The Nature of Storms Topic 7

Lake Effect Snow Caused by cold arctic air blowing across the “warmer” water of the Great Lakes. The cold arctic air picks up moisture from the “warmer” water. The moisture is deposited as snow when it hits land.

Lake Effect Snow - NY In order for NY to receive Lake Effect snow the cold winds must blow across Lake Ontario or Lake Erie from the N, NW, W or SW. The wind direction across the lake determines which location in NY will receive the snow.

Why doesn’t NY get Lake Effect snow if the winds are from the E or S? If the wind blows from the NE, E, or SE, then Canada will receive the snow. Wind from the South would be too warm to cause Lake Effect snow in NY and there isn’t a lake South of NY (PA is South of NY).

Problems caused by Lake Effect Snow Reduced visibility Lost work and school days Costs $$ to remove the snow Hazardous driving conditions

How Thunderstorms Form 3 conditions necessary: Lots of moisture available Note: condensation releases latent heat energy therefore the cloud is warmer than the surrounding air Mechanism to lift the air The atmosphere must be unstable The cloud must remain in air that is cooler than it

Air-Mass T-storms Form due to an unequal heating of Earth’s surface within an air mass. 2 types: Mountain T-storms Occur when an air mass rises up a mountain, this is called orographic lifting. Sea-breeze T-storms Occur near coasts due to extreme temp. differences over land and water.

Orographic Lifting

Sea breeze

Frontal T-storms Produced by advancing cold fronts that push warm air up rapidly.

Stages of Development Cumulus stage Mature stage Dissipation Stage

Cumulus stage Air rises vertically creating updrafts Brings moisture to the upper cloud Condensation occurs releasing heat energy Cool air Warm air

Mature stage Preciptation occurs causing the air to cool . The cool air is more dense so it sinks rapidly creating downdrafts. Convection cell. Equal downdrafts and updrafts.

Dissipation stage Downdrafts take over. The cool air cuts off the t-storms supply of warm, moist air.

Lightning Forms when friction between the downdrafts and updrafts causes atoms to lose their electrons making them positively charged and others to gain electrons making them negatively charged.

Stepped Leader/Return Stroke A stepped leader is an invisible channel of negatively charged air. It moves from the cloud to the ground. When it nears the ground a channel of positively charged ions, called the return stroke rushes upward to meet it.

Dubai, on the Arabian Peninsula

Downbursts Violent downdrafts that are concentrated in a local area. Macroburst: destructs 4-5km Microburst: destructs 3km or less

Hail Precipitation in the form of balls or lumps of ice. Hail formation: http://www.classzone.com/books/earth_science/terc/content/visualizations/es1805/es1805page01.cfm?chapter_no=visualization

Tornado A violent, whirling column of air in contact with the ground. Before it reaches the ground it is called a funnel cloud. Tornadoes are made visible by dust and debris in the air. Form when there is a sudden change in wind direction and speed (called wind shear) creating a horizontal rotation.

Tornado cont’d. If the horizontal funnel cloud forms near updrafts in a t-storm then it can become vertical. Air rotation accelerates, causing air to be removed in the center lowering the centers pressure.

Fujita Tornado Intensity Scale Category Damage Wind speeds F0 light 40–72 mph F1 moderate 73–112 mph F2 significant 113–157 mph F3 severe 158–206 mph F4 devastating 207–260 mph F5 incredible 261–318 mph

Tropical Cyclones Names: 2 conditions for formation: Cyclones – Indian Ocean Typhoons – Pacific Ocean Hurricanes – Atlantic Ocean, Caribbean Sea, and Gulf of Mexico 2 conditions for formation: Abundant supply of very warms ocean water Disturbance to lift warm air and keep it rising

Where do they occur?

How do they move? They move according to wind currents that steer them.

Stages Stage 1: Tropical Disturbance Stage 2: Tropical Depression Weak low pressure system Few develop into full-fledged hurricanes Stage 2: Tropical Depression When a disturbance acquires a cyclonic circulation around a center of low pressure. Stage 3: Tropical Storm When winds exceed 65 km/hr in a depression.

Stage 4: Hurricane When the winds in a storm exceed 120 km/hr and the air pressure continues to drop. Development of a calm center called the eye. The strongest winds are concentrated in a band immediately surrounding the eye called the eyewall.

Saffir-Simpson hurricane scale http://hosted.ap.org/specials/interactives/_national/hurricanes/index_categories.html

Satellite image of Hurricane Katrina

Storm Surge Occurs when hurricane-force winds drive a mound of ocean water toward coastal areas. Forms on the right side of a storm in the northern hemisphere, relative to its eye. When combined with high tide it can cause enormous damage.