Tornadoes, powerful rotating columns of air, can strike with devastating force. While many people associate tornadoes with daytime severe weather, they can also occur after the sun sets, posing unique challenges and increased risks. These nighttime events often catch communities off guard. Understanding the characteristics of tornadoes that form at night is important for safety and preparedness.
The Frequency of Nighttime Tornadoes
Tornadoes occur at any time of day or night. Research indicates that approximately 27.3% of tornadoes happen during nighttime hours, defined as between sunset and sunrise. Despite this lower frequency compared to daytime events, these nocturnal tornadoes contribute disproportionately to fatalities. They are responsible for nearly 40% of all tornado-related deaths and 42.1% of killer tornadoes.
Nighttime tornadoes are more common in specific regions, particularly across the Southeast United States. States within the lower Arkansas, lower and mid-Mississippi, and Tennessee River valleys experience a higher percentage of these events. For instance, Tennessee sees nearly 46% of its tornadoes at night, with Arkansas and Kentucky also experiencing over 40% of their tornadoes after dark. While spring is generally the peak tornado season, nocturnal tornadoes can also exhibit a notable peak in months like November, aligning with earlier sunsets.
How Nighttime Tornadoes Form
The atmospheric conditions that foster nighttime tornadoes differ from those that drive their daytime counterparts. During the day, solar heating warms the ground, creating instability as warm air rises. After sunset, this surface heating diminishes, but other mechanisms can sustain severe weather. A primary factor contributing to nocturnal tornado formation is the presence of a low-level jet (LLJ).
The low-level jet is a concentrated band of fast-moving air found about 1 to 2 kilometers above the surface. This jet often strengthens at night, especially over regions like the Great Plains and extending into the Southeast. The LLJ efficiently transports warm, moist air northward from the Gulf of Mexico. This continuous influx of moisture and energy provides the necessary fuel for thunderstorms to persist and intensify, even as surface temperatures cool.
Beyond providing moisture, the low-level jet also creates significant wind shear. Wind shear refers to changes in wind speed or direction with height in the atmosphere. This differential in wind provides the rotational component essential for supercell thunderstorms, which are often tornado-producing. Even with reduced surface instability, the strong wind shear and elevated moisture transport associated with the LLJ can lead to the development of storms capable of producing tornadoes throughout the night.
Increased Dangers After Dark
Nighttime tornadoes are generally considered more dangerous than those occurring during daylight hours. This heightened risk stems from several factors that reduce public awareness and response capabilities. A primary concern is severely limited visibility; the darkness makes it nearly impossible to visually confirm a tornado. This lack of visual cues hinders observation and reporting, reducing warning lead time.
Most people are asleep during the night. Individuals are less likely to hear or respond to warnings, increasing their vulnerability. Studies have shown that tornadoes occurring at night are more than twice as likely to result in fatalities compared to daytime tornadoes. For example, one study found that approximately one in every 20 nighttime tornadoes turned deadly, compared to about one in every 50 daytime tornadoes.
Challenges in warning dissemination further compound the danger. Outdoor tornado sirens, while valuable, may not be heard indoors, especially when people are sleeping. The flow of “ground truth” information from the public to meteorologists slows significantly at night, potentially leaving forecasters with less real-time data. People may also be in more vulnerable structures, such as mobile homes, which offer less protection against tornadic winds.
Nighttime Tornado Safety
Preparing for tornadoes that may strike at night requires proactive planning and multiple methods for receiving warnings. Relying solely on outdoor sirens is not advisable, as they may not be audible indoors, particularly while asleep. Instead, a NOAA Weather Radio is a valuable device, designed to emit a loud alarm that can wake residents and provide continuous weather information, even during power outages. Wireless Emergency Alerts (WEA) on cell phones also provide warnings and can override silent modes.
Developing a pre-determined shelter plan is also important, and every household member should know the designated safe location. The safest place during a tornado is an underground area, such as a basement or storm cellar. If an underground shelter is not available, the lowest floor of a building, in a small interior room or hallway away from windows, provides the best protection. The goal is to place as many walls as possible between yourself and the outside.
When a tornado warning is issued at night, especially if you are asleep, immediately seek your designated safe place. Ensure cell phones are charged and have alert volumes turned up before going to bed during severe weather threats. Do not attempt to open or close windows, as this does not protect the structure. Once in shelter, crouch low, cover your head with your arms, or use protective coverings like a mattress, thick blankets, or a helmet to shield yourself from flying debris. If residing in a mobile home, evacuate to a sturdy building or a designated community shelter, or lie flat in a ditch if no other options are available.