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Storms Table of Contents Formation of Precipitation Although all clouds contain water, only some produce precipitation. A raindrop large enough to reach the ground without evaporating contains roughly a million times the water of a cloud droplet. Therefore, in order for precipitation to form, millions of cloud droplets must somehow join together into drops large enough to sustain themselves during their descent. There are two mechanisms to explain this phenomenon: the Bergeron process and the collision-coalescence process. The Bergeron process is named for its discoverer, Tor Bergeron. Because water suspended in air does not freeze until it reaches nearly -40oC, much colder than it would freeze on land. This water is said to be supercooled. Airplanes collect ice when they pass through a liquid cloud made up of supercooled droplets because supercooled water freezes rapidly if agitated. Supercooled droplets will freeze quickly if they come in contact with freezing nuclei, or particles with a crystal form similar to that of ice (not unlike condensation nuclei necessary for cloud formation). Ice crystals are solid, and therefore are held together more tightly than molecules that make up liquid water. As solid crystals form, they form freezing nuclei for more solid crystals to form. Eventually, the crystals are large enough to fall. Even a summer rain may have begun as a snowstorm in the clouds overhead. In clouds below the freezing level, especially in the tropics, precipitation occurs as a result of a different process called the collision-coalescence process. Droplets of water larger than cloud droplets can form if giant condensation nuclei (such as sea salt) are present. As large droplets begin to form, they collide with other large droplets and eventually coalesce into a droplet large enough to fall to the ground without evaporating.

Homework Assignment #1 Fill in the blanks:

A raindrop large enough to reach the ground without evaporating contains roughly ______times the water of a cloud droplet.

The ______is the way that rain droplets can form in supercooled clouds. In this method, supercooled water droplets collect more and more water and get larger.

The ______is the way that rain droplets can form below the freezing point, but giant ______must be present for water to cling to. Formation of Clouds In order for clouds to form water must condense (change from a gas to a liquid) or sublimate (change from a solid to a gas). Additionally, there must be a condensation nuclei, which is a solid particle in the atmosphere that provides a solid surface onto which the liquid or solid water molecules accumulate. There are four ways that air can be cooled enough to condense or sublimate. 1. Adiabatic cooling - temperature cools as the air becomes less dense

2. Air cools because it has been mixed with other air masses

3. Air cools because it is lifted by another air mass into a cooler region

4. Advective cooling -temperature cools as the air moves over a cold surface Types of Clouds Clouds are categorized by characteristics such as: Altitude, Appearance, Origin High Clouds • Cirrus - high altitude wispy clouds

• Cirrocumulus - distinct patchy and/or wavelike appearance

• Cirrostratus - blanket sky in ill-defined sheets

Middle Clouds • Altocumulus - distinct cloud elements

• Altostratus – uniform, diffuse coverage Low Clouds • Cumulus – puffy, noticeable vertical development.

• Stratocumulus - widely scattered, clustered together, or layered w/ little vertical development.

• Stratus - lowest of the low clouds, appear as an overcast deck but can be scattered

• Fog - low stratus cloud in contact with the ground Multi-Layer Clouds Nimbostratus • Extends vertically through low and middle levels • associated with large areas of continuous precipitation

Cumulonimbus  can produce lightning, thunder, heavy rains, hail, strong winds, and tornadoes  can span all cloud layers  anvil-shaped tops form because of stronger winds at higher levels of the atmosphere RAIN GUAGE – device to measure precipitationHomework Assignment #5 1. Describe the conditions that are necessary for clouds to form.

2. List the four processes of cooling that can lead to cloud formation. Define each.

3. Compare cirrus, cumulus, and stratus clouds by completing this chart: Cloud Altitude Description of shape Drawing of shape Cirrus

Cumulus

Stratus

4. What kind of cloud causes thunderstorms?

5. How are clouds and fog different?

6. What instrument measures rainfall amount? Thunderstorms

Lightning is a huge, hot spark of electricity that shoots out of a thunderstorm cloud. Thunder is the sound made when air heated by lightning explodes.

Look for: Puffy white cumulus clouds that grow taller and flatten out at the top, forming anvil- shaped thunderheads. Caution: Lightning can be deadly. It is best to go indoors when lightning is nearby. If caught outdoors, stay away from high places and do not go under a tree. Crouch down on two feet, do not lie flat.

How Far Away? Light travels 186,000 miles per second, while sound travels only 1/5 of a mile per second. That is why you see the lightning before you hear the thunder. The farther away the lightning occurred, the more time elapses between seeing the lightning and hearing the thunder. It takes 5 seconds for thunder to travel one mile.

• Flash Floods/Floods are the number one killer associated with thunderstorms with nearly 140 fatalities a year. • A thunderstorm is considered severe if it produces hail at least 3/4-inch in diameter, winds of 58 mph or higher, or tornadoes. • Severe Thunderstorms – long lived thunderstorms; formed in strong vertical wind shears; associated with gust fronts, high winds, and tornadoes • A severe thunderstorm watch is issued by the National Weather Service when the weather conditions are such that a severe thunderstorm is likely to develop. • A severe thunderstorm warning is issued when a severe thunderstorm has been sighted or indicated by weather radar. At this point, the danger is very serious and everyone should go to a safe place, turn on a battery-operated radio or television, and wait for further information • 3 stages of thunderstorm development: 1. Cumulus - Warm humid air rises, cools, and condenses into cumulus cloud(s), Condensing water releases latent heat making cloud warmer than air around it. Insufficient time for precipitation to form. Updrafts keep water droplets and ice crystals suspended. 2. Mature - Cell = updraft + downdraft 3. Dissipating - Updrafts weaken and downdrafts dominate. Light precipitation, weaker winds • During a thunderstorm, positive and negative electrical charges build up in the clouds. • The discharge of the electricity between clouds is lightning. • Lightning is as hot as 30,000°C, hotter than the sun. • The heated air expands and explodes producing thunder. Tornado Rapidly whirling, funneled-shaped cloud (called a vortex) that reaches down from a storm cloud to touch Earth’s surface. Since air is invisible, the vortex that we see is water droplets, dust, and debris sucked up from the ground. Caution: Take shelter in a basement or in the center of a building immediately when tornado warnings are issued for your area. Stay away from windows. • Usually brief, touching the ground approximately 15 minutes or less • Wind speeds may reach 480 km/hr (230 mph) • Counterclockwise in both the Northern and Southern Hemispheres • Stages of Tornado Formation: • Overshooting top • Wall cloud descends below base of cloud and slowly rotates • Funnel cloud descends from wall cloud • Funnel cloud touches ground • Tornado Watch – tornadoes are possibly in your area • Tornado Warning – tornadoes have been seen in the sky or on weather radar • Occur most often in the US • Approx. 800 tornadoes a year • Tornado Alley - Warm humid air mass moves north from Gulf of Mexico and meets a cold dry mass that is moving south from Canada; SD, Iowa, Nebraska, Kansas, Missouri, Oklahoma, TX, NM, and Arkansas • Enhanced Fujita Scale- Measuring intensities of tornadoes based on a scale developed by Dr. Theodore Fujita at the University of Chicago in the 1960s • EF0 Gale 65-85mph • EF1 Weak 86-110 mph • EF2 Strong 111-135 mph • EF3 Severe 136-185mph • EF4 Devastating 186-200 mph • EF5 Incredible over 200 mph • The deadliest tornado on record in the United States was the tri-state tornado outbreak of March 18, 1925. Several tornadoes demolished portions of Missouri, Illinois, and Indiana, killing 747 people and injuring over 2000 more. Homework Assignment #2

1. What should you do if you’re caught outside during a lightning storm?

2. Why do you see lightning before you hear thunder?

3. What is the difference between a severe thunderstorm watch and a severe thunderstorm warning?

4. In what direction do tornadoes spin in NJ?

5. What is the difference between a tornado watch and a tornado warning?

6. Where is tornado valley? Tropical Storms and Hurricanes • Tropical storms form over warm ocean waters. They begin as a tropical disturbance which is nothing more than a cluster of big thunderstorms. When this growing storm begins to rotate, it becomes a tropical depression. When wind speeds reach 40 miles per hour it is called a tropical storm. • Hurricanes are tropical storms that have winds of 119 km/hr • Usually occur between June and November in the eastern US • Hurricanes that occur in western US are called typhoons. • Size can be 220-700 km in diameter • Eye – center of a hurricane • Storm Surge - Hurricane pulls up ocean water an average of 1 cm per 1 mb of air pressure drop; Regions along shores and coasts are inundated with water being pushed ahead of the hurricane • Stages of Hurricane Development: 1. Tropical Disturbance Mass of thunderstorms begin to organize Light wind circulation 2. Tropical Depression Wind speed 20-34 knots Central low pressure developing with rotation of thunderstorms 3. Tropical Storm 35-64 knots Strong central low pressure Increasing wind speeds Forward movement across oceans 4. Hurricane 64 knots or more Well developed central low pressure; eye may be visible Moving to west along with global winds (NE or SE Trades) Can move up to 50 knots over open ocean Highest wind speed on the forward traveling side • Saffir Simpson Scale – measures hurricane intensity by comparing wind speed and air pressure • Tropical Storm – 39-73 mph winds • Category 1 – 74-95 mph • Category 2 – 96-110 mph • Category 3 – 111-130 mph • Category 4 – 131-155 mph • Category 5 – 156 mph and up • Naming Hurricanes:

• Initial tracking of hurricane was by latitude and longitude

• WWII – use of military terms to identify individual hurricanes (Alpha, Bravo, Tango…) • Early 1950s – use of female names • Late 1970s – Male and female names are used; alternate male/female name for Pacific or Atlantic hurricanes Winter Storms • Ice storms – when temperatures below a raining cloud are very cold, the raindrops become supercooled which means they cool to below 32 degrees F and freeze when they hit the ground and other objects. Called freezing rain, precipitation from these ice storms covers streets, houses, and trees with layers of heavy ice. • Sleet forms when falling snow melts and then refreezes before it hits the ground. It consists of small pellets of ice that bounce and make tapping sounds when they hit the ground. • Hail is a mixture of liquid and frozen precipitation. Formed inside cumulonimbus clouds, hailstones are composed of layers of ice and can be quite large when strong gusts of upward moving air keep them inside the cloud. As they blow around in the cloud, they collide with raindrops, adding layers and growing before they fall to the earth. • Frost – ice crystals that form on a surface such as the ground or the leaves of plants are called frost. Frost forms when the air temperature drops below freezing and the water vapor in the air freezes into ice crystals. Frost is devastating to crops. When temperatures drop below freezing, the water inside leaves and stems freezes. Some farmers burn large drums of oil in their orchards on cold nights to ward off frost. • Snow forms when cloud temperatures are below freezing. Depending on temperature nd humidity, snowflakes can have lots of shapes. Snow that starts and stops, sometimes many times over, is called a flurry or snow shower. More intense brief snowfall, sometimes accumulating to several inches or more, is called a snow squall. • Blizzards are caused by the combination of heavy snowfall, cold temperatures, and strong winds. • Nor-Easters – most of the snow in New England and the mid-Atlantic states comes from northeasters which are storms with strong northeast winds that form over the Atlantic Ocean. These storms often dump large amounts of snow. Homework Assignment #3:

1. Where do tropical storms form?

2. List the stages of hurricane development:

3. What is a storm surge?

4. What is the difference between a tropical depression and a tropical storm?

5. What is the difference between a tropical storm and a hurricane?

6. How does freezing rain form?

7. How does sleet form?

8. How does hail form?

9. How does frost form?

10. How does snow form? Lab: Chasing Hurricane Andrew Background Hurricanes are the most destructive storms on earth. They develop from tropical storms (cyclones) and are Classified as hurricanes when their winds reach 64 knots ( ~ 71 mph or 119 kph). Hurricanes include a small central region known as the eye, where the winds are light and there are few clouds. Moving out from the eye, a narrow band of intense thunderstorms, heavy rains, and strong winds is encountered. This band is called the eye wall. Beyond the eye wall are strong but diminishing spirals of the same weather. Hurricanes are huge storms. Typically they are about 500 km in diameter, and they usually last for a week or more. Hurricanes contain tremendous amount of energy. They gather this energy from warm ocean waters in the tropics. As the warm, humid air rises, it cools and condenses, releasing heat (called latent heat). This heat warms the surrounding air, making it lighter and causing it to rise farther. As the warm air rises, cooler air flows in to replace it, causing wind. This cooler air is warmed by the ocean, and the cycle continues. This heat from warm ocean water is the fuel that hurricanes run on. For this reason, hurricanes diminish and die when they move inland or move into colder waters. In addition to the high winds – gusts up to 172 knots (about 192 mph or 320 kph) – and the torrential rains, hurricanes produce what is known as a storm surge. The circular winds, together with the low-pressure eye and high-pressure outer regions of a hurricane, create a mound of water in the center of a hurricane. The storm surge causes considerable flooding and is responsible for most hurricane damage and deaths. Weather satellites in orbit above the Earth can easily detect hurricanes. Satellite data, along with data from radar and aircraft, is used to follow developing hurricanes. Through tracking, we can tell where a hurricane has been. We also can estimate where it will go in the near future. When it appears that a hurricane is moving toward land, the National Weather Service (NSW) issues watches and warnings. A hurricane watch means that hurricane conditions are likely in the watch area within 36 hours. A hurricane warning means that these conditions are likely within 24 hours. People living in low coastal areas that could be affected by a storm surge need to evacuate as soon as watches and warnings are issued. In August 1992, Hurricane Andrew caused a tremendous amount of human suffering and billions of dollars of damage in the Bahamas, the Southern tip of Florida, and parts of Louisiana. This hurricane was unusual because it stuck the United States twice. After coming ashore in Florida, it passed over the Gulf of Mexico – regaining strength in the warm Gulf waters – then hit the coast of Louisiana. This activity contains the actual tracking data collected on Hurricane Andrew.

Procedure

1. Look at the data in the different parts of the table marked “Track of Hurricane Andrew.” It contains three types of information: a. Date/Time: Data was collected on Andrew every six hours beginning August 16 through August 28. Only a portion of the data is presented here. Time is given in the military convention; for example, 1200 is 12:00 noon, and 1800 is 6:00 pm. b. Position: This is the position of the eye of the hurricane by latitude and longitude. It is important to remember that the storm is much bigger than the eye. The winds extend out beyond the eye about 100 km in all directions (about ½ the area of one 5o longitude-latitude square on the map). c. Wind Speed: This is the maximum speed of the winds in the hurricane, not the speed with which the hurricane is actually moving. Wind speed is given in knots (kt). 1 kt = 1.15 mph = 1.85 kph.

2. Plot the data given in the tracking table on the map your teacher has supplied. Make a dot for each position of Andrew, and then connect the dots. For each position at the beginning of a day (time = 0000), draw a small star or asterisk over the dot. You will be asked to stop and plotting data periodically and issue hurricane warnings and watches based on the path of the hurricane you have plotted. REMEMBER: A hurricane warning means hurricane conditions are likely for a location within 24 hours. A hurricane watch means hurricane conditions are likely for a location within 36 hours. Questions/Conclusions 1. Where did Andrew do the most damage before striking Florida?

2. Describe the motion of the storm displayed on your tracking map from the first point you plotted to the last.

3. What happens to the direction of Andrew after it struck Louisiana?

4. What happened to the wind speed in Andrew after it came aground in Louisiana? Why did this happen?

5. Judging from the wind speed, when did Andrew become a hurricane and when should it have been downgraded to a tropical storm?

6. In terms of damage done, why was it so devastating for Andrew to hit the southern part of Florida? Why might it have been less destructive it if had hit Farther north on the coast of the United States; for instance, Georgia or South Carolina? Position Wind speed Date/Time Lat. (oN) Lon. (oW) (knots) ______Aug 21/0000 23.2 62.4 45

0600 23.9 63.4 45

1200 24.4 64.2 50

1800 24.8 64.9 50

Aug 22/0000 25.3 65.9 55

0600 25.6 67.0 60

1200 25.8 68.3 70

1800 25.7 69.7 80

Aug 23/0000

Stop! Question 1: Based on how far the storm has traveled over the last 24 hours and its direction so far, for which locations would you issue hurricane warnings and watches? You can tell how far the hurricane has traveled in the last 24 hours by looking at the distance between the last two stars or asterisks you have drawn on the map. Don’t forget that the size of the hurricane is much larger than the dots you have drawn.

Position Wind speed Date/Time Lat. (oN) Lon. (oW) (knots) ______Aug 23/0600 25.5 72.5 105

1200 25.4 74.2 120

1800 25.4 75.8 135

Aug 24/0000

Stop! Question #2. Based how far the storm has traveled over the last 24 hours and its direction so far, which locations would you issue hurricane warnings and watches? Position Wind speed Date/Time Lat. (oN) Lon. (oW) (knots) ______Aug 24/0600 25.4 79.3 120

1200 25.6 81.2 110

1800 25.8 83.1 115

Aug 25/0000

Stop! Question #3. Based how far the storm has traveled over the last 24 hours and its direction so far, which locations would you issue hurricane warnings and watches?

1200 27.2 88.2 115

1800 27.8 89.6 120

Aug 26/0000

1200 30.1 91.7 80

1800 30.9 91.6 50

Aug 27/0000

Intentional Weather Modification Weather modification is deliberate human intervention to influence and improve the atmospheric processes and events that constitute the weather – that is, to aim the weather at human purposes. From earliest recorded times, people have used prayer, wizardry, dancing, and even black magic in attempts to alter the weather. Until modern times, however, most attempts at weather modification remained largely in the realm of the mystic. By the nineteenth century such devices as smudge pots, sprinklers, and wind machines to fight frost were in use. During the American Civil War, observations that rainfall apparently increased following some battles led to experiments in which cannons were fired into clouds to bring more rain. Unfortunately, these experiments, as well as many others, proved unsuccessful. Weather modification strategies fall into three broad subdivisions. The first relies on the injection of energy by force. The use of powerful heat sources or the intense mechanical mixing of the air (such as by helicopters) are both examples of techniques of fog dispersal attempted at some airports that fall into this category. The second subdivision involves the alteration of land and water surfaces in order to change their natural interactions with the lower atmosphere. One commonly discussed but still theoretical example of this technique is the blanketing of a land area with a dark substance. If it were done, the amount of heat absorbed by the surface would increase and lead to stronger upward air currents that, in turn, might aid cloud formation. Finally, the third subdivision involves triggering, intensifying, or redirecting the atmosphere’s natural energies. The seeding of clouds with such agents as dry ice and silver iodide for many purposes, including precipitation enhancement, represents the primary example in this category. Because it offers a relatively inexpensive and easily used technique, cloud seeding has been the main focus of modern weather modification technology. Modern weather modification is used for various purposes. First, it can be used to disperse fog and clouds to improve visibility. The US Air Force has practiced weather modification for many years at various airbases, and commercial airlines have modified weather at selected airports in the northwest US. Additionally, weather modification can be used to suppress hail and reduce the damage caused by it. Finally, weather medication can be used to prevent frost from killing crops. The first scientific breakthrough in intentional weather modification came in 1946 when Vincent J. Schaefer discovered that dry ice dropped into a supercooled cloud spurred the growth of ice crystals. Shortly after Schaefer’s discovery, it was learned that silver iodide could also be used for cloud seeding. The similarity in the crystalline structures of silver iodide and ice accounts for silver iodide’s ability to initiate the growth of ice crystals. Thus, unlike dry ice, silver iodide crystals act as freezing nuclei rather than as a cooling agent. This substance has an advantage over dry ice in that it can be supplied to clouds from burners on the ground as well as from aircraft. If either substance is to be successful, certain atmospheric conditions must exist. Clouds must be present, for seeding cannot generate them. In addition, at least the top portion of the cloud must be supercooled to below 0oC.