ATM 103: Module 7.2- Storm Evolution and Impacts Web Page

Tropical Cyclone Formation: Key Ingredients

-Warm ocean waters ≥ 27°C (80°F) supply heat & moisture

-High humidity in lower-mid troposphere supports convection

-Coriolis force enables rotation (forms 5°-20° latitude)

-Low vertical wind shear maintains storm structure

-Pre-existing disturbance (e.g., tropical wave) initiates uplift & rotation

-All factors must align for formation & intensification

2019 North Atlantic Hurricane Season

-Duration: May-November

-Total: 18 named storms, 14 tropical storms, 5 hurricanes, 3 major hurricanes (Category 3+)

-Most intense: Dorian (Cat 5), Lorenzo (Cat 5), Humberto (Cat 3)

-Notable: Andrea formed before official start

-Damage: ≈ $11.4B USD

Hurricane Formation

-Low-pressure center forms as warming lowers surface pressure

-Outward flow aloft promotes vertical growth

-Winds <39 mph: Tropical depression; ≥39 mph: Tropical storm (named)

Hurricane Decay

-Energy source lost (latent heat cutoff)

-Occurs over cooler water or land

-Unfavorable upper-level winds disrupt storm

-Without warm, moist air, circulation weakens rapidly

Tropical Disturbance & Depression

-Disturbance: clusters of strong thunderstorms; ~90% dissipate

-Tropical Depression: winds <39 mph; weak low-pressure; rotation develops; receives numerical designation

Tropical Storm & Hurricane

-Tropical Storm: winds 39-73 mph; circulation organizes; named

-Hurricane: winds >73 mph; may form distinct eye; winds decrease outward from core

Deadliest & Most Damaging U.S. Hurricanes

-1900 Galveston Hurricane: ~8,000 deaths

-Hurricane Katrina (2005): $161B damage, 1,800+ deaths

-Hurricane Maria (2017): ~3,000 deaths, major infrastructure loss

-Hurricane Harvey (2017): $125B damage

-Hurricane Sandy (2012): $70B losses

-Hurricane Ian (2022): 150+ deaths, $110B damage

Saffir-Simpson Scale

-Ranks hurricanes by wind speed, pressure, storm surge

-Category 1: minimal damage (74-95 mph)

-Category 3+: major hurricanes, devastating impacts

-Category 5: catastrophic destruction (≥157 mph)

Hurricane Wind Damage

-High winds cause structural damage & turn objects into missiles

-Can spawn tornadoes, adding localized destruction

Hurricane Storm Surge

-Dome of water pushed ashore by strong surface winds

-Typically 65-80 km wide near landfall

Often more destructive than wind alone

Heavy Rain & Flooding

-Torrential rainfall can cause flash floods, mudflows

-Inland flooding worsened by mountainous terrain

-Rainfall measurement often relies on radar

Mountainous Region Impacts

-Orographic lifting forces moist air upward → enhances precipitation

-Can trigger flash floods & mudslides even as winds weaken

Deadliest Tropical Cyclones

-Extreme winds, storm surge, heavy rainfall; affect coastal & inland areas

-1900 Galveston (TX): ~8,000 deaths

-Bhola Cyclone (Bangladesh, 1970): ~500,000 deaths

-Hurricane Katrina (2005): ~1,800 deaths

-Cyclone Nargis (Myanmar, 2008): ~138,000 deaths

1900 Galveston Hurricane

-Deadliest U.S. hurricane; struck Galveston, Texas

-Dates: Aug 27 - Sep 17, 1900

-Wind: 145 mph; Pressure: 936 mbar; Storm surge: 15 ft

-Death toll: 6,000-12,000 people