Summertime thunderstorms bring all varieties of severe weather: Flash floods, dangerous lightning, hail, damaging winds and tornadoes. After a severe thunderstorm that has downed trees and powerlines, how can meteorologists tell if the wind damage was due to a strong, localized downdraft (called a downburst) or a tornado? What "forensic tools" does a meteorologist use to determine the culprit and notify the public?

Severe thunderstorm winds come in several flavors, including tornadoes and downbursts. Tornadoes are narrow vortices with low pressure in the center. A spiral flow of air rushes toward the core of the tornado, and this creates damaging winds. Wind speeds are typically 75-150 mph for weak tornadoes, and up to 250 mph in the most damaging tornadoes. Because the tornado moves along the ground (it is carried along by the motion of its parent thunderstorm), tornadoes produce a characteristic narrow swath of damage. From high up, the damage path looks like a straight line etched across the landscape.

Downbursts are an intense type of downdraft in a thunderstorm. The cool blast of rain-chilled air that blows through tries before the rain arrives is the downdraft. A downburst is an especially violent descending bubble of air, usually less than 1 mile in diameter. When this bubble of air hits the surface, it spreads outward in all directions along the ground. This creates a powerful outrush of damaging winds. Imagine aiming a garden hose with a nozzle toward the ground...the starburst blast of water hitting the ground is similar to the downburst. Downbursts produce damaging winds in the range of 70-150 mph, very similar to weaker tornadoes.

Which brings us to the point of the question. In the aftermath of a severe thunderstorm, when you go outside and observe tree limbs and power lines down, and maybe entire trees uprooted, how can you decide if the wind damage was caused by a tornado or downburst? Many people quickly conclude that a tornado must have been the cause. But unless someone observed a funnel cloud, this may not be the case.

A meteorologist will use eyewitness accounts, if available and reliable, to determine if a tornado or rotating cloud was observed. The meteorologist will also review tapes of the weather radar images that show the structure of the thunderstorm. These can give valuable clues as to whether there was rotation present, or a pattern that suggests that straight-line or downburst winds were at work.

But the best way to determine tornado or downburst is do conduct a damage survey from the air. This is most often done using a small single-propeller aircraft or helicopter. The meteorologist can observe the entire damage pattern from an altitude of several hundred feet, and take digital photos. If a tornado was responsible for the damage, then there will be a narrow path of fallen trees through forested regions. Trees will fall inward, in a direction pointing toward the tornado path, because the low pressure of the tornado vortex causes debris to stream inward toward it. For a downburst, the meteorologist looks for a broad, starburst-shaped pattern of damage in the trees. Here, all the trees will be uprooted in a direction that points away from the impact point of the downburst on the surface....like the detonation of an explosive. The damage area will also be circular or elliptical in shape, and will not resemble the narrow path of a moving tornado.

In this manner, the meteorologist applies some "forensic techniques" in the aftermath of a storm to diagnose its windy culprit. Remember...in a tornado, the wind and debris move inward toward the center, and then upward; in a downburst, the flow is reversed...air first moves downward to the surface from aloft, then spreads outwards along the ground.

This week's question comes from Dr. Jeffrey Halverson. Dr. Halverson investigates severe storms at the NASA Goddard Space Flight Center, serves as the Education and Outreach Scientist for NASA's Tropical Rainfall Measurement Mission satellite, and teaches courses on meteorology at the University of Maryland Baltimore County. He holds a PhD in Environmental Sciences and writes a column on interesting weather phenomena in the bi-monthly publication Weatherwise. Dr. Halverson is also an avid amateur astronomer and enjoys hiking throughout the Mid Atlantic to better understand the region's complex geology.