Minnesota DOT Office of Aeronautics
Published in Midwest Flyer – June/July 2017
Every spring brings about changes in the landscape, the weather, and even in people. The warmth of the strong and the increased number of daylight hours are welcomed with open arms. Even the early spring flowers and plants open their buds and spread their tiny petals or leaves like unfolding arms raised toward the sky to say ‘thank you.’
As the beauty of spring reveals itself and the scenery changes daily, it is easy to be drawn to the gentleness of the moments and the subtle sweet scents of the first flowers that bloom. But spring also brings the clashes of strong atmospheric fronts as they move rapidly to occupy the space of an opposing front.
Each day the sun pours more and more energy into the atmosphere, quickly warming the air and the uncovered ground. But in the early part of spring, winter hasn’t yet given up its reign. Often blasts of cold air (a cold front) can move quickly from the north to ram into the growing warm fronts. When that occurs, the differing temperature and pressure gradients of the two air masses can cause violent wind patterns to occur. The violence can manifest itself as severe turbulence, or as wind shear. It can also cause the most violent winds to quickly develop near the surface into a rapidly spinning, potentially very destructive vortex called a tornado.
Tornadoes can occur in any season. However, they are most likely to occur during the spring and summer months. Tornadoes are always a part of strong thunder cells, though not all thunderstorm cells create tornadoes. Tornadoes may be seen as the classic funnel shaped cloud that reaches the ground. While generally easy to see in the daylight hours, tornadoes may be essentially invisible after dark until they strike powerlines, for instance, and a flash of light or even a flash of lightning may for a moment reveal a tornado’s presence. Even in the daylight hours, a tornado can be difficult to see.
Some tornadoes that have not yet touched the ground, might not have picked up any dust, dirt or debris and may appear white or as gray as the background. Still others can be hidden behind a deluge of rain that wraps completely around the funnel cloud, thoroughly obscuring it from view. These rain-wrapped tornadoes can and do continue moving in the direction of the parent thunderstorm and their width and speed across the land can vary greatly at any time.
Tornadoes have been recorded in every state in the continental United States and also in southern Alaska. Some record sized tornadoes have been shown to be in excess of one mile wide! They can stay on the ground for many miles causing monumental devastation to populated areas in just seconds. The strongest tornadoes can have circulating winds of up to 300 mph, though the most common have winds that vary from 90 to 150 mph.
It is important for everyone to remember that tornadoes can develop very quickly, with little or no warning. Stay weather alert for danger signs including dark, often greenish sky; frequent lightning; large hail; a large, dark, low-lying cloud (particularly if rotating); and a loud roar, similar to a fast-moving freight train.
Be sure you know where you would go to have the highest available level of protection from a tornado, for every place you spend a reasonable amount of time, such as home, work, school, or a place of worship. If and when you see approaching storms or any of the danger signs, be prepared to take shelter immediately because you never really know when the violent winds might come.
More information is available in two free FEMA publications:
Taking Shelter from the Storm: Building a Safe Room for Your Home or Small Business (FEMA P-320, Third Edition, August 2008): www.fema.gov/media-library/assets/documents/2009?id=1536
Design and Construction Guidance for Community Safe Rooms (FEMA P-361, Second Edition, August 2008): www.fema.gov/media-library/assets/documents/3140?id=1657
A copy of the ICC/National Storm Shelter Association (NSSA) Standard for the Design and Construction of Storm Shelters can be purchased and downloaded from this website: http://shop.iccsafe.org/icc-500-2008-icc-nssa-standard-for-the-design-and-construction-of-storm-shelters-2.html.