Straight line winds are a common, yet often misunderstood, threat associated with severe thunderstorms. These powerful, non-rotating wind events can cause damage rivaling that of a weak tornado, frequently leading to misidentification after a storm passes. The destructive force behind straight line winds originates high within a thunderstorm, making them a serious weather hazard.
Defining Straight Line Winds
Straight line winds are strong bursts of wind that move horizontally along the ground in a single direction or a fan-like pattern. Unlike the swirling vortex of a tornado, the air in these events flows outward from a central point. To be classified as a severe thunderstorm wind event by the National Weather Service, these winds must reach or exceed 58 miles per hour (93 kilometers per hour). The name “straight line” comes from the destruction pattern left behind, where trees and debris are pushed over in the same general direction, lying parallel to one another.
The Meteorological Engine: Downbursts and Downdrafts
The formation of straight line winds begins with a downdraft, which is the sinking air within a thunderstorm. As warm, moist air rises, water vapor condenses into precipitation, cooling the surrounding air significantly as it descends. This cooling is often enhanced by evaporational cooling, where falling rain evaporates, making the air denser and heavier. This dense, cold air accelerates rapidly downward toward the surface, creating a powerful downburst. When this concentrated column of sinking air hits the ground, it spreads horizontally in all directions, creating the damaging straight line wind known as an outflow.
Categorizing Straight Line Wind Events
Straight line wind events are classified based on the scale and size of the downburst they originate from. The two primary classifications are microbursts and macrobursts.
A microburst is a smaller, localized event, defined by a damage path less than 2.5 miles (4 kilometers) in diameter, typically lasting two to five minutes. Macrobursts are larger counterparts, affecting an area greater than 2.5 miles in diameter and lasting longer, generally from five to 30 minutes.
These events are further categorized by the presence of precipitation at the surface. A wet burst is accompanied by heavy rain, while a dry burst occurs when the rain evaporates completely before reaching the ground, often leaving only a visible ring of dust.
Identifying Damage Patterns and Safety Measures
Damage Patterns
Assessing the damage after a severe storm is the most reliable way to distinguish straight line winds from a tornado. National Weather Service survey teams look for evidence of rotation in the debris fields. If trees are blown over in a parallel fashion, or fanned out from a central point, straight line winds are confirmed. In contrast, tornado damage leaves a chaotic, convergent pattern, with debris scattered in multiple directions due to the storm’s rotation. Straight line winds can be destructive, capable of removing roofs and causing significant structural damage.
Safety Measures
When severe thunderstorm warnings are issued, the public should seek immediate shelter, treating the threat seriously. This involves moving to an interior room on the lowest floor of a sturdy building, away from windows. If driving, pull over safely away from trees and power lines, pointing the vehicle into the wind if possible.