Severe weather encompasses meteorological events that pose significant threats to life, property, and the environment. Thunderstorms, tornadoes, hurricanes, and hail are classified as severe weather phenomena. The National Weather Service defines weather conditions by thresholds: wind gusts of 93 km/h (58 mph) or higher, hail of 1 inch or larger in diameter, and frequent cloud-to-ground lightning. Heavy rainfall of 1 inch or more in a short period, flash flooding, and winter storms producing heavy snowfall, freezing rain, or sleet are considered severe weather events.
Historical severe weather events have caused destruction and loss of life. The 1900 Galveston Hurricane remains the deadliest U.S. natural disaster, killing 8,000-12,000 people with 233 km/h (145 mph) winds and a 15-foot storm surge. The 1931 China Flood resulted in an estimated 3 million deaths and displaced 15 million people. Heat waves have had severe impacts, with the 2003 European heat wave claiming an estimated 70,000 lives. The April 2011 Super Outbreak of Tornadoes produced 362 tornadoes across 21 states, leading to 348 deaths and $11 billion in damages.
Common severe weather conditions include thunderstorms, tornadoes, hurricanes, blizzards, ice storms, floods, windstorms, heatwaves, droughts, fog, dust storms, hail, and lightning. Each of these conditions presents unique hazards and requires specific safety measures.
Severe weather is caused by complex atmospheric and terrestrial interactions. Ocean and air warming create instability, fostering convection currents that generate winds, thunderstorms, and tornadoes. Low-pressure areas form, pulling in surrounding air. Rising greenhouse gas levels intensify weather events by trapping more atmospheric heat and increasing evaporation. Increased atmospheric moisture content fuels heavier precipitation events. The combination of increased moisture, lift, instability, and changes in temperature, wind, and air pressure creates intense severe weather occurrences.
What is the definition of severe weather?
Severe weather encompasses meteorological events. Classified phenomena include thunderstorms, tornadoes, hurricanes, and hail. These events pose threats to life, property, and the environment. Severe weather differs from regular weather in its potential for harm. The National Weather Service defines and tracks severe weather occurrences.
Severe weather conditions are characterized by extreme atmospheric circumstances exceeding thresholds. Wind gusts of 93 km/h (58 mph) or higher, hail of 25 mm (1 inch) or larger in diameter, and frequent cloud-to-ground lightning characterize severe weather events. Heavy rainfall of 25 mm (1 inch) or more in a period, flash flooding, and winter storms producing heavy snowfall, freezing rain, or sleet are severe weather phenomena.
Severe weather terminology is crucial for effective communication and warning systems. Severe weather watches are issued when conditions are favorable for severe weather development. Severe weather warnings indicate imminent or occurring severe weather, signaling the need for immediate shelter. The National Weather Service uses a tiered system to categorize severe weather risk, ranging from slight to high risk levels.
Severe weather damage results in losses exceeding $10 billion in the United States. Severe weather impacts include property destruction, agricultural losses, environmental degradation, power outages, transportation disruptions, and communication breakdowns. Severe weather safety measures involve monitoring forecasts, seeking shelter, avoiding travel, and staying away from windows during events. Emergency management officials and meteorologists use tools and technologies to monitor and predict severe weather events.
What is the difference between severe weather and regular weather?
Severe weather produces conditions like tornadoes, hail, and winds exceeding 97 km/h (60 mph). Severe thunderstorms cause significant damage and disruption. Regular weather refers to conditions like sunshine and clouds. Regular weather poses no threats. Severe weather risks lives and property. Regular weather impacts are minimal. Meteorologists define severe weather by thresholds.
Severe weather has a limited duration, lasting 24 hours or less, while regular weather patterns persist for days. The National Weather Service issues severe weather watches and warnings, whereas forecasters provide weather forecasts for conditions. Severe events like tornadoes, blizzards, and hurricanes have potential for widespread destruction. Weather is dangerous and does not necessitate protective measures like seeking shelter or evacuating.
Wind speed thresholds for severe weather exceed 93 km/h (58 mph) sustained or 113 km/h (70 mph) gusts. Hail size thresholds for weather are 25 mm (1 inch) or larger in diameter. Tornadoes, rotating columns of air touching the ground, are indicators of severe weather conditions. Severe weather causes significant social disruption, including evacuations, closure of schools and businesses, and disruption of transportation systems.
Severe weather poses a threat to human life and property. Flying debris in severe weather causes injury or death. Flooding from weather increases risk of drowning. Downed power lines in weather pose electrocution risk. Lightning kills an average of 70 people each year in the United States. Tornadoes kill an average of 70 people per year in the United States.
Meteorologists find severe weather challenging to forecast. Atmospheric complexity makes severe weather prediction difficult. The National Weather Service classifies severe weather into categories, including severe thunderstorms, hurricanes, and floods. Weather services issue warnings for severe conditions, such as tornado warnings, severe thunderstorm warnings, and flash flood warnings.
What are the most severe weather events in history?
Heat waves in history occurred in India in 1896, in North America in 1936, and in Europe in 2003. The 1896 Indian heat wave killed over 438 people with temperatures reaching 49°C (120°F). The 1936 North American heat wave killed over 5,000 people during a drought. The 2003 European heat wave killed an estimated 70,000 people.
The most severe weather events in history are listed in the table below.
| Event Name | Year | Description | Deaths | Economic Impact | Location |
| September Gale | 1815 | A hurricane with winds up to 160 km/h (99 mph) and a 3-meter storm surge | 38 | $1 million (approximately $17 million in 2020 USD) | Northeastern United States, specifically New York and New Jersey |
| Year Without a Summer | 1816 | Global cooling event caused by Mount Tambora eruption, leading to temperatures 3°C (37.4°F) below average, crop failures, and famine | 71,000 | $100 million (approximately $1.5 billion in 2020 USD) | Worldwide, especially Western Europe and North America |
| Galveston Hurricane | 1900 | Category 4 hurricane with 233 km/h (145 mph) winds and a 4.6-meter storm surge | 8,000-12,000 | $20-30 million (approximately $600-900 million in 2020 USD) | Galveston, Texas, USA |
| Hurricane Wilma | 2005 | Category 5 hurricane with 257 km/h (160 mph) winds and a 6.7-meter storm surge | 35 | $29 billion | Florida, USA, specifically Miami-Dade and Broward counties |
| Hurricane Sandy | 2012 | Category 2 hurricane with 129 km/h (80 mph) winds and a 4.2-meter storm surge | 147 | $70 billion | New York and New Jersey, USA, specifically the New York City metropolitan area |
| Hurricane Georges | 1998 | Category 4 hurricane with 249 km/h (155 mph) winds and a 4.3-meter storm surge | 604 | $5.9 billion | Florida, USA, specifically the Florida Keys and Miami-Dade County |
| 1936 U.S. Heat Wave | 1936 | Heat wave with temperatures reaching 49°C (120°F) across 15 states, lasting 12 days | 5,000 | $1 billion (approximately $18 billion in 2020 USD) | United States, specifically the Great Plains and Midwest regions |
| Australia Heat Waves | 1938-39 and 1896 | Heat waves with temperatures up to 52°C (126°F), lasting 10-14 days | 400 each | $10 million (approximately $150 million in 2020 USD) | Australia, specifically New South Wales and Victoria |
| 2006 North American Heat Wave | 2006 | Heat wave with temperatures up to 46°C (115°F), lasting 10 days | 600 | $1 billion | Western United States and Canada, specifically California, Arizona, and British Columbia |
| 1931 China Flood | 1931 | Flood when the Yangtze River burst its banks, affecting 15 provinces and 51 million people | 3 million | $1 billion (approximately $18 billion in 2020 USD) | China, specifically the Yangtze River Basin |
| 2018 California Fire Tornado | 2018 | Destructive fire tornado formed during the Carr Fire, with winds up to 240 km/h (149 mph) and a 1-kilometer diameter | 8 | $1.6 billion | California, USA, specifically Shasta and Trinity counties |
| 1974 F5 Tornadoes Outbreak | 1974 | Spawned 148 tornadoes across 13 states, including 30 F5 tornadoes, with winds up to 320 km/h (199 mph) | 330 | $3 billion | United States, specifically the Midwest and Southeast regions |
| Camp Fire | 2018 | fire spread over 153,000 acres, destroying 18,804 structures and forcing 52,000 evacuations | 85 | $16.5 billion | California, USA, specifically Butte County |
| April 2011 Super Tornado Outbreak | 2011 | produced 362 tornadoes across 21 states over four days, including 216 EF3-EF5 tornadoes, with winds up to 320 km/h (199 mph) | 348 | $11 billion | United States, specifically the Southeast and Midwest regions |
What are the severe weather conditions?
Severe weather conditions can pose significant risks to human life and property. Some of the most common severe weather conditions include the following.
- Thunderstorms
- Tornadoes
- Hurricanes
- Blizzards
- Ice Storms
- Floods
- Winter Storms
- Heatwaves
- Droughts
- Extreme Fog
- Dust Storms
- Hail
- Lightning
1. Thunderstorms
Thunderstorms are weather events characterized by thunder, lightning, rain, and winds. Thunderstorms form when warm moist air rises into cooler air, creating towering cumulonimbus clouds reaching heights over 10,000 meters. Cumulonimbus clouds generate electrical charges through collisions of water droplets and ice crystals, resulting in massive electrostatic discharges known as lightning. Thunder is the sound produced by lightning heating the surrounding air, creating a shockwave that travels.
Thunderstorms vary in intensity from weak non-severe storms to severe storms producing hail, winds, and tornadoes. Severe thunderstorms produce hail at least 25 mm (1 inch) in diameter and wind gusts of 93 km/h (58 mph) or higher. Thunderstorms release energy from water vapor condensation, with a lightning bolt containing up to 1 gigajoule of electrical energy. Heavy rainfall from thunderstorms occurs at rates up to 100 mm/h (4 in/h), leading to flash flooding.
Thunderstorms occur in tropical and subtropical regions, with the Great Plains and Southeast United States experiencing the highest frequency. Thunderstorms last around 30 minutes and cause $10,000 in damages. Thunderstorms play a role in Earth’s climate system by distributing heat and moisture around the globe and helping regulate Earth’s temperature.
Weather forecasters issue thunderstorm warnings when storms are imminent or occurring, and thunderstorm watches when conditions favor storm development. Safety experts recommend monitoring forecasts during thunderstorm threats and seeking shelter in buildings. Lightning strikes cause an average of 47 deaths per year in the United States, posing dangers to life and property. Winds from thunderstorms knock down trees and power lines, while tornadoes spawned by thunderstorms cause devastation.
2. Tornadoes
Tornadoes are rotating columns of air extending from thunderstorms to the ground. Tornadoes form from thunderstorms in warm, moist, unstable air conditions. Supercell thunderstorms produce destructive tornadoes. Mesocyclones draw air and tighten into rotating columns. Rotating columns extend from thunderstorm clouds to the ground as tornadoes.
Tornadoes rotate at speeds up to 483 km/h (300 mph). Tornadoes rotate counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Tornado paths average 10-15 kilometers (6.2-9.3 miles) in length and range from 10 meters (0.01 mile) to 1.6 kilometers wide (1 miles). Tornado diameter measures 100-200 meters (0.06-01 miles). Tornadoes last 10-15 minutes. Tornadoes lift cars, demolish houses, and strip trees.
The Enhanced Fujita Scale classifies tornado intensity. EF0 tornadoes have wind speeds of 105-137 km/h (65-85 mph), while EF5 tornadoes exceed 322 kmh (200 mph). EF3 to EF5 tornadoes cause damage to structures and vegetation.
Tornado Alley includes Texas, Oklahoma, Kansas, Missouri, and Nebraska. The United States experiences 1,200 tornadoes annually. Late afternoon and evening see most tornado activity. Spring is peak tornado season in Tornado Alley. Tornadoes occur year-round when conditions are right.
The National Weather Service issues tornado watches and warnings. Tornado warnings indicate confirmed or imminent tornadoes. Weather agencies issue tornado alerts through emergency systems. Tornado shelters provide safe refuge during storms. Basements and interior rooms provide tornado shelter. Tornado shelters withstand winds and flying debris. Tornado safety involves seeking shelter.
3. Hurricanes
Hurricanes are intense tropical cyclones with winds of at least 119 km/h (74 mph). Hurricanes form over ocean waters when sea surface temperatures reach at least 26.5°C (80°F). Hurricanes have a well-defined circulation consisting of thunderstorms and strong winds. The eye of the hurricane is a cloud-free area at the center with low pressure and winds.
Hurricane winds reach speeds of up to 240 km/h (150 mph). Hurricane pressure at the center drops as low as 900 mbar (26.58 inHg). Hurricanes are classified into five levels based on wind speed, pressure, and damage. Hurricane tracking is unpredictable but they move westward in the tropics and curve poleward in mid-latitudes.
Hurricane landfall brings damage and flooding to coastal areas. Hurricane damage includes shattering windows, ripping off roofs, and leveling homes and buildings. Hurricane impact extends to severe economic losses, injuries, fatalities, and lasting mental health effects on affected communities. Hurricane preparedness involves monitoring forecasts, creating emergency kits, evacuating if ordered, and securing outdoor items. The Atlantic hurricane season runs from June 1 to November 30, though hurricanes occur year-round in ocean basins.
4. Blizzards
Blizzards are winter storms characterized by heavy snow, strong winds, and low visibility. Blizzard conditions include winds over 56.3 km/h (35 mph), heavy snowfall, and visibility under 0.4 km (1/4 mile) for at least 3 hours. Blizzard storms bring snowfall rates of 51-102 mm (2-4 inches) per hour and wind gusts over 97-113 km/h (60-70 mph). Blizzard weather creates whiteout conditions and dangerous travel situations.
Characteristics of blizzards include weather conditions. Blizzard snowfall accumulates at rates of 25.4-51 mm (1-2 inches) per hour or more. Blizzard winds sustain speeds of 56-80 km/h (35-50 mph) with gusts up to 97-129 km/h (60-80 mph). Blizzard temperatures drop below -12°C (10°F), increasing the risk of hypothermia and frostbite.
Visibility and duration are critical factors in defining blizzards. Blizzard visibility reduces to less than 0.4 km (1/4 mile) due to blowing snow. Blizzard duration ranges from several hours to several days. Blizzard warnings indicate imminent or occurring hazardous conditions lasting for at least 3 hours.
Safety concerns during blizzards are present. Blizzard conditions lead to impassable roads from drifting snow. Blizzard impacts include power outages and disruptions to life. Blizzard safety requires staying indoors and avoiding travel. Blizzard preparedness involves having emergency supplies and winter vehicle readiness.
5. Ice Storms
Ice storms are winter weather events characterized by freezing rain that coats surfaces with ice. Freezing rain falls and accumulates a layer of ice, requiring 1/4 inch or more of ice accumulation. Ice storms derive their severity from long precipitation duration, allowing heavy ice buildup. High winds accompany ice storms due to low pressure systems.
Ice storms damage infrastructure by downing trees, utility poles, and communication towers from ice weight. Power outages lasting days are common during ice storms. Hazardous driving and walking conditions are created by ice accumulation. Fire and carbon monoxide poisoning risks increase from improper generator and heater use during outages. Buildings, vehicles, and infrastructure suffer damage from ice accumulation. Transportation, communications, and economic activity are disrupted by ice storms.
Ice storms impact life. Traffic speeds are reduced by 50% during ice storms, according to the Federal Highway Administration. Annual economic losses of up to $10 billion are caused by ice storms, as found by a National Oceanic and Atmospheric Administration study. Health is affected through hypothermia and frostbite risks. Hypothermia occurs within 30 minutes below 0°C (32°F), as stated by the Centers for Disease Control and Prevention.
Ice storm safety requires staying indoors, avoiding travel, keeping phone lines open, maintaining emergency kits, and awareness of health risks. Government declarations, evacuations, and shelter openings are activated during ice storm emergencies. Power sources, food and water stocks, and first aid preparedness are essential for ice storm emergencies.
Ice storm weather involves freezing rain, freezing drizzle, and sleet. Precipitation rates vary: freezing rain (0.01-0.1 inches/hour), freezing drizzle (0.001-0.01 inches/hour), and sleet (0.1-1.0 inches/hour). Ice storm severity classifications include light (0.01-0.1 inches), moderate (0.1-0.5 inches), heavy (0.5-1.0 inches), and extreme (over 1.0 inches). The National Weather Service issues ice storm warnings for ice accumulation, life disruptions, and duration.
Ice storms impact power infrastructure through outages and grid disruptions. Ice accumulation adds 500 pounds per power line span, according to the National Weather Service. Tree damage from ice storms reaches up to $1 billion, as found by an Arbor Day Foundation study. Building damage costs from ice storms are $10,000, according to the Insurance Information Institute.
6. Floods
Floods are overflows of water onto dry land. Heavy rainfall, snowmelt, dam failures, and storm surges cause floods. Floods occur or develop, submerging entire areas and causing damage.
Flash floods happen within minutes or hours. Intense rainfall or dam failures trigger flash floods. Flash floods are among the dangerous floods. Flash floods occur with little warning, posing severe risks to life and property.
River floods develop when rivers overflow their banks. Heavy rainfall and snowmelt lead to river flooding. River floods are common in spring due to snowmelt and ice jams. Coastal areas face storm surge flooding from tropical storms and hurricanes.
Floods damage buildings, roads, bridges, and infrastructure. Flood damage costs $40 billion annually in the United States. Floods account for 40% of all natural disasters. Floods displace people and result in loss of life. The International Disaster Database reports floods cause 6,800 deaths annually.
Flood warning systems save lives and reduce damage. The National Weather Service issues over 4,000 flood warnings annually. Flood watches warn of flooding. Flood warnings indicate imminent or occurring flooding. Flood alerts send emergency messages to mobile devices and activate sirens.
Flood control measures mitigate flood impact. Levees and floodwalls contain water and protect against flooding. The United States has over 100,000 miles of levees. Dams and reservoirs control floods. Wetland restoration absorbs water and reduces flood risk.
Flood risk affects over 2.3 billion people worldwide. One in 5 people live in flood-prone areas. Proximity to water bodies, topography, soil type, and land use contribute to flood risk. Low-lying coastal regions and river deltas are susceptible to flooding.
7. Windstorms
Windstorms are weather events characterized by sustained winds exceeding 55 km/h (34 mph). Wind gusts during windstorms reach higher velocities, causing damage and disruption to the environment, infrastructure, and human populations.
Windstorms impact vast geographical areas, affecting regions with tropical cyclones, such as hurricanes or typhoons. These weather events occur, with annual global losses estimated between $10 billion to $50 billion. Windstorms cause power outages due to downed power lines and damaged transmission infrastructure. Property destruction from windstorms includes roof loss, structural collapse, and debris dispersal. Environmental effects of windstorms involve damage to trees and exacerbation of existing weather conditions like heavy rainfall or drought. Economic consequences are significant, with windstorms responsible for an average of $10 billion in damages each year in the United States.
Windstorm conditions involve a combination of atmospheric and meteorological factors. Wind speeds exceed 55 km/h (34 mph), with classifications varying based on intensity. Weather phenomena include low-pressure systems, tropical cyclones, winter storms, and derechos. Windstorms form and develop in unstable atmospheric conditions, associated with wind shear, which leads to the formation of tornadoes or derechos. Low-pressure systems, cold fronts, warm fronts, and thunderstorms contribute to the development of windstorms, creating weather patterns.
8. Heatwaves
Heatwaves are prolonged periods of hot weather. Heatwaves last for days or weeks, with temperatures above average highs for a region. Heatwave conditions are defined as 3-5 days exceeding the 90th percentile of average high temperatures. Heatwaves are characterized by high air temperature, high humidity, and intense sunshine.
Heatwaves effects are far-reaching. Heatwaves cause heat-related illnesses like heat exhaustion and heat stroke. Heatwaves are responsible for an estimated 12,000 to 15,000 deaths per year in the United States. Heatwave impacts extend beyond health. Heatwaves alter ecosystems, increase wildfire risk, and lead to crop damage and reduced yields. Heatwaves strain power grids from increased air conditioning use. Heatwaves cause infrastructure damage and disrupt transportation systems.
Heatwave duration varies. Heatwave duration includes 1-3 days, 4-7 days, or more than 8 days. Some heatwaves have persisted for longer periods in some cases. The 2003 heatwave lasted for 10 days. The 2019 Indian heatwave persisted for over 30 days.
Heatwave stress is exacerbated by factors. High humidity levels increase the perceived temperature during heatwaves. Urban heat island effect intensifies heatwave conditions in cities. Heatwave stress is measured using indices like the Heat Index, which considers temperature and humidity to estimate perceived temperature.
9. Droughts
A drought is a period of below-average precipitation resulting in water shortage. Droughts impact water availability by reducing it 20-50% according to the IPCC. Droughts impact agriculture by reducing crop yields 10-30% according to the FAO. Droughts impact economies by causing up to $10 billion in annual losses according to NOAA.
Drought causes include climate change altering precipitation patterns. Drought causes include climate variability like El Niño and La Niña events. Other causes include human activities like groundwater over extraction and deforestation. Droughts last from months to 10 years according to NASA. Drought frequency is expected to increase 20-50% by 2050 in some regions due to climate change according to the IPCC. Drought severity is measured using indices like the Palmer Drought Severity Index and Standardized Precipitation Index. Droughts affect up to 50% of a country’s land area according to the FAO.
Drought effects include degradation through soil erosion and increased wildfire risk. Other effects include health issues like water-borne diseases and heat stress. Droughts also impact migration and unrest. Drought consequences include food insecurity from crop failures and reduced food availability. Droughts consequences include water scarcity affecting consumption and industry. Other consequences include economic instability impacting growth and poverty levels.
Drought strategies include water conservation, irrigation and drought crops. Other strategies include preparedness and early warning systems to reduce impacts. Drought disasters include wildfires, dust storms and water scarcity emergencies. Drought conditions are characterized by low rainfall, high temperatures and water scarcity. Drought conditions impact ecosystems, agriculture, and human societies.
10. Extreme fog
Extreme fog is a fog that reduces visibility to less than 0.4 km (1/4 mile). Visibility in fog conditions drops to less than 10 feet, making driving hazardous. Extreme fog lasts for hours or days, with density exceeding 10 times that of normal fog.
Geographic factors influence extreme fog formation. Valleys, coastal areas, and low-lying terrain are prone to fog conditions. Temperature inversions trap cool air under warm air, contributing to extreme fog development. Humidity above 90% and moisture-rich air from water bodies play a role in extreme fog formation.
Extreme fog creates transportation hazards. Drivers must slow down, use low-beam headlights, and maintain following distance in fog. Road closures, flight delays, and cancellations are common during extreme fog events. The Federal Highway Administration reports fog contributes to 31,000 crashes annually in the United States.
Extreme fog accidents result in consequences. Over 500 fatalities and 16,000 injuries occur annually due to fog in the United States. Multi-vehicle collisions are a risk in extreme fog conditions. The 1997 New Jersey Turnpike extreme fog accident involved 80 vehicles and injured 42 people. The 2011 I-10 highway extreme fog accident in Texas involved 140 vehicles and killed 2 people.
Economic consequences of extreme fog are substantial. Transportation disruptions lead to lost productivity and increased costs. The Federal Aviation Administration reports fog contributed to over 1,000 aviation accidents from 2001 to 2010. Fog detection systems and variable speed limits are implemented to mitigate the impacts of extreme fog conditions.
11. Dust Storms
Dust storms are weather events characterized by strong winds carrying amounts of dust and debris. Dust storm particles range from 0.01 to 0.05 millimeters in diameter. Dust storm conditions include strong winds exceeding 50 km/h (31 mph), low humidity below 20%, and temperatures above 30°C (86°F). Dust storm weather is associated with thunderstorms, frontal systems, low-pressure systems, and windstorms.
Dust storm hazards pose risks to transportation, aviation, and daily life. Dust storm impacts include reduced visibility leading to accidents, power outages, equipment damage, and disruption of activities. Dust storms alter ecosystems, damage crops, and affect water quality. Dust storms exacerbate respiratory conditions, increase the risk of cardiovascular disease, and cause eye and skin irritation. Dust storms impact agriculture by reducing crop yields up to 50%, damaging soil, and affecting livestock health.
Dust storm duration varies from minutes to days. Dust storms are natural phenomena occurring in arid and semi-arid regions worldwide. They occur in the American Southwest, Middle East, Australia, North Africa, Indian subcontinent, and African Sahel region. Dust storms occur at any time of year, with increased frequency during periods of drought, strong winds, and low humidity.
12. Hail
Hail is solid precipitation in the form of ice pellets. Hailstones form when updrafts in thunderstorms carry water droplets high into freezing temperatures. Water droplets accumulate layers of ice as they move up and down through the storm, creating alternating clear and cloudy layers within the hailstone.
Hail size ranges from pea-sized (6-8 mm) to baseball-sized (7-10 cm) or larger. The highest recorded hailstone reached 20 cm (7.9 in) in diameter and weighed 1.9 kg (4.2 lbs). Hail forms in spherical, ellipsoidal, and irregular shapes. Hailstones grow through a process called accretion, where water droplets freeze upon contact with the ice core.
Severe thunderstorms produce hail 1 inch or larger in diameter. These storms generate winds of 93 km/h (58 mph) or greater and spawn tornadoes in some instances. Hail forecast models use atmospheric conditions and radar data to predict hail storms. Doppler radar systems detect hail presence, estimate size, and predict trajectory.
Hail frequency varies by region, with the Great Plains and Midwest experiencing frequent hail storms. Meteorologists refer to these areas as “Hail Alley.” Hail storms occur in mid-latitudes due to favorable moisture and updraft conditions. Some regions experience several hail storms per year.
Hail causes damage to crops, vehicles, and infrastructure. Hail damage costs in the United States exceed $1 billion. Large hailstones dent vehicles, damage roofs and windows, and pose safety risks to exposed people. Hail injuries range from cuts to serious head trauma, with an average of 24 fatalities per year in the United States.
Hail accumulation disrupts transportation and affects life in impacted areas. Some hail storms produce drifts up to 1 meter (3.3 feet) deep. Hail intensity is measured by hailstone diameter and the amount of damage caused. Hailstones and winds result in damage.
Meteorologists classify hail size by comparing it to objects like dimes, golf balls, or baseballs. Hail precipitation is measured to determine storm intensity. Impact assessments evaluate the extent of damage caused by hail storms to property, agriculture, and infrastructure.
13. Lightning
Lightning is a massive electrostatic discharge occurring between clouds and ground or within clouds during a thunderstorm. Lightning strikes reach temperatures up to 30,000 Kelvin (29,727°C, 53,540°F), hotter than the sun’s surface. Lightning flashes extend up to 10 kilometers (6.2 miles) in length and travel at speeds up to 270,000 kilometers per hour. Lightning forms when air rises, creating cumulonimbus clouds reaching 10,000 meters. Water droplets and ice crystals in clouds transfer electrons, generating charge separation. Cloud tops become positively charged, while lower parts and ground become negatively charged. Charge separation creates an electric field, breaking down air molecules. Lightning flashes follow the conductive pathway created by the electric field breakdown.
Lightning flashes occur in forms, including intracloud, cloud-to-cloud, cloud-to-ground, and ground-to-cloud. Lightning strikes contain enough energy to power a 100-watt light bulb for 200,000 hours. Lightning damage causes damage to buildings and bridges, triggers power outages, and ignites forest fires and wildfires. Lightning effects include electromagnetic pulses disrupting electronic devices, shockwaves damaging buildings, and thermal effects leading to fires and explosions. Lightning disrupts communication and transportation systems.
Lightning safety is crucial during storms, with 1 in 700,000 odds of being struck in a given year. People have 1 in 8,000 odds of being struck by lightning in their lifetime. Experts recommend monitoring weather forecasts and seeking shelter during lightning storms. Lightning safety guidelines include seeking shelter in buildings or vehicles, avoiding open areas and tall objects, and staying away from windows and doors. The National Weather Service recommends the “30/30 Rule” for lightning safety. People must seek shelter if thunder follows lightning within 30 seconds. The United States experiences 47 lightning-related deaths and 400 injuries annually. Lightning strikes cause an estimated 24,000 deaths and 240,000 injuries each year.
What causes severe weather?
Severe weather results from atmospheric and terrestrial conditions. Ocean and air warming create instability. Environments foster convection currents. Winds, thunderstorms, and tornadoes are generated. Low-pressure areas form, pulling in surrounding air. Florida’s location near warm Atlantic and Gulf waters makes it prone to hurricanes and tropical cyclones.
Several atmospheric processes lead to severe weather development. Thermals generate when surface heating is uneven, creating areas of warm air that rise and form towering cumulus clouds. Temperature changes cause cold fronts to move into warm air areas, forcing rapid air rise and cloud formation. Wind changes contribute to weather by creating wind shear that forces air to rise, cool, and condense. Air pressure changes in low-pressure systems cause air to rise, cool, and condense, leading to cloud and precipitation formation.
Rising greenhouse gas levels intensify severe weather events. Increased greenhouse gasses trap more atmospheric heat, leading to enhanced evaporation from water bodies. Intensified evaporation results in higher atmospheric moisture content, which fuels heavier precipitation events. The combination of increased moisture, lift, instability, and changes in temperature, wind, and air pressure creates intense severe weather occurrences.
How does air pressure affect the formation of severe weather?
Low-pressure systems create rising air, forming clouds and storms. Warmer surface air in low-pressure areas tends to rise. Moisture condenses as air rises, developing clouds. High humidity increases cloud formation. Air mass mixing from pressure system movements pushes warm and cold fronts together. Blending of air masses generates strong winds, heavy precipitation, and tornadoes.
Atmospheric instability plays a role in severe weather formation. Temperature fluctuates in unstable conditions, with differences of up to 10°C (18°F) occurring within minutes. Instability increases as air pressure decreases, with a 5 mb drop resulting in a 10% increase in instability. Convection deepens as instability grows, allowing for the formation of towering cumulonimbus clouds associated with severe thunderstorms.
Severe weather develops as a result of these atmospheric conditions. Thunderstorms form when instability and rising air create updrafts and downdrafts. Hurricanes develop in tropical regions when air pressure is low enough to sustain a rotating system of clouds and precipitation. Hurricane centers have air pressures as low as 950 mb. Tornadoes form under specific atmospheric conditions, including warm, moist surface air and wind shear. Tornado centers experience low pressures, reaching as low as 900 mb.
Scale weather changes occur due to air pressure variations. Air masses with different temperatures and humidity levels collide, forming fronts and producing severe weather. Weather patterns change as high and low-pressure systems move across regions. Wind patterns shift in response to air pressure changes, impacting the movement of weather systems. A 1 millibar drop in air pressure increases the likelihood of severe thunderstorms by 1%.