EARTH SCIENCE UNIT 6 LESSON 14 WEIRD WEATHER

OBJECTIVES

Describe causes of severe weather. Recall appropriate safety precautions during severe weather conditions.

VOCABULARY

desertification the spread of desert environments

drought a long period of unusually dry weather

wind shear when winds change direction and speed up into the atmosphere

Lesson Outline

Thunderstorms are common in warm, moist climates. They usually produce rain or hail, heavy winds, thunder, and lightning.

Late in the summer, hurricanes are common over warm tropical waters. Hurricanes have wind speeds over 74 mph, with a calm center or eye. They get their energy from the heat of tropical waters and die out once over cool water or land.

When warm, moist air masses clash with polar air masses, a horizontal spinning column of wind can form—wind shear. As the warm air rises into the storm clouds, it can turn the spinning column into a vertical vortex, or tornado.

Floods can be produced by tropical storms, violent wave activity along coasts, and earthquakes. Some scientists think global warming is causing more storm activity and flooding.

Drought is low rainfall for an extended period. Any climate can have a drought if rainfall is below average. Water shortages can also occur whenever water consumption is greater than precipitation. In either case, water conservation efforts are needed.

Stay up-to-date on appropriate safety tips for severe weather conditions because severe weather can happen quickly.

What are Thunderstorms?

How do you know when there's a thunderstorm? Well, the first clue is thunder, which is the loud, crashing noise that can vibrate the ceilings of your home. And, with thunder, there is always lightning. This is because thunder is the sound that is created when lightning passes through the air. Lightning is created by one particular type of cloud, a cumulonimbus cloud, also called a thunderhead.

When a cumulonimbus cloud forms, it separates electrical charges in the air.

At the top of the cloud, a positive charge forms, and, at the bottom of the cloud, a negative charge forms. Movement of precipitation within the cloud causes these separated charges to strike one another, creating lightning.

When lightning strikes, it heats the air, causing air molecules to expand rapidly. Collisions among the expanding air molecules and the surrounding air produce a loud noise. Even though it may seem like thunder and lightning are not happening at the same time, they really are. You simply see the lightning first because light travels faster than sound.

Most thunderstorms bring strong winds and heavy rain, and sometimes hail. Thunderstorms occur when warm, humid air near the surface of the earth is thrust rapidly up into the sky. As this warm air rises, it cools, and the water vapor in the air condenses to form clouds, mainly cumulonimbus clouds.

Thunderstorms usually occur in the summer when there is more moisture and heat energy in the air. In the tropics, they can occur year around due to the warm and moist climate. However, tropical thunderstorms usually last only a short time.

If you are in a car during a thunderstorm, stay there. Lightning does not usually strike a closed car. If you are outside, take shelter from the storm in a building. If you cannot do that, crouch in a low place, and because lightning often strikes tall trees, you should not stand under a tree.

If you are in a boat or swimming, get to land as soon as possible. The electricity of lightning travels through water. If you are inside a building, do not touch electrical equipment. Stay away from open windows and doors. Thunderstorms can put on quite an impressive show; remember these safety precautions and think about the powerful science of weather the next time you see a storm approaching.

Did you know. . ?You can estimate how long you have to take cover from a thunderstorm by counting the number of seconds between when you see lightning and hear thunder. Divide the number of seconds by five. This will tell you how many miles away the storm is from your location.

Hurricane

A hurricane is a very strong storm that forms over the ocean. Hurricanes can cause serious damage to buildings, trees, and cars. For example, in 2005, Hurricane Katrina flooded about 80% of New Orleans and neighboring areas.

A hurricane starts as a tropical storm or cyclone that forms over warm, tropical waters. Remember, the energy of a storm comes from latent heat (when water vapor condenses, it releases latent heat). In hot places, like the tropics, the water evaporates more quickly, more moisture is held in the air, and the moisture contains more heat. Latent heat, which water vapor releases as it starts to cool in the air, slows the condensation process. The added heat causes the warmer air to rise higher and faster than the surrounding air. The energy can create convection currents. Winds begin to circulate, and a well-defined wall of cumulonimbus clouds, called the "eye wall," forms.

The eye wall is where the most severe winds and rainfall happen in a hurricane. At the center or "eye" of the storm is where winds are sucked upward and swirl out of the top of the storm, like a huge tornado. It's like dipping a cone in a cotton

candy maker. The cone is the eye of the storm, and the cotton candy forming around the cone in a circular pattern is the clouds forming into an eye wall.

The eye of a developing storm is calm with light winds. However, in the eye wall, winds can blow 28-74 miles per hour. Once these winds increase to over 74 miles per hour, the storm is a hurricane. The National Hurricane Center gives the storm a name, like Hurricane Katrina or Hurricane Charley.

For more information . . .Find the latest and greatest on hurricane and typhoon activity at: National Hurricane Center.

Hurricanes are heat engines. They need warm waters to power their "engines." So, hurricanes usually don't form until late summer. In the Northern Hemisphere, the hurricane season usually runs from June 1 to November 1. Once a hurricane moves across cool waters or land, they lose their strength and fade.

Hurricane safety

If you have a hurricane in your midst, listen to local news on whether to evacuate. If not, stay indoors and away from windows. If the hurricane seems to have passed, be careful, because you may just be in the "eye" of the storm and another blast is coming!

Tornados

You might be familiar with tornadoes if you live in the Great Plains and Midwest regions of the U.S. Or, you may have seen one in the movie, Wizard of Oz, when Dorothy from Kansas (the Midwest) is caught in a tornado that carries her to Oz. What you probably don't know is how a tornado gets started in the first place.

Tornadoes usually develop with severe thunderstorms on flat plains—where there aren't mountains or other geographic features to deter wind flow. They happen when very different air masses collide, like warm, moist, tropical air masses meeting dry, cold, polar air masses. The cold air mass drops down into the warm air mass, which causes winds to change directions and speed, called wind shear. When this happens, a rolling, horizontal column of wind develops. As the warm air is pushed upward into the atmosphere, the horizontal spinning column of air is thrust upward with it, creating a vertical spinning column of air, or tornado, within the thunderstorm.

Before a thunderstorm develops, changes in wind speed and direction create a horizontal, rotating column of air.As air rises inside the updraft of the growing storm cloud, this region tilts vertically.The spinning column, now two to six miles wide, extends through much of the cloud system and can form strong, violent tornadoes.

Globally, there are about 1,000 tornadoes each year. About 800 of those tornadoes occur in the U.S., because of the flat plains between the Arctic Ocean and Gulf of Mexico. The wind speed of a tornado can exceed 250 mph, which can destroy anything in its path. Tornadoes are actually white and cloudy in color, but they often look darker because of all the debris they pick up! Warm months are tornado season, particularly April, May, and June. This is because the warm temperatures allow polar air masses to come down as far south as Texas.

Tornado safety  Always pay attention to updates on tornado warnings and watches. If a tornado is

headed your way, find a safe place inside that is away from glass and potential flying objects. If you are in a two-story house, go to the lowest level. A basement is usually the best place to be. If you are in a car or mobile home, abandon it and find a safe place inside a building. If you're outside, find a ditch or low-lying area and lie flat in it.

What are Floods?

Floods occur when rivers or ocean water flows onto normally dry land and destroys property, and they can happen without a moment's notice. [Atmospheric view of a hurricane] For example, in 1969, a hurricane from the Gulf of Mexico headed north toward Virginia. At the same time, cold northern storms heading south met and clashed with the warm, moist hurricane.

This produced 31 inches of rain within 6 hours. Virginia's creeks and rivers could not hold all the water, so a flood occurred. The rain even caused a landslide that buried farms under 30 feet of soil. One hundred twenty-five people died in the flood, and many others were left homeless.

The most common cause of floods is tropical storms that are carried over land. Tropical storms can hold so much moisture because they are generated by warm surface waters, like those found in the Gulf of Mexico. When they reach land, tropical storms can drop heavy rain, which then causes the overflow streams and rivers.

Floods also occur along coastal regions. Sometimes, winds over the oceans can cause very choppy, fierce waves and these waves can overflow the coast. Or, a volcano or earthquake along the ocean floor can create a giant wave, known as a tsunami. Tsunamis have flooded and destroyed many coastal areas.

Some meteorologists are concerned that climate change has contributed to flooding and will continue to have a significant impact. [Flooded farm] In 1990, record floods in Texas are thought to have come from increased evaporation of water in the Gulf of Mexico, which strengthened storms. If climate change intensifies, some scientists anticipate that sea levels will also rise and flood many coastal areas.

If you are in an area that can flood, always have sandbags handy to place around your house. Pay attention to flood warnings on TV, and, if flooding occurs, turn off all utilities to avoid electric shock. If you are driving, avoid storm drains and ditches and, especially, do not drive around police blockades! If

needed, evacuate your home or car and flee to higher ground. Floods can happen in the blink of an eye, and it is important to be prepared.

Drought

In contrast to a flood, you can have a drought, where little rainfall happens for an abnormally long period. If rain does fall, it usually isn't enough to be absorbed into the ground and just quickly evaporates from the dry, warm winds in the region. Droughts usually occur in hot, dry areas around 30° latitude. Along these latitudes, warm tropical air masses descend. As they descend, the air becomes hotter and drier. With westerly winds blowing this hot, dry air into neighboring regions, droughts can follow. But, droughts can occur anywhere there's below average rainfall for several months. If precipitation doesn't balance water consumption, it can lead to falling water tables and loss of surface water.

The effects of drought can be increased by human activities. People contribute to desertification—the spread of desert environments—by cutting trees, building houses, and overusing land. People also affect local water availability by building dams, canals, and other projects that change water distribution.

Drought safety Droughts don't just appear quickly, like thunderstorms or tornadoes can.

Droughts are long periods with low rainfall, so you should be aware from announcements by local authorities if a drought is affecting your town. If so, restrict water usage according to guidelines set forth by your community. There are many ways you can conserve water.