadoes form in the spring during the late afternoon and evening.the late afternoon and evening.In the central United States, a region In the central United States, a region called “Tornado Alley” extends from called “Tornado Alley” extends from northern Texas through Oklahoma, northern Texas through Oklahoma, Kansas and MissouriKansas and Missouri