Section 4: Recurrent Weather Even a relatively mild weather system can become destructive and dangerous if it persists for long periods of time. K What I Know W What I Want to Find Out L What I Learned

Essential Questions What are the problems associated with recurring weather patterns? What atmospheric events cause recurring weather patterns? How do heat waves and cold waves differ? Recurrent Weather Copyright © McGraw-Hill Education

Vocabulary Review New Fahrenheit scale drought heat wave cold wave windchill index Recurrent Weather Copyright © McGraw-Hill Education

Floods An individual thunderstorm can unleash enough rain to produce floods, and hurricanes also cause torrential downpours, which result in extensive flooding. Floods can also occur when weather patterns cause even mild storms to persist over the same area. Copyright © McGraw-Hill Education Recurrent Weather

Floods Low-lying areas are most susceptible to flooding, making coastlines particularly vulnerable to storm surges during hurricanes. Rivers in narrow-walled valleys can rise rapidly, creating high-powered and destructive walls of water. Recurrent Weather Copyright © McGraw-Hill Education

Droughts Droughts are extended periods of well-below-average rainfall. Droughts are usually the result of shifts in global wind patterns that allow large, high-pressure systems to persist for weeks or months over continental areas. Recurrent Weather Copyright © McGraw-Hill Education

Droughts Because the sinking air prevents humid air from rising, condensation cannot occur, and drought sets in until global patterns shift enough to move the high-pressure system. Recurrent Weather Copyright © McGraw-Hill Education

Droughts Heat waves An unpleasant side effect of droughts often comes in the form of heat waves, which are extended periods of above-average temperatures. Heat waves can be formed by the same high-pressure systems that cause droughts. Recurrent Weather Copyright © McGraw-Hill Education

Droughts Heat waves Because of the dangers posed by a combination of heat and humidity, the National Weather Service (NWS) routinely reports the heat index. The heat index assesses the effect of the body’s difficulty in regulating its internal temperature as relative humidity rises. Recurrent Weather Copyright © McGraw-Hill Education

Add link to Interactive Table from p. 363 here. The Heat Index Concepts In Motion FPO Add link to Interactive Table from p. 363 here. Recurrent Weather Copyright © McGraw-Hill Education

Cold Waves The opposite of a heat wave is a cold wave, which is an extended period of below-average temperatures. Cold waves are also brought on by large, high-pressure systems. However, cold waves are caused by systems of continental polar or arctic origin. Recurrent Weather Copyright © McGraw-Hill Education

Cold Waves Because of the location and the time of year in which they occur, winter high-pressure systems are much more influenced by the jet stream than are summer high-pressure systems. The winter location of the jet stream can remain essentially unchanged for days or even weeks. This means that several polar high-pressure systems can follow the same path and subject the same areas to continuous numbing cold. Recurrent Weather Copyright © McGraw-Hill Education

Cold Waves Windchill index The effects of cold air on the human body are magnified by wind. Known as the windchill factor, this phenomenon is measured by the windchill index. Recurrent Weather Copyright © McGraw-Hill Education

Cold Waves Windchill index The windchill chart was designed to show the dangers of cold and wind. Recurrent Weather Copyright © McGraw-Hill Education

Review Essential Questions Vocabulary What are the problems associated with recurring weather patterns? What atmospheric events cause recurring weather patterns? How do heat waves and cold waves differ? Vocabulary drought heat wave cold wave windchill index Recurrent Weather Copyright © McGraw-Hill Education