Severe Weather 1.Thunderstorms 2.Tornadoes 3.Hurricanes

Thunderstorm Formation Three Conditions: 1.Abundant source of moisture. As moisture condenses heat is released. So cloud is warmer than air around it. How is it moving? 2.Mechanism to lift the air. 3.Atmosphere must be unstable. Air must continue to cool with altitude so that the cloud stays warmer than the surrounding air.

Limits to Growth 1.Rising air meets stable air that it cannot overcome. 2.Rate of condensation is not producing enough heat.

Air-Mass Thunderstorms Air rises because of unequal heating of Earth’s surface within one air mass – Max >> during mid- afternoon TWO TYPES: 1.Mountain: air mass moving up side of mountain 2.Sea-breeze: temperature differences between air over land and air over water.

Frontal Thunderstorms Produced by advancing cold fronts and sometimes warm fronts Cold Fronts: -Air rises rapidly -Lines of thunderstorms -Get lift from push of cold air -Can persist long into night Warm Fronts: -Air rises gradually -Relatively mild thunderstorms

Classified by direction of air movement Cumulus: air rises vertically; clouds Mature: downdrafts & updrafts; rain cools air Dissipation: smaller downdrafts; no more energy Stages of Thunderstorms

Stages of Development 1.Cumulus Stage: – Air rises vertically upwards – Creates updrafts = moisture goes to the upper parts of the cloud 2. Mature Stage -- Strong updrafts and downdrafts What do you think causes downdrafts? --As precipitation falls, it cools the air around it. This newly cooled air sinks rapidly = downdraft

Stages of Development 3. Dissipation Stage: -- Storm loses energy -- lingering downdrafts The convection cell can only exist if there is a steady supply of warm, moist air. – Knowing this, What do you think is the ultimate cause of a thunderstorm losing energy? DOWNDRAFTS! – They spread in all directions when they reach the surface and cool the surface air.

Severe Thunderstorms Continuous supply of surface moisture – Cold fronts moving into warm areas Instability of air – Increases strength of updrafts and downdrafts Supercells: intense, rotating updrafts Only about 10% severe in US

Thunderstorm Hazards 1.Lightning:  Electricity caused by rapid rush of air in a cumulonimbus cloud. (friction)  Heats air to 30,000 °C  What is thunder?!  Sound from heated air expanding and contracting

Lightning

Thunderstorm Hazards 2. Floods:  Rain falls faster than it can be transported or absorbed 3.Hail:  Water droplets encounter ice pellets and freeze  Ice pellets can get caught in the up and downdrafts

Hail Storm Video ews/samaras-hail-storm?source=relatedvideo ews/samaras-hail-storm?source=relatedvideo

Thunderstorm Hazards 4. Wind:  Downbursts: concentrated in one area  Macrobursts and microbursts

TORNADOES chasers/videos/tornado-near-miss/

Tornadoes Form when wind speed and direction change suddenly with height – Wind Shear Funnel cloud: a tornado before it reaches the ground Why can we see tornadoes? – Dust and debris that is drawn into the swirling air

Tornado Formation Wind Shear can cause horizontal rotations near Earth’s surface. Updrafts can tilt this column of wind vertical and speed up their rotations. As rotation speed increases, wind is removed from the center = low air pressure Pressure gradient produces violent winds XExpb3pY

Tornadoes

F-scale: path of destruction, wind speed, and duration – F0-F5 Only 1% reach F4-F5; Most are F1 When do most tornadoes occur? – Spring. Why? – Large temp. contrasts between polar and tropical air Where do most tornadoes occur? – Central United States. Why? Think air masses! – Cold cP air collides with warm mT air from the gulf

April 27, 2011 Tornadoes uQ uQ ARQ ARQ

Tornado Alley

Fujita Scale

SAFETY! Warning signs: – Dark, greenish skies – Hail – Towering wall of clouds – Roaring noise SEEK SHELTER! biggest-storm/draper-text

TROPICAL CYCLONES  Large, rotating, low-pressure storms  Hurricanes = strongest  Coriolis effect causes rotation

TROPICAL CYCLONE FORMATION CONDITIONS:  Abundant supply of warm ocean water  Disturbance to lift warm air and keep it rising  What oceans would cyclones NOT form in?  South Atlantic and South Pacific oceans. Why?  Cooler waters

Stages of Development 1.DISTURBANCE:  Tropical disturbance causes air to rise 2.DEPRESSION:  Cyclonic circulation at a low pressure center 3.TROPICAL STORM:  Winds exceed 65 km/hr (40 mph) 4.HURRICANE:  Air pressure falls; winds can reach 120 km/hr (75 mph)

Classifying Hurricanes

Hurricane Map

Hurricane Paths

HURRICANE FORMATION  Water evaporates, latent heat is stored  Heat is released as air rises and clouds form  Disturbances produce more precipitation; more energy is released  Rising air creates low pressure at ocean’s surface  Coriolis effect causes moving air to turn counterclockwise in the northern hemisphere  Cyclonic rotation of a tropical cyclone

HURRICANE HAZARDS Storm surge: mound of ocean water towards coast Flooding: caused by vast amounts of rain CsQ&safety_mode=true&persist_safety_mode =1 CsQ&safety_mode=true&persist_safety_mode =1

Droughts are extended periods of well-below- normal rainfall Floods can occur when storms and hurricanes unleash enough rain and remain over an area for an extended time Both are considered recurring weather. Floods and Droughts

Heat waves usually come with drought and involve extended periods of above-normal temperatures Cold Waves are extended periods of below- normal temperatures Both are brought on by high-pressure systems except cold waves are brought on by continental polar or arctic air masses. Heat and Cold Waves