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Section 2Fronts and WeatherKey Concept Weather results from the movement of air masses that differ intemperature and humidity.What You Will Learn Differences in pressure, temperature, air movement, and humidity cause changes inweather, including severe weather. Severe weather and floods can cause property damage and death.Why It MattersSevere weather can cause property damage, injury, and death.Have you ever wondered how the weather can change so quickly? Changes in weatherare caused by the interaction of air masses. An air mass is a large body of air that hassimilar temperature and moisture content throughout.FrontsWhen different air masses meet, the less dense air mass rises over the denser airmass. Warm air is less dense than cold air is. So, when a warm air mass and a cold airmass meet, warm air generally rises. The area in which two or more air masses meetis called a front. Figure 1 shows the four different kinds of fronts.Figure 1 The Four Main Types of Fronts

Cold FrontsA cold front forms where cold air moves under warm air. Because the warm air is lessdense, the cold air pushes the warm air up. Cold fronts can move quickly and bringheavy rain or snow. Cooler weather follows a cold front. The cooler weather is broughtby the cold, dry air mass behind the cold front that pushed up the warm air mass.Warm FrontsA warm front forms where warm air moves over cold, denser airthat is leaving an area. The warm air replaces the cold air as thecold air moves away. Warm fronts generally bring drizzly rain.They also are followed by clear, warm weather.Occluded FrontsAn occluded front forms when a warm air mass is caught betweentwo colder air masses. One cold air mass moves under and pushesup the warm air mass. The cold air mass then moves forward untilit meets the other cold air mass. The advancing cold air massesmoves under and pushes up the other cold air mass. Sometimes,though, the two colder air masses mix. An occluded front bringscool temperatures and large amounts of rain and snow.What kind of weather would you expect anoccluded front to produce?Stationary FrontsA stationary front forms when a cold air mass and a warm airmass move toward each other. The warm air mass is commonlyforced over the cold air mass. However, there is not enough forcefor either air mass to advance relative to the other. So, the twoair masses remain separated. Stationary fronts happen whenthere is not enough wind to keep the air masses pushing againsteach other. A stationary front generally causes many days ofcloudy, wet weather.

Air Pressure and WeatherYou may have heard a weather reporter talking about areas of lowpressure and high pressure. These areas of different pressure causechanges in the weather. An area that has lower air pressure than theareas around it do is called a cyclone. Cyclones are areas where airrises. As the air in the center of a cyclone rises, the air cools. Cloudsform and may cause rainy and stormy weather.Areas of high air pressure are called anticyclones. Anticyclones are areaswhere air sinks. As the air sinks, it gets warmer and its relative humiditydecreases. As a result, the sinking air in an anticyclone brings dry, clearweather. Figure 2 shows how an anticyclone can form a cyclone.Figure 2 As the colder, denser air spirals out of the anticyclone, the airmay spiral in toward areas of low pressure called cyclones.Describe the different kinds of weather thatcyclones and anticyclones can form.

ThunderstormsA thunderstorm is an intense local storm that forms strong winds, heavyrain, lightning, and thunder. Two atmospheric conditions are needed toform thunderstorms: warm and moist air near Earth’s surface and anunstable atmosphere. The atmosphere is unstable when cold air is overwarm air. When the rising warm air reaches its dew point, the watervapor in the air forms cumulus clouds. If the warm air continues to rise,the cloud may grow into a dark, cumulonimbus cloud. One such cloud isshown in Figure 3.Figure 3 A typical thunderstorm, such as this one over Los Angeles,California, generates an enormous amount of electrical energy.LightningHave you ever touched someone after scuffing your feet on the carpetand received a mild shock? If so, you have experienced how lightningforms. While you walk around, friction between the floor and your shoesbuilds up an electric charge in your body. When you touch someoneelse, the charge is released. Lightning forms in a similar way. Lightningis an electric discharge that happens between a positively charged areaand a negatively charged area. This process is shown in Figure 4.

Figure 4 The upper part of a cloud usually carries a positive electriccharge. The lower part of the cloud carries mainly negative charges.ThunderWhen lightning strikes, the air along its path is superheated. Thesuperheated air expands rapidly. The rapidly expanding air causes theair to vibrate and release energy as sound waves. The result is thunder,which is the sound caused by the fast expansion of air along thelightning strike.TornadoesTornadoes happen in less than 1% of all thunderstorms. A tornado is a rapidly,spinning column of air with high wind speeds and low central pressure and thattouches the ground. The beginning of a tornado can be seen as a funnel cloud that

pokes through the bottom of a cumulonimbus cloud. The funnel cloud becomes atornado when it touches down on Earth’s surface, as shown in Figure 5.Figure 5 How a Tornado Forms

HurricanesA large, rotating tropical weather system that has wind speeds of at least 120 km/h iscalled a hurricane. A hurricane is shown in Figure 6. Hurricanes are the most powerfulstorms on Earth. Hurricanes range in size from 160 km to 1,500 km in diameter. Theycan travel for thousands of kilometers.Figure 6 Although hurricanes are very destructive storms, the eye at the center of thetropical storm is relatively calm.Where Hurricanes FormMost hurricanes form in the areas between 5 and 20 north latitude and between 5 and 20 south latitude. These storms form over warm, tropical oceans. At higherlatitudes, the water is too cold for hurricanes to form.How Hurricanes FormA hurricane gets its energy from the evaporation and condensation of water vapor.Once formed, the hurricane is fueled through contact with the warm ocean water. Heatfrom the sun causes ocean water to evaporate. The evaporation adds moisture to thewarm air. As the warm, moist air rises, the water vapor condenses and releases largeamounts of energy. A group of thunderstorms forms and moves over tropical oceanwaters. The thunderstorms produce a large vortex, or whirl of air.

The hurricane continues to grow as long as it is over warm ocean water. When thehurricane moves over colder waters or over land, the storm loses energy. This loss ofenergy is the reason that California does not experience hurricanes. Hurricanesapproaching California quickly die out over the cold California coastal waters.How does the sun’s energy power hurricanes?Effects of Severe WeatherSevere weather is weather that can cause property damage, injury, and sometimesdeath. Hail, lightning, high winds, tornadoes, and flash floods are all part of severeweather. Hailstorms can damage crops and cars and can break windows. Lightningstarts thousands of forest fires and kills or injures hundreds of people and animalseach year. Winds and tornadoes can uproot trees and destroy homes.Floods caused by heavy rains cause millions of dollars in property damage every year.Flash flooding is also a leading cause of weather-related deaths. Most damage fromhurricanes results from flooding caused by heavy rains and storm surges. A stormsurge is a rise in sea level that forms in the ocean during a storm. The storm surgecrashes onto shore, endangering lives and causing property damage. Hurricane Katrinain 2005 caused more damage and deaths to the southeastern coast of the UnitedStates from flooding than from high-speed winds.How do floods affect humans?Severe-Weather SafetyDuring severe weather, one of the most important things to do is to listen to your localradio or TV stations. Severe-weather announcements will let you know a storm’slocation. They also tell you if the storm is getting worse. During most kinds of severeweather, it is safest to stay indoors away from windows. However, in some situations,you may need to evacuate. During a flood warning, if you are in a low-lying place, suchas the one shown in Figure 7, you should move to higher ground. Never enter

floodwaters. Even shallow floodwater can be dangerous if it is moving fast.Figure 7 In 1998, heavy rains caused flooding in the town of Petaluma, California.Section Summary Thunderstorms are weather systems thatproduce strong winds, heavy rain, lightning,and thunder. Lightning is a large electric discharge thatoccurs between two oppositely chargedsurfaces. Lightning releases a great deal ofenergy and can be very dangerous. Tornadoes are small, rapidly rotating columnsof air that touch the ground and can cause

severe damage. A hurricane is a large, rotating tropicalweather system that has high wind speeds. In the event of severe weather, it is importantto stay safe. Listen to your local TV or radiostations for updates, and remain indoors andaway from windows.

Fronts When different air masses meet, the less dense air mass rises over the denser air mass. Warm air is less dense than cold air is. So, when a warm air mass and a cold air mass meet, warm air generally rises. The area in which two or more air masses meet is called a front. Figure 1 shows the four differen

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