Severe thunderstorms, which are capable of producing large hailstones- also known as hail, damaging winds, and tornadoes, can pose serious threats to human life, safety, and property in New York City.

Thunderstorms are normally localized events. According to the National Weather Service (NWS), the average thunderstorm in the United States is 15 miles in diameter and lasts an average of 30 minutes. Non-severe thunderstorms have winds of less than 58 mph or less and can produce lightning, rain, and sometimes, small hail.

About 10 percent of thunderstorms in the United States are classified as severe, with winds of at least 58 mph and/or large hail of at least 1 inch in diameter.  Severe thunderstorms happen when warm, moist air collides with colder air. As the warm air rises, the moisture condenses and builds up energy as it forms into a thunderstorm cloud. This pent-up energy releases as a violent thunderstorm.  In severe cases, the storm’s structure creates a vertical updraft that can rotate for hours, leading to hazardous tornadoes, high winds and hailstones.

Hail are falling particles of ice. Hail develops as warm, moist air rises in the upper troposphere and cools below the freezing point. The water vapor then condenses into ice crystals. These ice crystals remain suspended by high-velocity updraft winds, growing larger, and eventually fall to the ground as hail, at speeds that can approach 100 mph or more.

Generally, the size of hailstones are correlated with the severity of the thunderstorm. In New York City, the diameter of hail typically ranges from 0.20 inches to 2.0 inches.

Severe storms with high winds lead to several hazards -- broken tree limbs, downed power lines, and flying debris, which can lead to power outages, transportation disruptions, damage to buildings and vehicles, and personal injury and death.

A tornado is a violently rotating column of air that has winds of 65 mph to over 300 mph. These short-lived storms generally appear as whirling funnel-shaped clouds extending from the base of a thundercloud down to the ground. Tornadoes are initially transparent with extremely strong winds – a danger because they cannot be easily seen. As they pick up debris and dust, and as their water vapor condenses, they acquire a grayish color.

Tornadoes are the most violent atmospheric phenomenon to occur over land, and over a small area, they are the most destructive. In a matter of seconds, tornadoes can uproot trees, demolish buildings, and turn harmless objects into deadly missiles. Predicting them can be difficult, so there may only be a few minutes for officials to warn people to seek shelter and take other precautions for their safety.

A tornado’s path of destruction can be more than 1 mile across and 50 miles long. Each year in the United States, an average of 1,200 tornadoes strike, causing around 60 to 65 fatalities and 1,500 injuries.

Not all high-wind events are associated with severe thunderstorms and tornadoes. Other types of windstorms that affect New York City are the result of complex interactions in the atmosphere. High winds are a byproduct of certain processes in the air -- for example, when atmospheric conditions push air from high to low pressure areas, and when strong cold and warm frontal systems interact.

Two other causes of high winds that are familiar to New Yorkers – hurricanes and nor’easters - are included in the Coastal Storm hazard profile.

Link to: Hazard Profile – Coastal Storms

High winds can create a range of hazards -- downed trees and power lines, flying debris, and building collapses. Any of these hazards may lead to power outages, transportation disruptions, damage to buildings and vehicles, and personal injury and death.

Flying debris is the primary cause of damage during a windstorm. Even if a building remains structurally sound, broken window glass can injure people inside and outside the building and allow the wind to cause extensive damage inside.