Thunderstorms, Hurricanes and Tornadoes

Thunderstorm Facts Right now there are about 2,000 thunderstorms occurring worldwide There are about 45,000 thunderstorms per day!! U.S. gets 100,000 per year!

Severe Thunderstorms Torrential Downpours Strong winds at greater than 58 mph Hail ¾ inch diameter (or larger) Frequent lightning

Severe Thunderstorm Watch Issued by National Weather Service This means that conditions are favorable for a thunderstorm Length is multiple hours.

Severe Thunderstorm Warning Issued By Local Weather Service Means a severe storm has been spotted by doppler radar Usually only an hour

Lightning

Types of Lightning Cloud-to-ground (CG) Intra-cloud (IC) Ribbon lightning Sheet lightning Heat lightning Ball lightning

Cloud-to-ground (CG) Lightning

Properties of a Cloud-to-ground Flash Peak current of 30 to 40 kAmps Temperatures of over 30,000  F Channel diameter of 1 to 2 cm Number of return strokes: 1 to 20 (avg ~4)

Properties of an Intra-cloud Flash Much more frequent than CG flashes Not well known (difficult to study)

Intra-cloud (IC) Lightning

Ribbon Lightning

Sheet Lightning

Heat Lightning

Ball Lightning

Lightning Safety Stay indoors, away from doors and windows. Do not take baths or showers. Do not use phones with cords.

Cars are pretty safe…

Thunder

How Far Away Was That Lightning? When you see the flash, start counting seconds. When you hear the thunder, stop counting. Every 5 seconds is equal to 1 mile.

Tornadoes

F-Scale F0 – Light damage F1 – Moderate damage F2 – Considerable damage F3 – Severe damage F4 – Devastating damage F5 – Incredible damage (Auntie Em!)

Tornadoes in Pennsylvania From there were: – 694 tornadoes in Pennsylvania 22 F4 2 F5 –27 tornadoes in Lancaster County 1 F3 No F4 or F5

Tornado Watch Conditions are favorable for a tornado to form

Tornado Warning A tornado has formed.

Tornado Safety Get indoors and stay there Stay away from doors and windows Get to lowest level possible Don’t get near things that can fall on you

Hurricanes

Formation Regions

Typical Paths

When to expect Hurricane season is June through November Peak is in September

Formation of Hurricanes Tropical Depression Tropical Storm Hurricane

Tropical Depression Cluster of organized thunderstorms Not very compact Sustained winds of at least 30 mph Given a number

Tropical Storm Winds above 39 mph Given a name More organized If hits land, most damage caused is by rainfall.

Hurricane Winds over 74 mph Well defined center of circulation Eye Wall Damage from includes: –Wind –Storm surge –Rain

Pressure in hurricane A way to tell strength of hurricane is by its pressure Measured in millibars (mb) Higher pressures are weak, lower are stronger

Naming System A list of names from A-W (excluding Q) Alternate from male to female 6 lists, repeated in cycle Big name storms replaced

Saffir-Simpson Scale Designed by Herbert Saffir and Bob Simpson Divides hurricanes into categories based on maximum sustained winds.

Category 1 Winds mph Storm Surge- 4-5 ft. Pressure- 986mb Damage- mobile homes, trees, minor costal flooding

Category 2 Winds mph Storm surge- 6-8 ft. Pressure mb Damage- Some roof and window. Damage to piers on coast. Small crafts. vegetation

Category 3 Winds mph Storm surge ft. Pressure mb Damage- structure damage to homes and buildings. Mobile homes destroying. Major coastal flooding with a potential for inland flooding.

Category 4 Winds mph Storm surge ft. Pressure mb Damage- Complete roof damage. Major erosion to beaches. Further inland flooding.

Category 5 Winds- Greater than 156 mph Storm surge- over 19 ft. Pressure- less than 920 mb Damage- Roofs torn off, buildings collapse, major inland flooding.

Hurricane Watch A hurricane poses a possible threat within the next 36 hours

Hurricane Warning Hurricane conditions expected within 24 hours

Most Damaging Storms 1) Katrina (2005)- 75 billion 2)Andrew (1992) 34 billion 3)Charley (2004)- 14 billion 4) Ivan (2004)- 13 billion 5) Wilma (2005)- 12 billion..9) Agnes billion

Hurricane Safety Best plan is to evacuate –Towns have voluntary evacuation during a watch, and some switch to mandatory during a warning. Plan early in advance

IF you choose to stay Board all windows Prepare Flashlights, avoid candles Stay tuned to radio Stay indoors