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Chapter 2: “Understanding Weather” Humidity Humidity – the amount of water vapor in air Air’s ability to hold water vapor changes as the temperature of the air changes. Increased temperature means the air can hold more water. Decreased temperature means the air can hold less water. Relative Humidity Relative Humidity –ratio of water vapor in the air to the maximum amount the air can hold 2 factors that affect relative humidity: 1. Temperature 2. Humidity Measuring Relative Humidity How to measure relative humidity? Psychrometer – a tool to measure relative humidity; contains a wet and a dry bulb. 1) Wet-bulb thermometer –bulb covered with wet cloth 2) Dry-bulb thermometer –bulb is dry The difference in temperature readings between the thermometers can be used to determine the relative humidity. Determining Relative Humidity Dew Point Dew point is the temperature at which air can no longer hold any more water PRECIPITATION Relative humidity = 100% Clouds Clouds are groups of millions of tiny water droplets or ice crystals 2 Steps to Cloud formation: 1. Hot air rises 2. Air cools and condenses - water droplets form Clouds Are Made Of… Ice crystals – form when water condenses below 0 C = 32 F Water droplets – form when water condenses above 0 C = 32 F Clouds Clouds are classified by: 1) Altitude • Low clouds • Middle clouds • High clouds *NIMBO or NIMBUS = clouds producing precipitation* Low Clouds Found below 2,000 meters & made of only water droplets Three Types: 1)Stratus Clouds 2)Nimbostratus Clouds 3)Stratocumulus Clouds The lowest clouds “Overcast” Individual clouds have ill-defined edgesLayered Cover large areas Can block the sun Dark stratus cloud, overcast Produces continuous rain Gray, rainy day Lumpy & Low lying Cover much of the sky Form patches or rows of clouds with some blue sky in-between. Flat; don’t look like popcorn Middle Clouds Found between 2,000 meters and 6,000 meters They are made of water droplets or ice crystals Alto- indicates most middle clouds Four Types: 1) Cumulus 2) Cumulonimbus 3) Altocumulus 4) Altostratus White & Puffy Usually have flat bottoms Distinct edges Popcorn-like; NOT flat Big & Dark cumulus cloud Thunderstorm clouds Hail, tornadoes Tallest clouds Usually have a anvil shaped top because off strong winds at higher altitudes Grayish, white Layered cumulus clouds Distinct cloud elements patchy, and scattered OR in bands Uniform & Scattered Coverage Difficult to detect individual features Grayish blue High Clouds Found above 6,000 m Temperature are very cold and made of ice crystals Cirro- is used to indicate all high clouds Three Types: 1) Cirrus Clouds 2) Cirrocumulus Clouds 3) Cirrostratus Clouds Thin & feathery, wispy White Very high altitudes Made of ice crystals Thick white wisps/layers Extremely high in the air Distinct patchy or wavelike appearance . Sheet-like & cover entire sky Composed of ice crystals Usually transparent (sun/moon can easily be seen) #1 #2 #3 #4 #5 #6 #7 #9 #8 #10 #5 #2 #1 #3 #4 #6 #8 #7 Water in the Air Precipitation – solid or liquid water that falls to Earth’s surface from clouds More common = Rain & Snow Less common = Sleet & Hail Recap!!! What is weather? What does it greatly depend on? Weather is the condition of the atmosphere at a certain time and place. It depends greatly on the amount of water in the air. What is the difference between humidity and relative humidity? Humidity is the amount of water vapor in the air. Relative humidity is the ratio of the amount of water vapor in the air to the maximum amount the air can hold. A B Stratus Cumulus Which two main cloud shapes are seen in this picture? C D Cirrus Cumulus Which two main cloud shapes are seen in this picture? Cirrus Clouds Stratus Clouds Recap!!! What does nimbo or nimbus mean for clouds? It produces precipitation What does stratus mean for clouds? They are layered What are each of the following clouds made of? 1) Low Clouds 1) Water droplets only 2) Middle Clouds 2) Water droplets/ice crystals 3) High Clouds 3) Ice crystals only What are the two most common forms of precipitation? Rain and snow Air Masses Air Masses – a very large volume of air that has a certain temperature & moisture content Fronts – the place where two or more air masses meet When air masses meet, the less dense air mass rises over more dense air mass. Since warm air is less dense, warm air masses usually rise above cold air masses Fronts The four main kinds of fronts are: 1) Cold Fronts 2) Warm Fronts 3) Occluded Fronts 4) Stationary Fronts Cold Fronts Cold air mass moves under a warm air mass and pushes the air up Cold air mass replaces warm air mass Moves quick & brings heavy precipitation Warm Fronts Warm air mass moves in over a cold air mass leaving an area. Warm air mass replaces cold air as it moves away Bring light rain & followed by clear, warm weather Cold Front = cold air mass pushes warm air mass away Warm Front = warm air mass replaces a cold air mass Occluded Fronts Warm air mass is caught between 2 cold air masses. Brings cool temperatures, rain, and snow Stationary Fronts Cold air mass & a warm air mass move toward each other. Neither has enough energy to push the other away. Remain in same place causing cloudy weather Occluded Front = warm air mass is trapped between two cold air masses, which move together and push war air away Stationary Front = air masses stay in one place Air Pressure and Weather Cyclone – air mass formed that has lower pressure than surrounding air. Air rises and cools Forecast: Clouds can form & cause rainy or stormy weather Anticyclone – air mass that has higher pressure than the surrounding air • Air sinks and gets warmer • Relative humidity decreases • Forecast: Warm sinking air brings dry, clear weather Cyclones and anticyclones can affect each other. Air moving out from anticyclones move towards areas of low pressure. This movement can form a cyclone. Recap!!! What does air mass depend on? Water Content (Moisture) & Temperature What are the four different kinds of fronts? Cold Fronts, Warm Fronts, Occluded Fronts, Stationary Fronts Recap!!! What is the difference between Cyclones and Anticyclones? •Cyclones lower pressure than surrounding air, so air cools as it rises. •Anticyclones higher pressure than surrounding air & warms as it sinks. How are front and air masses related? A front is where two or more air masses meet. Therefore, without air masses…there would be no fronts. Severe Weather What are the different types of severe weather? 1. Thunderstorms 2. Hurricanes 3. Tornadoes Thunderstorms Thunderstorms –intense storm with strong winds, heavy rain, & thunder Many happen along cold fronts Conditions required: 1) Warm, moist air near Earth’s surface 2) Unstable area of the atmosphere Thunderstorms 4 Steps to a Formation of a Thunderstorm 1. Body of cold air is found above body of warm air 2. Warm air rises & cools as it mixes with cool air 3. When warm air reaches its dew point, water vapor condenses & forms cumulus clouds 4. If warm air keeps rising, clouds may become dark cumulonimbus clouds What is the difference between a severe thunderstorm watch and a severe thunderstorm warning? • 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 Thunderstorm Safety •Avoid touching metal objects •Install metal lightening rods •Find a low area away from trees •Stay away from water Number of days each year in which thunderstorms occur. Note maximum (90) in Florida. Lightning 5 steps to the formation of lightning: 1. As a cloud grows bigger, parts of it begins to develop electrical charges 2. Upper parts = positively charged 3. Lower parts = negatively charged 4. When one charge gets big enough, electricity flows from one area another or the ground 5. These electrical currents are lightning http://www.usatoday.com/weather/resources/graphics/2008-05-18-lightning-flashgraphic_N.htm Thunder 3 Steps to the Formation of Thunder 1. When lightning moves through the air, the air gets very hot and expands rapidly 2. As it expands, it makes air vibrate 3. These vibrations release energy in the form of sound waves, called thunder. Tornado • Rapidly whirling, funneled-shaped cloud that reaches down from a storm cloud to touch E’s surface • Most frightening & disruptive storm • Usually brief, touching the ground approx. 15 minutes or less • Wind speeds may reach 480 km/hr (230 mph) • Counterclockwise in both the Northern and Southern Hemispheres Tornadoes 1. Wind moving in 2 directions causes a lyaer of air in the middle to being to spin like a roll of toilet paper. 2. The spinning column of air is turned to a vertical position by strong updrafts of air in the cumulonimbus cloud. The updrafts of air also begin to spin. 3. The spinning column of air moves to the bottom of the cumulonimbus cloud and forms a funnel cloud. 4. The funnel cloud becomes a tornado when it touches ground. Tornadoes Fewer than 1% of thunderstorms produce tornadoes About 75% of the world’s tornadoes happen in the United States Most happen in the spring and early summer Most last for only a few minutes Average wind speeds are between 120 km/h and 180 km/h Tornado Alley Where is Tornado Alley?? An area of the United States where tornadoes are most frequent Between Rocky & Appalachian Mountains z Tornado Alley Why is Central United States considered Tornado Alley? 90% of tornadoes hit this region of the US Cold, dry air from Canada/Rocky Mountains meets warm, moist air from the Gulf of Mexico and hot, dry air from deserts They are combined with atmospheric instability to produce intense tornadoes Tornado Alley The core of Tornado Alley consists of: Texas Panhandle Oklahoma Kansas Nebraska South Dakota Colorado Eastern Plains Ohio Valley The southern Great Lakes The Tennessee Valley The lower Mississippi Valley A map Tornado Alley outlined. This area has been defined as the area where there has been approximately at least one significant tornado per day, per decade. Tornado Safety •Listen for watches & warnings •Basement level, or to the middle room on ground floor •Stay away from windows •Lie in a ditch Hurricanes Hurricanes – large, rotating tropical weather systems Wind speeds over 120 km/h Can be 160 km to 1,500 km in diameter and travel thousands of miles Most powerful storms on Earth Also called typhoons & cyclones Form over warm, tropical oceans Hurricanes 7 Steps in the Formation of a Hurricane 1. Begins as a group of thunderstorms traveling over topical ocean waters 2. Winds traveling in two different directions meet and cause the storm to spin – Tropical Depression Tropical Depression 3. Because of the Coriolis Effect, they rotate counterclockwise in the North and clockwise in the South Tropical Storm • 4. A group of thunderstorms form and produce a large, spinning storm – Tropical Storm Hurricane Formation continued • 5. The hurricane forms as the storm gets stronger • 6. It will continue to grow as long as it is over warm ocean water • 7. When they move over cold water or land they lose energy which is why they aren’t common in middle of continents Hurricane Damage A storm surge is a rise in sea level that happens during a storm A storm surge from a hurricane can be up to 8 m high Hurricane Safety •Evacuate •Move into the interior room and stay away from windows Recap!!! Why do thunder and lightning usually happen together? When lightning moves through the air, the air gets very hot and expands rapidly. This makes the air vibrate which releases energy in the form of sound waves, called thunder. Where do most tornadoes happen. Why? 90% of tornados occur in Tornado Alley. This is a result of when the cold, dry air from Canada and the Rocky Mountains meets warm, moist air from the Gulf of Mexico and hot, dry air from deserts, which combines with atmospheric instability to produce intense tornadoes. Recap!!! Which two conditions are required for thunderstorms to form: 1) warm, moist air near Earth’s surface 2) an unstable area of the atmosphere What type of energy powers hurricanes? How do hurricanes lose energy? They are powered by solar energy. The sun’s energy causes ocean water to evaporate. When they move over cold water or land they lose energy. What is a storm surge? Which type of severe storm is most commonly associated with this? A storm surge is a rise in sea level that happens during a storm. A hurricane can cause a storm surge to be 8 m high.