Just finished a very very basic essay on Tornadoes for my Weather and Climate class. Figured I would post it for anyone who is interested in a primer on tornadoes...
Tornado Alley - Cyclones in the United States
Growing up in the mid-west United States, one generally feels pretty protected from most of the natural disasters that you hear about on the news; landslides in California, Avalanches in Colorado, Hurricanes along the coasts. But there are two natural disasters, or occurrences as they are not always disasters, that happen almost every year: tornadoes and flooding. And while floods are fairly predictable, as they happen along river ways and streams generally in the Spring months, tornadoes are a bit more unpredictable. Still, to this day tornado prediction is quite difficult even for highly trained storm chasers and meteorologists (4). Even when the conditions are perfect, a tornado may not occur... other times, a tornado may form in minutes, seemingly out of nowhere. It is this fascinating event, known by many names: tornado, twister, vortex, cyclone that is the basis of my research.
A tornado can be described as a rotating column of rising air that is in contact with both the ground and a cloud. This can either be a cumulonimbus or on occasion a cumulus cloud (cumulonimbus clouds form from cumulus clouds are are the typical cloud invisioned when one thinks of a thunderstorm)(3). Tornadoes usually run across the ground for several miles and can span from several yards to 2-3 miles wide (3). The winds produced by a tornado have been known to reach 300 miles per hour (1), but typically blow at around 100 miles per hour or less (3). Tornadoes come in many shapes and sizes, and are traditionally imagined as a funnel shape, with the smallest end of the funnel at the ground. They can also form to look like long skinny ropes attaching the ground to the sky, or have multiple vortices - smaller tornadoes rotating around a central location (3). They can also vary in color, generally depending upon what debris the tornado picks up, or may be nearly invisible at other times (3).
The formation of a tornado generally occurs during a thunderstorm. “Thunderstorms develop in warm, moist air in advance of eastward-moving cold fronts”(1). Tornadoes are produced when the two air masses with different characteristics run into one another (5). Within tornado alley (most of the mid-west United States), this usually means dry continental air from the west meeting warm moist air in the central plains (5). Along where these air masses meet, a ‘dryline’ is formed. Many tornadoes and severe storms develop along a ‘dryline’ (5). Clouds are formed when air rises, causing water to condense in the air. This physical state change releases heat, fueling the thunderstorm, particularly the ‘updraft’ (6). Supercell thunderstorms have particularly strong updrafts, which can create a vortex known as a mesocyclone, the prelude to a tornado. These are generally 2-6 miles wide and once one forms, there is approximately a 1 in 2 chance that the storm could become a tornado within half an hour (6). Mesocyclones can exist as early as 20 minutes to one hour before the creation of a tornado (7). The cyclone (tornado) itself is formed when the lower, warmer air mass punches a hole in the cooler air mass above it. It does this because the warmer air is less dense and naturally rises. The warm air then rises dramatically, as it pushes through the cooler layer above it, and, much like in the bathtub when you pull the drain, a vortex forms. As the air moves upward, a low pressure system is created, pulling air towards the tornado in all directions. Tornadoes in the northern hemisphere usually spin counter-clockwise due to the coriolis effect, which is caused by the rotation of the earth. On rare occasions tornadoes in the Northern hemisphere spin clockwise due to specific wind shear characteristics in certain storm conditions, but this is considered quite rare (7)(9).
This main type of tornado is created from a supercell thunderstorm (a long lived storm feeding off an updraft that is tilted and rotating), and is the most common type of tornado (7). There are several less common types of tornadoes that occur without the presence of a supercell storm: gustnadoes, landspouts and waterspouts (7). Gustnadoes are what many people would call a ‘dust devil’ - small whirls of dust or debris near the ground without the presence of a condensation funnel. These usually originate with the gust of wind in front of a storm (7). Landspouts are skinny, long, rope-like tornadoes that form when a thunderstorm is still growing and there is not yet a rotating updraft. Instead, the spinning motion begins at the ground instead of in the sky. Waterspouts are much like landspouts, but they occur over water (8).
In the United States, on average nearly 800 tornadoes are reported, resulting in around 80 deaths per year (1). In 2008 1,691 tornadoes killed at least 120 people, resulting in one of the most active tornado years on record (2). As population in the midwest expands, and people continue clearing forested areas for farmland and development, the human-tornado incidence rate is bound to increase. Only through the advancement of tornado detection technology and advanced warning systems, are we able to alert people faster than ever of coming storms. Even with millions of dollars being spent each year on tornado and weather research, predicting tornadoes accurately is still extremely difficult, and even basic questions about how tornadoes form are still being asked to this day. It is on this basic level of cyclonic knowledge that we will base our research on for years to come.
(1)"Tornadoes...Nature's most violent storm." NOAA. Available from
http://www.nssl.noaa.gov/edu/safety/tornadoguide.html. Internet; accessed 11 April 2011.
(2)Gray, Will. "Stormchasers: Studying twisters up close." New Scientist 205 no. 2749 (2010): 40-43.
(3)"Storm Encyclopedia : Tornadoes." The Weather Channel. Available from
http://www.weather.com/encyclopedia/tornado/form.html. Internet; accessed 11 April 2011.
(4)Frankel, Leora. "Funnel Vision." Discover 30 no. 5 (2009): 34-39.
(5)Oblack, Rachelle. "The Fujita Scale." About. Available from
http://weather.about.com/od/tornadoe1/ss/fujita_6.htm. Internet; accessed 11 April 2011.
(6)Marshall, Brian. "Tornadoes and Thunderstorms." How Stuff Works. Available from http://science.howstuffworks.com/nature/climate-weather/storms/tornado2.htm. Internet; accessed 11 April 2011.
(7)"Tornado Basics." National Severe Storms Laboratory. Available from
http://www.nssl.noaa.gov/primer/tornado/tor_basics.html#. Internet; accessed 11 April 2011.
(8)"Understanding Waterspouts." USA Today. Available from
http://www.usatoday.com/weather/tornado/wtspouts.htm. Internet; accessed 11 April 2011.
(9)"Inside a Tornado On the Road, In the Lab." National Geographic 205 no. 4 (2004): 29-31.