Lesson 1: Basic Thunderstorm Meteorology and Terms

An excerpt from the USAF Flying Safety Magazine (June 1998)

Ever wonder what it is like to be INSIDE a thunderstorm in an aircraft?

It isn’t a friendly picture. It isn’t a place you want to be. Your reading material for this lesson took you inside a hurricane with the "Hurricane Hunters" of the 53rd Weather Reconnaissance Squadron. That unit offers a "virtual flight" into a hurricane as well. If you go there, pay close attention to the potential hazards around the aircraft. I want to repeat again that YOU don't want to be there in YOUR aircraft.

As a military pilot I had that unfortunate experience. My aircraft was number two in a two-ship formation. The navigator was using the radar to follow the lead around a squall line, we penetrated a nearby thunderstorm that reports stated were rising to over 50,000 feet. Once inside we encountered just about the worst ride I can say I’ve ever experienced in an aircraft. It started with a brief bout with heavy precipitation. Next came the pounding of hailstones. Severe turbulence forced us up and down over 400 feet within seconds. Then there came the bluish glow of "St. Elmo’s Fire" on the cockpit windows. Finally a very loud bang accompanied a very bright flash just outside of the cockpit. The AC electrical system quit. We went to battery power until our craft was out of the storm. Once we were clear of the weather, we reset two generators.

Upon landing we found a six-inch hole in the radome. We had had been hit by lightning, but we were also very fortunate. It could have ended a lot worse. After all, we still had many more good years of aviation left in us.

Introduction to Thunderstorms

At any instant there is, on average, at least one aviator who is looking squarely at a thunderstorm on radar or out the window of the aircraft while flying. Almost once a second, on average, a lightning strike between the ground and a cloud occurs in the United States. Over 100 lightning strikes take place every second over the Earth, where over 44,000 thunderstorms are occurring right now ... significant hazards to aviation and ground operations. There is a very good chance you’ll be facing a potential thunderstorm encounter within the next month or two. During that encounter, you will face the many and powerful hazards of a thunderstorm, including strong winds and windshears, heavy precipitation, lightning, hail, and tornadoes. Are you ready?
The definition of a thunderstorm is pretty basic, yet misunderstood by many. The weatherman’s definition of a thunderstorm is any local storm with lightning and thunder, produced by a cumulonimbus cloud, usually producing gusty winds, heavy rain and sometimes hail. However, what the weather observer primarily uses to identify a thunderstorm is ... thunder! That’s all, just hearing thunder, according to the handbook published for all observers.

Cumulonimbus clouds or "CBs" are vertical columns of cloud mass with rain descending from them which could potentially be thunderstorms. But until the first thunder is heard, there technically is not a thunderstorm at the airfield.
Weather manuals do allow observers to report thunderstorms using other criteria when the airport environment’s regular noise would hamper the detection of thunder. Weather observers can also use the presence of lightning in the immediate vicinity (5 NM), or hail, to identify when a thunderstorm is impacting an airfield.

The weather observation will stop reporting thunderstorms 15 minutes after the last reporting criteria is observed.
This, however, begs one of aviation weather’s biggest questions. How do the new automated weather observing systems found on airports sense thunderstorms? The answer right now is that unless a human is augmenting the system, it doesn’t usually. This is changing as we speak.

A Review of Thunderstorm Meteorology

What does it take to make a thunderstorm? While thunder is key to the storm’s identification, there are a few basic ingredients needed to create the phenomenon. We can imagine the whole process as an engine sustained by fuel and activated by a trigger.

An (unstable atmosphere) is the first ingredient and the "engine" that keeps the process going. Instability occurs when there is air that is warmer than the atmosphere around it. Under those conditions the warmer air is lighter and will rise, expand and cool to the same temperature as its environment. As the air cools it transfers energy to the surrounding air. When the air cools to the dew point temperature a visible cloud forms. While rising air is the "engine," it needs a source of "fuel."

Moisture in the form of water vapor is the second ingredient in our recipe and the fuel for the process. The more moisture there is, the better the environment is for creating a thunderstorm. With more moisture, the dew point temperature is higher so clouds will form with less cooling. There will also be more energy to release to the surrounding atmosphere during the cooling process (the solar energy stored in the water when the sun evaporated it from lakes, rivers, and oceans.) Warm, moist air is the fuel that keeps the unstable atmosphere creating thunderstorms, but we still need the trigger.

The final ingredient is a mechanical device, the "trigger," that will initially lift the air up so that the atmosphere’s instability will keep it rising. There are actually a number of triggering mechanisms. Mountainous terrain, fronts, or colliding airflows force air upward.

All (weather fronts) (cold, warm, stationary, or occluded) can be sources of uplift for the initial development of thunderstorms. At the frontal boundaries, warmer air rises over cooler air masses to create upward motion. Because cold fronts have a steeper slope, the uplifted air moves faster which can create more severe thunderstorms. Frontal storms are also hazardous because the thunderstorms can be embedded and unseen within stratiform clouds that also form.

Associated with rapidly moving cold fronts is the source of some of the strongest thunderstorms, the squall line. Here large-scale wind flows converge between 50 and 300 miles ahead of the cold front, and have nowhere to go but up. This strong and rapid movement upward creates a thin band of very unstable air that extends in a long line. The thunderstorms here are very active and potentially quite hazardous.

Another source of uplifting motion comes from the movement of moist air over rising terrain features or "orographic" lift. The thunderstorm will usually form on the windward side of the terrain if the air is unstable, and the storms are usually embedded within layers of clouds near the peaks.

The collision of moving air, or convergence, plays a role in thunderstorms. Since solar heating of the land occurs unevenly, some areas will be warmer while other areas are cooler. Air rises over the warmer areas, and is replaced in low levels by air converging from surrounding cooler areas. These converging airflows collide and force an uplifting motion. The squall lines mentioned above are dangerous examples; sea breeze convergence fronts along coastlines are a tamer example. Convergence also occurs when cooler air from nearby thunderstorms descends to the ground, spreads out, and pushes under the warmer air, lifting it upward to form a whole new thunderstorm. Sometimes, the descending air from different storms meet and force warmer air upward.

Once the air is lifted by one of these mechanisms, other processes account for the growth and development of the individual thunderstorm cell.
So where do thunderstorms form in the United States? Are they more common one place than another? Check out the National Severe Storms Laboratory's Thunderstorm Climatology Website and find out where things COULD be happening as you read this. You can also go to the real-time radar link you found this past week and find out where things ACTUALLY are happening.

Thunderstorm Development

There are three major stages of development to the life of a thunderstorm. The whole process lasts from only 20 minutes to several hours. Watching the development of a single-cell thunderstorm through all three stages gives us a chance to understand the forces involved in creating this aviation hazard.

The first, or updraft stage, begins with a simple cumulus cloud. During this initial stage, the updraft that carries the moist air aloft can be as rapid as 3,000 feet per minute and extend from the ground to several thousand feet above the cloud. The heat energy released as the air cools, expands the bubble of unstable air. As the air moves upward, cloud droplets collide with others and grow in size. The suspended water can be in liquid form well above the altitude at which water freezes, due to the energy released in the growing cloud. Towering (cumulus clouds), TCU, are now visible.

In the mature stage of the thunderstorm, the liquid droplets grow to a size where they can no longer be suspended aloft by the updrafts within the cloud. Precipitation begins and drags cooler air from the higher altitudes down with it. This creates a downdraft within the cloud. This colder air accelerates groundward at up to 2,500 feet per minute. As precipitation descends, drier air mixes into the cloud in a process called "entrainment," causing some of the rain to evaporate. Cooling accompanies the evaporation and accelerates the descent. When the downdraft strikes the Earth’s surface, it spreads out to create a gust front with strong windshears and damaging winds. If the downburst is less than 2.2 NM wide (4 km) it is called a (microburst); a larger downburst is called a macroburst. Updrafts gain intensity to the point that some storm clouds can grow at up to 8,000 to 10,000 feet per minute. With updrafts and downdrafts located close to each other, large droplets that were carried aloft to be frozen in the higher portions of the atmosphere, fall and collect more moisture only to be snatched by an updraft and carried aloft again. This cycle can eventually form hail. Throughout the mature stage, the movement and collisions of the air molecules and water droplets create electrical fields within the cloud, producing lightning (the cause of thunder) and therefore a thunderstorm. Turbulence is severe within the cloud. At its maximum intensity, a thunderstorm top reaches the tropopause and ice crystals spread out in the faster winds of the higher altitudes to create the familiar anvil formation.
You might want to check out the microburst animation sequence at the National Severe Storms Laboratory website.
Finally, in the dissipating stage, downdrafts form throughout the cloud, decreasing the uplifting taking place. The source of energy to sustain the storm is removed. The intensity then decreases until all that is left is the floating cirrus anvil.

Most individual (thunderstorm cells) last from 20 minutes to an hour within a system of multi-celled clusters of CBs. The gust front usually produces additional uplifting action ahead of the thunderstorm, creating a new one that will have a life of its own. Where there is a lot of moisture available, the cluster will grow to a large size called a mesoscale convective complex or MCC. It is important to understand that new thunderstorms form wherever gust fronts create lifting, but the whole system moves in a direction steered by winds in the middle altitudes.

When there is stronger wind aloft the mature thunderstorm will tilt. In this situation the process of growth and maturity can be sustained for a long time. This is the "supercell," a source of some of the severest weather produced by thunderstorms.

Supercells exist because of the strong windshears created between lower level warm moist air and the dry upper-altitude winds, and the very strong rotation of air moving upward within the cell. The difference in wind speeds and directions also form a horizontal rotation much like the formation that creates roll clouds or causes waves to curl on a shoreline. When the horizontal rolling motion is tilted vertically, the portion that is rotating in the same direction as the winds within the cell (usually counterclockwise in the Northern Hemisphere) adds its motion to the storm’s spin and a tornado forms. Sustained by large amounts of warm moist air lifted into the path of the cell by a large gust front up to 15 miles ahead of the cell, the formation can grow to over 60,000 feet in altitude and punch through the tropopause in a formation called an "overshooting top." These cells can live long enough to travel hundreds of miles along the surface of the Earth, dragging damaging tornadoes with them.

A tornado is not the only form of damaging winds from large thunderstorms. Besides the gust fronts and the havoc they can play on aviation, downbursts taking place in a larger scale system can create very damaging straight-line winds called "derechos." A derecho is a system of downbursts produced by convective weather systems that, because there are so many downbursts so close to each other in time and space, the net effect is a destructive straight-line wind that could reach over 100 knots.

Whether severe or not, all thunderstorms have the vertical motions that create electrical hazards. As the liquid and frozen water droplets collide in the violent vertical motions of a towering cumulus cloud growing to maturity, electrical fields are created in the cloud. There is a thin area of negative charge at the top of the cloud. Below it, within the anvil, is an area of positive charge. Around the freezing level is a strong area of negative charge as water exists in all three states. Finally, along the bottom, there is a thin area of positive charge. Underneath the cloud, and traveling with it, is a very strong area of positive charge.

At a point, the differences between the high concentration of negative charge at the freezing level and a nearby positively charged area is so great, that nature seeks to neutralize the differences. Most of the time, it is a positively charged area within a cloud, whether the same or different one, that is used. Some of the time, it is the positively charged ground that is used.

Whichever way it happens, the electrons from the negative area move away from the cloud in small 10 to 50 foot movements called the "stepped leaders." Because air is so resistant to the flow of electrons, it takes lots of electrons to flow to find the path of least resistance, making a forked pattern. Additional 15 to 25 footsteps take place from stepped leaders as the path grows towards the positively charged area. When the stepped leader gets close to the target, a path of positive charge is drawn towards the negative charge. If the target is the ground, the "streamer" ascends through a high point such as a flagpole, lightning rod, tree, or the occasional human. When contact is made between the negative leaders and the positive streamer, there is a mass migration of positive charge along the entire path created by the leaders. As the positive and negative charges collide, the one-eighth to six-inch thick pathway heats up to 10,000 degrees Celsius. (That’s hotter than the surface of the sun.) The heat energy creates light and the rapid expansion of the air around it. We see lightning. We hear thunder. This whole process has taken less than a second to occur, and since the pathway through the air is now less resistant to electrical flow, it can be repeated up to three or more times in a second as "dart leaders" travel down the same path as the step leaders. This gives (lightning) a flickering appearance. We hear thunder "rolling" as the soundwaves from different parts of the flash reach us at different times. We have a thunderstorm on our hands.

If you are out on the ramp and want to know how close lightning is use the "five second rule". For every five seconds the thunder takes to reach you after the flash, the lightning is one mile away.

 Study Questions:
1. What defines a thunderstorm?
2. Can an observer use other criteria to report a thunderstorm at the airport?
3. When will thunderstorms stop being reported at an airport?
4. What 3 ingredients create and keep a thunderstorm building?
5. What are some ways that uplift occurs?
6. What is a TCU?
7. What defines a microburst versus a macroburst?
8. How is hail formed?
9. What are the names of the three stages of a thunderstorm's life cycle?
10.. What is an MCC?
11. What is a "supercell?"
12. What is the difference between a tornado and a derecho?
13. If thunder occurs 15 seconds after a flash of lightning, how far away was the stroke?

Go to Lesson 2