How Do Tornadoes Form? | How Tornadoes Form & Where
Knowing how tornadoes form can be huge for people living in tornado-prone areas. Here is how tornadoes form & where.
*This post only uses the SLIM acronym to explain. There is more to the complicated side of tornado forecasting*
Tornado forecasting saves many lives every year. The tornado watches/warnings/emergencies from the forecasting tell people to be aware or take cover. Basically the main knowledge you need to get a forecast is how a tornado forms.. Many factors & ingredients go into forming tornadoes. But you came here wondering “How do tornadoes form?” so…
This post is all about how tornadoes form & where.
SLIM Ingredients
There are four major ingredients for tornadoes to form…
- Shear
- Lift
- Instability
- Moisture
Shear
According to the National Weather Service, wind shear is “the change of wind speed or direction with distance or height.” Wind shear usually occurs around temperature inversions, lows, and troughs, around the jet stream, mountains, & downbursts. The shear can make thunderstorms last longer, be stronger, and rotate. Wind shear is a big part of the stages of thunderstorm formation. Low-level wind shear is especially associated with tornado formation. This leads us to the two main types of vertical wind shear…
- Speed Shear
- Directional Shear
Speed Shear
Speed shear is the change of wind speed with height. This particular type of shear can make storms last longer. The different speeds of the wind at different heights make the updraft of the thunderstorm tilt. This results in the precipitation being on another side of the storm away from the updraft. With this, the updraft doesn’t get suffocated and the storm can last longer. Speed shear can also affect the tornado’s direction of movement, not just how they form. This type of shear can often be caused by interactions between the jet stream and winds at 850 MB. The jet stream winds can be found at 500 mb.
Directional Shear
Directional shear is the change of wind direction with height. The directional shear is significant for the formation of tornadoes. This type of shear causes the updraft to rotate, which in addition, could lead to a tornado dropping down. For the best possibility of tornadoes, you would want the direction of the wind to rotate at least 90 degrees in the atmosphere as you go up in the troposphere.
Lift
Atmospheric lift is the process of air rising higher into the atmosphere. Updrafts lift the air to create clouds & precipitation. Here are the three types of atmospheric lifting mechanisms…
Orographic Lifting
Orographic lifting is present when air is forced to a higher elevation by a landform. Examples of landforms can be large hills, mountains, or even large valleys. The valley can cause the air to dip down but then go up further when it comes out. This can result in various conditions like precipitation & clouds. The clouds that the lifting creates can turn into a storm complex. Lots of different cloud formations occur from this type of lift. Some wrap around the mountain, some hover over it, some blanket it! There are many cloud formations from this type of lift.
Convergence
Convergence occurs when two air masses collide and the warm air is forced upward. This is often seen in weather fronts. When it happens in weather fronts, it is called frontal lifting. An example of frontal lifting would be cold fronts. The cold air shoves into the warm air & forces it upward, creating rapidly rising clouds. Tornado formation is prime in areas of convergence.
Convection
Convection happens when the sun’s rays hit & warm the ground. The air above the ground then begins to warm up, too. The warm air rises and creates clouds. This also happens often during severe weather events. That’s why stormy weather likes to have sunshine before it.
Instability
The definition of instability, according to the National Weather Service, is “The tendency for air parcels to accelerate when they are displaced from their original position; especially, the tendency to accelerate upward after being lifted”. Lifted things would be warmer than the surrounding air in an unstable atmosphere. The type of instability I will talk about is called “convective instability”. Convective Instability is the ability of air to move vertically. In an atmosphere where the convective instability is stable, vertical air movement is difficult. When there is unstable air, the amount of air moving vertically can be larger with it also being easier to move in that way.
The atmosphere is called unstable when the dry adiabatic lapse rate temperature decrease is above 9.8C per kilometer and stable when below. One index to measure instability that is widely used by the weather community is the CAPE (Convective Available Potential Energy) index. CAPE measures the positive buoyancy of air parcels usually with J/kg.
Moisture
Atmospheric moisture is how much water vapor the air holds. There are two main ways of looking at moisture: dew point and relative humidity. The dew point is the temperature where water vapor below that temperature begins to condense into water vapor. The relative humidity is how much water vapor is in the air expressed in a percentage. It is compared to how much water vapor the air can hold at that temperature. Higher moisture levels can indeed help develop tornadoes in multiple ways. For instance, more latent heat could be released, condensation level decreases, and it makes clouds!
