Tornadoes are one of nature’s most destructive forces. These violently rotating columns of air extend from thunderstorm clouds down to the ground, wrecking havoc along their paths. But how exactly do these whirling winds get their start?
What causes tornadoes?
Tornadoes form when wind patterns in thunderstorms start spinning and rotating in a vertical fashion. Specifically, they are born when an updraft of warm, moist air begins to rise and rotate within a supercell thunderstorm.
Supercell thunderstorms are created when warm, moist air at lower levels gets wedged under cooler, drier air above. This causes the warm air to rapidly rise, creating the towering cumulonimbus clouds that thunderstorms are known for.
As the warm air rises and rotates, a horizontal spinning motion is induced in the atmosphere. When strong changes in wind speed and direction are present (called wind shear), this horizontal spinning can be tilted on its side and stretched vertically.
Updrafts within the supercell help accelerate this newly aligned vertical spinning motion. As it picks up speed, a concentrated vortex is formed. This is the beginning stage of a tornado.
How do tornadoes fully form?
For a full-fledged tornado to form, the vertical spinning vortex needs to extend from the cloud base all the way down to the ground. This is enabled by the concentrated low pressure area within the vortex itself.
The significantly lower pressure within the vortex causes the rotating air to rush inward at high speeds. The intense inward and upward motion creates a funnel-like shape as it extends downwards from the storm cloud.
Once the rotating winds reach all the way to the Earth’s surface, the tornado has officially formed. Winds inside the funnel can reach speeds of over 200 mph as the tornado carves a path along the land.
What weather conditions are needed?
Tornadoes require very specific atmospheric conditions in order to form. There are six main ingredients that need to be present:
- Moisture – Warm, humid air near the ground provides fuel for thunderstorms to form.
- Unstable air – Warm air that can rapidly rise within thunderstorms.
- Lift – A mechanism to lift the warm, moist air upwards, like a front or topography.
- Wind shear – Change in wind direction/speed at different heights creates rolling motion.
- Low-level jet stream – Strong winds about 1-2 km above ground.
- Supercell thunderstorm – Rotating updrafts and downdrafts.
When these ingredients come together in just the right way, they set the stage for tornado development. The presence of a supercell thunderstorm and strong wind shear are two of the most crucial factors needed.
What time of year do tornadoes occur?
In the United States, peak tornado season depends on the region:
| Region | Peak Months |
|---|---|
| Southeast | March-May |
| Northern Plains | May-August |
| Southern Plains | May-June |
States like Florida and Texas can see tornadoes as early as February when conditions are right. Overall, late spring is the most active time for tornadoes in much of the country.
The reasons tornadoes follow seasonal patterns has to do with weather conditions. In spring, warm, moist Gulf air starts to migrate north, colliding with lingering cold air masses. This instability in temperatures combined with strong jet stream winds leads to the abundant severe storms that spawn tornadoes.
Where do tornadoes tend to occur?
While tornadoes have been documented on every continent except Antarctica, they occur most frequently in the United States. More specifically, an area of the country nicknamed “Tornado Alley” sees the highest concentration of tornadoes:
- Northern Texas
- Oklahoma
- Kansas
- Nebraska
- Eastern Colorado
- Southwest Iowa
This region is perfectly suited for tornado development because it receives an optimal mix of the needed ingredients. Warm, humid air from the Gulf of Mexico clashes with drier air masses moving east across the Rocky Mountains, creating instability. Jet stream winds provide abundant wind shear. And the flat terrain allows thunderstorms to readily form and organize.
Other areas known for frequent tornadoes include the lower Mississippi River Valley, the upper Midwest, and Florida.
What types of tornadoes are there?
Tornadoes are rated on the Enhanced Fujita Scale, which assigns a category from 0 to 5 based on estimated wind speeds:
| EF Rating | Wind Speeds |
|---|---|
| EF0 | 65-85 mph |
| EF1 | 86-110 mph |
| EF2 | 111-135 mph |
| EF3 | 136-165 mph |
| EF4 | 166-200 mph |
| EF5 | Over 200 mph |
About 75% of all tornadoes are considered weak, rated either EF0 or EF1. But the most violent tornadoes, EF4 and EF5, account for only about 1% of occurrences.
Some other types of tornadoes include:
- Wedge Tornadoes – Widest at ground level, tapering higher up
- Rope Tornadoes – Narrow funnel shape from ground to cloud
- Multiple Vortex Tornadoes – Two or more small subvortices orbiting inside funnel
- Waterspouts – Tornadoes that form over water
Tornado formation stages
Meteorologists have identified a 3-step process in the formation of most tornadoes:
Developing stage
A concentrated area of rotation develops within a thunderstorm as winds change speed/direction. This mesocyclone can be seen on radar before a funnel appears.
Mature stage
The rotation extends towards the ground in the form of a visible condensation funnel. This indicates the tornado has formed and may strengthen.
Decaying stage
The tornado weakens and eventually dissipates after the inflow of warm air weakens or wind shear decreases. Funnel clouds thin and rope tornadoes may form briefly before the tornado dies out.
The lifespan of a tornado rarely exceeds 30 minutes, but some of the most extreme tornadoes can last over an hour. Indications on radar usually precede tornado formation by about 20-40 minutes.
Conclusion
In summary, tornadoes go through distinct stages as they form within severe thunderstorms. Specific ingredients like wind shear, instability, and high moisture all have to come together for tornado development. Once formed, these violently rotating columns of air can cause catastrophic damage if they travel over populated areas.
Understanding how tornadoes are born provides valuable insights for research and forecasting. As meteorologists continue to study supercell thunderstorm structures and evolution, forecast lead times for tornado warnings will hopefully continue to improve.