1. The Nature of Storms
Topic 7

2. 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.

3. 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.

8. 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).

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

10. 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

11. 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.

12. Orographic Lifting

13. Sea breeze

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

15. Stages of Development
Cumulus stage
Mature stage
Dissipation Stage

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

17. 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.

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

19. 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.

20. 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.

22. Dubai, on the Arabian Peninsula

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

27. Hail Precipitation in the form of balls or lumps of ice.
Hail formation:

30. 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.

31. 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.

32. 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

35. 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

36. Where do they occur?

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

38. 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.

39. 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.

41. Saffir-Simpson hurricane scale

42. Satellite image of Hurricane Katrina

43. 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.