The SIX ingredients that lead to tornado outbreaks, in order
Meteorologist
Thursday, May 7, 2015, 6:40 AM - Severe weather events are all unique, and each one presents its own particular forecast challenges. But all severe weather events are built from the same basic set of ingredients, and understanding what these are and how they relate to each other can help you understand your forecast better, and be more prepared for the next time there are storm clouds on the horizon.
As the severe storm season ramps up, let's take a close look at the processes and mechanics in the atmosphere that drive some of our most impactful types of weather, including heavy rain, lightning, hail, and tornadoes.
Moisture
The most fundamental atmospheric ingredient for severe weather is, not surprisingly, moisture. Without water vapor in the air, there can be no clouds, no precipitation, and certainly no storms. Where does this moisture come from? In the case of most severe weather events in North America, the answer is the Gulf of Mexico. The warm waters of the Gulf, under the strong spring sun, provide an ample source of water vapor for the atmosphere.
When conditions are right, even moisture from as far away as the Caribbean can be drawn northward across the U.S. and Canada. Weather systems that access this deep moisture tend to produce very heavy rain, with flooding as a major threat.
STORM HUNTERS MONTH: Starting May 5, 2015, the Weather Network's Storm Hunter team heads south to research some of the summer's most-severe weather. Tune in to TV for regular updates and visit The Storm Hunters homepage.
Storms grow from the bottom up, so when meteorologists analyze a potential storm environment we are most interested in the moisture at the lowest levels of the atmosphere, near the earth’s surface. We assess this moisture by looking at the surface Dew Point Temperature. This represents the temperature at which the air becomes saturated and droplets of water will begin to condense out of the atmosphere. The higher the Dew Point Temperature, the more moisture is available to fuel the formation of clouds and precipitation.
Lift
Moisture on its own is not enough to produce storms. We’ve all experienced humid days in the spring and summer, with high Dew Points and abundant moisture, and yet often on these days there’s not a cloud in the sky. For moisture to produce clouds and precipitation, a second ingredient is required: lift.
The water vapor in the atmosphere is a gas, and to form cloud droplets and eventually precipitation, a change has to take place. The vapor has to condense into a liquid. The most common way to get that to happen is to provide lift. A lifting mechanism in the atmosphere forces air to rise, which causes it to expand and cool. Fronts, strong winds aloft, and even the large-scale movement of airmasses can provide lift.
As the air is lifted it expands and cools until it reaches the Dew Point Temperature. At this point it becomes saturated and can no longer support the amount of water vapor it contains. The extra vapor condenses out into liquid droplets, forming clouds. If the lift is strong enough, and enough moisture is available, the clouds will eventually begin to produce precipitation.
Moisture + Lift = Clouds & Precipitation
Instability
A lifting mechanism can force air to rise, which is enough to produce ordinary, run-of-the-mill clouds and precipitation, but for strong storms to form forced lift is not enough. The air has to rise of its own accord. For that, we need another ingredient: instability.
Instability occurs where there is warmer air near the earth’s surface, and colder air high in the atmosphere. Warm air is less dense and so wants to rise – it has buoyancy. This is the same principle that makes a hot air balloon rise. Instability causes air to quickly accelerate upwards once it is lifted, which then enhances the processes of condensation and precipitation.
This rapidly rising air, fueled by instability, forms the updrafts of storms. Strong updrafts are what mark the difference between a thunderstorm and an ordinary rainshower. Updrafts provide the conditions necessary to produce heavy rain and even hailstones. Updrafts also create a separation of electrical charges which leads to lightning. In other words, updrafts are the engines that drive thunderstorms.
Moisture + Lift + Instability = Thunderstorms
Speed Shear
The old adage says that what goes up, must come down. This definitely holds true in the atmosphere. All that rising air from the updraft eventually has to sink back to the surface, once its instability is consumed. This sinking air forms the counterpart to the updraft¬—the downdraft. The downdraft is the region of a storm where precipitation is falling. It is full of dense, rain-cooled air that acts to cancel out instability.
If you want to maintain a strong, long lived thunderstorm, the last thing that you need is a downdraft stabilizing the environment. When the updraft and downdraft are too close to each other, the updraft will quickly get choked off and collapse. But in the right circumstances, the atmosphere provides a mechanism to keep the updraft and downdraft separated: speed shear.
Speed shear occurs when winds high in the atmosphere are faster than winds at the surface. This causes the updraft to tilt over, and the updraft and the downdraft become separated. The separation means that the updraft can develop and maintain its strength without interference. When speed shear is present in the atmosphere, storms can become stronger and longer lived.
Moisture + Lift + Instability + Speed Shear = Strong Thunderstorms
Directional Shear
The four previous ingredients are capable of producing strong thunderstorms with heavy rain, hail, lightning, and strong winds. However they are not capable of producing one of the most impactful types of severe weather—tornadoes. Tornadoes require a couple of additional ingredients, both related to generating rotation. The first of these is directional shear.
Where speed shear is a change in wind speed with altitude, directional shear is a change in wind direction with altitude. In the typical severe weather environment, this would mean winds at the surface out of the southeast, while winds high in the atmosphere are westerly.
As the updraft rises through an environment with speed shear, it feels the change in wind direction and begins to take on rotation. This rotation is what marks the difference between a typical thunderstorm and a supercell. The rotation causes the storm to take on a very particular set of characteristics, which a trained meteorologist can distinguish on radar. Supercells must be monitored very closely, because they are the parent storms which produce tornadoes.
Moisture + Lift + Instability + Speed Shear + Directional Shear = Supercells
Near-Surface Environment
Meteorologists have a very good understanding of what causes supercells to form, but not all supercells produce tornadoes. What is the difference between a tornadic and a non-tornadic supercell? That question is a subject of very active research in the atmospheric science community.
What we do know is that it depends very much on the characteristics of near-surface environment, and particularly the lowest 1000 meters of the atmosphere – the space in between the ground and the base of the cloud. High instability and strong directional shear in the lowest kilometer have shown to be very important factors, but there are many others which may play a role.
Ongoing research projects such as VORTEX (soon to launch its 3rd campaign) are attempting to take scientific instruments directly into the environments that produce tornadic storms, in order to better observe their characteristics. This is helping meteorologists develop better forecasting techniques and increase warning times for tornadoes. Identifying the important near-surface environments which produce tornadoes is a critical role for forecasters in severe weather events.
Moisture + Lift + Instability + Speed Shear + Directional Shear + Near-Surface Environment = Tornadoes
Putting It All Together
Severe weather is produced by the interaction of complex set of processes and mechanisms. All the ingredients have to come together in just the right way, and at just the right time. Lots of factors play a role in bringing the right ingredients together, including the overall atmospheric pattern, the time of year, and even geography.
This map from the U.S. National Climate Data Center shows the distribution of tornadoes across the globe. It highlights the regions where these severe weather ingredients most often come together, and as you can see the U.S. and Canada are a major hot spot. More tornadoes occur in North America than anywhere else in the world by far.
As we move into the transitional spring season and the atmospheric pattern becomes more active, outbreaks of severe weather are bound follow, driven by moisture and lift, instability and shear. Forecasters keep a close watch on weather data to track situations which might bring these severe weather ingredients together. Because when they do, the results can be explosive.
