How do thunderstorms form and how can you prepare for them?

March 16, 2022

Learn how thunderstorms are formed, what makes a severe thunderstorm, and how to prepare for Texas storm season!

Spring in Texas means lots of sunshine warming the lingering winter chill - the perfect recipe for thunderstorms. As we leave winter behind and move into spring, we also move into Texas severe weather season! 

Severe weather can take many forms in Texas, but it all begins with a thunderstorm. But how do thunderstorms form? What turns a regular storm cloud into a severe thunderstorm? How can you stay alert and ready your family for the coming storm season? In the first installment of our severe weather series, we'll explain everything you need to know about thunderstorms so you can be weather aware and prepared - read on!

How do thunderstorms form?

What makes a thunderstorm dangerous, exactly? Where are they most likely to occur and when? Why are some storms harmless and others devastating and dangerous? These are all great questions, and answering them is important if we want to know the answer to the most important question: How can I keep my family safe during thunderstorms and severe weather? 

Understanding the mechanics of how a thunderstorm forms can help you and your family make sense of these powerful weather events so that you can judge risks, make preparations, and make informed decisions when the time comes. 

Thunderstorms are a common weather phenomenon that can occur almost anywhere in the world. As a general rule, any time warm moist air collides with cold dry air, you're likely to have a storm of some kind. When those storms have enough power, or fuel, they can become thunderstorms, which range from harmless rain and rumbling to downright dangerous depending on the location, season, and initial environmental conditions. 

Next, we'll dive into a little more detail and explain how a thunderstorm is born!

Favorable conditions for thunderstorms

A diagram of how thunderstorms form

In order for a thunderstorm to form, they need three basic ingredients: moisture, unstable air, and a lifting mechanism. In other words, the air needs to have water in it, it needs to be able to rise through the atmosphere, and it needs something to help it rise.

Let's take a closer look at each of those ingredients.

Moisture. Moisture comes from a number of different places. At any given time, the air around you has some amount of moisture in it. It comes from lakes, rivers, streams, previous rain showers, and the ocean. As the sun heats it, it evaporates into water vapor in the air. 

Unstable air. Instability is a complicated subject, but generally, it refers to atmospheric conditions that allow warm air to rise high into the atmosphere. As you may remember from science class, warm air rises because it is less dense and therefore more buoyant. Air temperature changes (gets colder) as you increase in altitude. If that change is gradual, then air near the surface isn't able to rise high enough to trigger a thunderstorm. However, if the temperature decreases rapidly with altitude, then the warm moist air has a clear path upward.

Lift, triggering events. When the atmosphere is unstable, warm air near the surface has a clear path up into the atmosphere, but it often still needs some sort of help to get started in that direction. 

There are a number of ways this can happen, but during storm season, solar illumination is the main cause. In other words, the heat from the sun warms the ground and the air above it, which causes it to rise. Cold fronts can also be triggering events for thunderstorms as the mass of cool air can literally push the warm moist air up over it as it passes through. Thunderstorms can even form around geographic features, like hills and mountains. Like a cold front, the mountain forces the warm air up into the cool air, which creates a thunderstorm. 

These are the ingredients necessary to kick thunderstorm development into gear.

Life cycle of a thunderstorm

Once the ingredients are all in place, a thunderstorm begins its life! But storms don't just pop up, dump rain and lightning, and then disappear - they go through a life cycle, almost as if they were a living thing. Next, we'll look at the three main stages of a thunderstorm's life. 

Development stage. Remember, hot air is less dense than cold air, and therefore it rises (similar to a hot air balloon). Warm air is also capable of carrying more moisture, or water vapor, than cold air - air molecules are tightly packed in cold air and don't leave as much room for the water vapor droplets. As the warm moist air rises into the cooler air at higher altitudes, it cools and the water vapor is forced to condense into droplets, which together form cumulus clouds. Small amounts of latent heat are released through the process of condensation, which keeps the air moving upwards. 

Mature stage. This rising current of warm air is called an updraft, and depending on just how unstable the atmosphere is, it can continue to rise, climbing miles into the air. The warm air will eventually stop when it is no longer warmer than the surrounding air or when it meets a warmer mass of air, which causes the cloud to flatten out, giving it the appearance of an anvil.

When the water droplets cool enough, they freeze into small ice particles. At this point, if the updraft can no longer keep them in the air, they fall and melt on the way down, causing rain. However, if the updraft is strong enough, they will fall, melt, rise, freeze again, and repeat this cycle until they are heavy enough to fall as hail. 

As the rain falls, it cools and drags the surrounding air down with it, creating a downdraft. This cycle of rising and sinking air with simultaneous updrafts and downdrafts is called convection and is responsible for creating the towering cumulonimbus clouds we often associate with storm clouds. 

As you can imagine, the process of convection can become intense and may produce strong winds, lightning and if other conditions are present, possibly even tornadoes. 

Dissipating stage. In a normal thunderstorm, the final stages begin when the downdraft becomes the dominant force in the storm. After 20-30 minutes, most of the warm most air has cooled, condensed, and fallen as rain, and the storm rains itself out. As the downdraft hits the ground, it spreads out in all directions creating an outflow boundary, cooling the air around it and cutting itself off from its own supply of fuel: the inflow of warm most air. 

At this point, the thunderstorm is mostly spent, but may still present another danger called a downburst. Depending on the conditions, if the downdraft is strong enough, it can create a downburst, or microburst (depending on the size), which can be a brief but intense wall of air that pushes out from all directions as the downdraft crashes into the ground. 

Types of thunderstorms

So now we know how thunderstorms form and we've seen how they go through the various stages of their life cycle. However, it's also important to understand that not all thunderstorms are the same. 

Depending on the environmental and atmospheric conditions, thunderstorms can come in a few different varieties. Some are more mild and short lived, while others can become quite severe and persist for hours. 

What is a thunderstorm cell, or a storm cell? Before we take a look at the different types of thunderstorms, we need to understand what is meant by a thunderstorm "cell." You may have heard the weather forecast mention a cell before, but what is it exactly?

As we've discussed, every thunderstorm has an updraft and a downdraft. When we refer to a thunderstorm cell, we're referring to an updraft and downdraft pair, or couplet. These convective loops within a volume of air act like a single entity and are considered the smallest discrete part of a larger storm system.

A single cell thunderstorm with anvil cloud

Single-cell. These are the thunderstorms that you are likely to see in the later afternoon and early evening during the spring and summer.  You may also see them during the winter as cold fronts pass through and lift warmer air above them at the leading edge of the front. They are referred to as a single cell because they only have one updraft/downdraft couplet. They aren't particularly dangerous and are seldom severe. When they do become severe, they are called pulse storms and, as the name suggests, are very brief.  

A multi-cell thunderstorm system

Multi-cell thunderstorms, or multi-cell clusters.
This is the most common way that thunderstorms develop. They usually have a parent storm, or mother storm, which is the strongest mature storm in the center of the cluster.

As new cells develop on the upwind part of the storm (usually the west or southwest side), they gradually move to the center as they mature before finally moving to the downwind side of the storm (usually the east or northeast side). 

Multi-cell clusters do have the potential to become severe, the most intense storms being the mature cells in the center of the cluster. Sometimes, depending on the conditions, they can organize into a multicell line, also known as a squall line. 

A multi-cell squall line thunderstorm system

Multicell line, or squall line. Multicell line thunderstorms, or squall lines, are long thunderstorm systems that form on the edge of or in front of a cold front. Sometimes, they are part of a much larger cyclonic storm and form the tail of the classic comma-shaped convective system (such as a nor'easter or tropical cyclone). 

When watching the weather radar, a squall line appears like a long, slightly curving line of storms that can stretch for hundreds of miles, sometimes the entire length of the country. When they appear to bend more substantially, they are called a bow echo.

The thunderstorms along a squall line can often be severe, bringing hail, intense rain, damaging straight line winds, and occasionally weak tornadoes. Intense multicell thunderstorms in the summer are called derechos, which have sustained winds that can exceed 100 mph, making them something like an inland hurricane. 

A supercell thunderstorm, a severe thunderstorm

Supercell. Supercell thunderstorms, often referred to as a rotating thunderstorms, are the most dangerous types of thunderstorm, and occur when wind shear is present. Wind shear refers to variable wind speed and direction at different altitudes. In other words, wind shear is present when winds are blowing in different directions as you move up through the air. 

Wind shear creates invisible horizontal rotating tubes of air above the surface. When an unstable atmosphere is favorable for storm formation, the updraft of the thunderstorm begins to lift this horizontal tube, creating a vertical rotating updraft called a mesocyclone. The mesocyclone separates the cool downdraft from the warm updraft and creates a miniature low pressure system, which means the storm can get a steady supply of warm moist air without the rain-cooled air choking it off. The result is a powerful storm that can persist for hours - far longer than the 20 or 30-minute life of a regular thunderstorm. 

Supercells often look very different than a standard thunderstorm. While they usually display the classic anvil shape, the updraft is strong enough to push above it, creating a distinct bulge called the overshooting top. Because of the rotating winds, the clouds may appear like a large spacecraft-like disc or an inverted layer cake. 

Because of the dynamics of a supercell and their sheer power, they are frequently associated with torrential rains and flooding, giant hailstones, powerful straight line winds, and violent, destructive tornadoes. While almost all of the most powerful tornadoes come from supercells, only about 30% of supercells actually create tornadoes. 

Regions: Where do thunderstorms form?

Thunderstorms can occur almost anywhere on Earth, but there are some areas more prone to them than others. Similarly, within the United States, all 50 states have thunderstorms, but some have them far more often than others

They often form near the coast where moisture is abundant. For example, the portions of the states bordering the Gulf of Mexico have frequent thunderstorms. This is because of the warm, moist gulf air, but also because of the winds that come off of the water, called sea breeze. Like a miniature cold front, the sea breeze can provide lift to the air over land which warms during the day. The result is frequent thunderstorms along much of the coast. 

Supercells are most common in central part of the United States (the Great Plains) and form over Kansas, Nebraska, Oklahoma, and parts of Texas because of unique geographic features and weather patterns. Cool dry air comes down from Canada over the Rocky Mountains to meet the warm most air flowing up from the Gulf of Mexico. This area is known as Tornado Alley which is known not only for the tornadoes, but the storms that cause them.  

However, intense thunderstorms (including supercells) and storm systems also frequently form in the Ohio River Valley, Mississippi River Valley, which means states like Missouri, Iowa, Ohio, Arkansas, Louisiana, Mississippi, Alabama, and Kentucky are susceptible to severe weather. This is known as Dixie Alley. 

Texas has many different regions and varying climates, but almost all of them experience thunderstorms of all kinds, especially during storm season. 

When is thunderstorm season? 

If you've been following along this far, then you can probably get an idea for why certain seasons are more favorable for thunderstorms than others. While thunderstorms can happen during any season, they are most common in the spring and early summer, and to a lesser extent during the fall in some places. 

During the spring, cool dry air from the winter begins to meet warm moist air as the seasons change. Thunderstorms also need instability and as the days get longer and the sun shines more, it heats the ground, which means the air near the ground becomes warm and rises into the cooler atmosphere. Remember, thunderstorms happen any time warm moist air and cool dry collide, and that happens a lot during the spring. 

Spring also marks the migration of the subtropical jet stream. Jet streams are belts of high-speed winds that flow miles above the ground in the atmosphere. The subtropical jet stream generally marks the boundary between the warm and cool air masses in the northern hemisphere. Generally speaking, it spans across Mexico in the winter, and as the spring sun grows in strength, it travels north over the United States. Because of the strong winds within this river of air, the jet stream can add power and intensity to storm systems during the spring. 

In Texas, storm season, or severe weather season, begins in March and peaks around mid to late May, dropping off substantially in early to mid June. 

How dangerous are thunderstorms? 

Most of us have experienced many thunderstorms in our lives and most of them come and go without incident. The wind may whip, the thunder may boom, and the rains may pour down, but most thunderstorms are harmless relatively speaking. Sometimes, however, a thunderstorm becomes powerful enough to present real danger, and there are a number of different hazards possible when an intense thunderstorm blows through.

Dangers of thunderstorms

Floods and flash floods. We've talked about Tornado Alley, but Texas is also known as Flash Flood Alley. It may surprise you to learn that out of all the dangers associated with thunderstorms and severe weather, floods (particularly flash flooding) are responsible for the most fatalities

Because of the geography in many places in Texas, it is often the case that storms dump too much rain for the soil and waterways to handle. As a result, dry creek beds can become a raging torrent in minutes, so it is important to be prepared and be aware of flood safety best practices.

Tornadoes. There is no force of nature on Earth quite like a tornado. They can range from small, noodle-like twisters to massive wedges more than two miles in diameter. Weaker tornadoes can break windows and tear off roofing, but more powerful tornadoes can wipe a foundation clean.

Although the likelihood of being being in the path of a tornado is relatively small, it is important to be aware of the possibility, especially during storm season, and have a plan in place should you find your home in the path of a tornado-producing thunderstorm

Hail. While giant stones of ice falling from the sky might seem deadly, hail rarely results in fatalities. Property damage, on the other hand, is a much more common threat when it comes to hail-producing thunderstorms. Small hailstones can cause roof and shingle damage or dent your vehicle, but larger stones can completely crush a vehicle and ruin your roof entirely

However, hail can seriously injure a person, especially if they are caught out in the open with no shelter in sight. That's why it's so important to check the weather forecast during storm season before committing to outdoor activities where cover may not be available. 

Damaging straight line winds. Tornadoes aren't the only wind-related danger associated with thunderstorms - strong, straight line winds can be a serious hazard, too. Because of the convective nature of thunderstorms, powerful gusts of wind are not uncommon. 

Strong thunderstorms can break limbs and branches which can damage cars, powerlines, homes, and pose risks to both you and your property. 

Lightning. Thunderstorms wouldn't be thunderstorms without lightning! These powerful discharges of energy don't always hit the ground, but when they do they can be quite damaging. 

Downbursts and microbursts. As a thunderstorm reaches its final stages, they may not fade away so quietly. Downbursts and microbursts occur when the storm's updraft runs out of steam and can no longer keep the cool, rain-filled air aloft. This large mass of air crashes to the ground all at once, fanning out in all directions. The result is a burst of brief but intense wind and rain, which can break windows, rip shingles off of roofs, and even flatten trees. Because they happen rather suddenly, it's difficult if not impossible to forecast when or where they will happen. 

How to prepare for severe thunderstorms - Thunderstorm preparedness and safety

Now that we have a solid understanding of how and why thunderstorms form, where they form, and when they are most likely to form, we can talk about how to prepare for the dangers they can present.

Be weather aware: Thunderstorm education and awareness

The fist part of preparation is education and awareness; learning about severe weather and thunderstorms can help you make sense of situations so you know how and when to act. Although we may often joke about the inaccuracy of weather forecasts, they have come a long way in a short time, and are accurate enough to help us prepare. 

Let's take a look at a few things you can do to stay weather aware during severe weather season. 

Watch local weather forecasts. Local weather stations have been helping communities prepare for severe weather for decades, and they remain an important tool for the public. They take the time to review scientific data produced by weather models, and translate it into meaningful insights for you and your community. 

By tuning in at least a few times a week, you can get a general idea of what the weather is going to be like in the coming days. They'll let you know when threats are possible so that you can take steps to prepare. 

Follow local weather on social media. Social media has proven to be an important tool for staying up to date on current weather information. In addition to live broadcasts, many weather stations will share forecasts, alerts, and warnings on their social media accounts.

You can even find meteorologists who operate exclusively on social platforms like YouTube. They provide in-depth weather forecasts and even stream live coverage of major weather events as they happen. 

Tune in to NOAA, NWS, and the SPC. Weather can be difficult to predict, but fortunately those of us in the United States have the meteorologists of the National Oceanic and Atmospheric Association (NOAA) and the National Weather Service (NWS). Together, they provide 24/7 weather predictions to the public through the Storm Prediction Center (SPC), which monitors the weather around the clock and issues watches and warnings when severe weather is possible. In addition to providing information and resources to local weather stations, NOAA also has radio broadcasts that you can tune into for real-time information about severe weather and weather emergencies. 

If you want a closer look at some of the predictions that even your local weather stations look to, you can check out the Storm Prediction Center and their Day 1 Convective Outlooks. These are short to mid-range predictions for the likelihood of severe weather, like thunderstorms, wind, hail, and tornadoes, across the country.

SPC outlooks shade regions of the country that may have severe weather, and they illustrate the threat of severe weather with a 5-tier system: 1: Marginal, 2: Slight, 3: Enhanced, 4: Moderate, and 5: High. With these outlooks, you can get a general idea of what kinds of weather to expect in your area. 

Download weather apps. In addition to social media, weather apps are yet another way that technology can help you prepare for thunderstorm and severe weather. Many local weather stations have their own app, which feature real-time doppler radar feeds, 7 day forecasts, and can often give you alerts when severe weather warnings occur in your area. 

What is the difference between a severe thunderstorm watch and a severe thunderstorm warning? 

If you've watched the weather during the spring storm season, you may have heard that your area is under a "severe thunderstorm watch," or even a "severe thunderstorm warning." But what's the difference between the two? What does it mean for a thunderstorm to be severe?

NOAA considers a thunderstorm to be severe when it produces winds that equal or exceed 58 miles per hour and/or hail that is one inch or larger in diameter (quarter-sized hail). 

A severe thunderstorm watch is issued when the conditions in a given area are favorable for such a storm. They typically last 4 to 8 hours and are usually issued before the weather in question actually begins. If your area is under a severe thunderstorm watch, it is important to remain alert and prepare to act if a storm approaches.

A severe thunderstorm warning is issued when an existing storm is shown to have those conditions, either indicated by radar or observed directly by a trained spotter. When issued, a severe thunderstorm warning will include the main threat associated with the storm, such as hail, damaging winds, and/or flash floods. They are usually issued for the duration of an hour, but may be renewed if the conditions persist. 

If your home is in the path of a storm which has been given a severe thunderstorm warning, it's important to have a plan of action in place to keep you and your family safe. 

Severe thunderstorm emergency preparedness and planning

In the event of severe weather warning, make sure you and your family have a plan of action in place. 

Family preparedness plan. This involves preparing steps to find and communicate with family members to make sure they're aware of dangers and in a safe place. You'll also want to find a safe place in your home or identify a safe meeting place (such as a storm shelter) if your home won't provide sufficient shelter (like mobile homes, campers, and RVs). Talk with your family to determine who will be responsible for various tasks, such as calling other family members, gathering supplies, and ushering family members to shelter. It's always helpful to get basic first aid training for as many people in your household as possible.

Once a plan is in place, run a drill once or twice a year just to make sure everyone is aware of what their role is. Not only does this allow you to operate in a calm, smooth fashion in the event of an emergency, it gives you the opportunity to work out an problems before they arise.

Communication plan. When a severe weather emergency arises, you'll want to have a plan in place to ensure you can get in touch with and account for everyone in your family or household. While everyone with a phone should have all the relevant phone numbers stored, it might be helpful to have emergency contacts posted somewhere like the fridge. This way everyone in your family can easily contact friends, family, neighbors, police, fire, and other emergency services. It's also important to make sure that places like your children's school or your work have up-to-date emergency contact information.

Emergency supply kit.  Thunderstorms can cause all sorts of problems, such as power outages, so having an emergency kit on hand is a must. You'll supply your kit with flashlights, batteries, radios, first aid, fully-charged phones, and possibly a little food and water just in case.

Secure things outside/prepare your property. When a severe thunderstorm warning is put in place, you won't have time to go outside and secure things, so it's important to always maintain a yard free of hazards during storm season. 

This involves securing furniture and outdoor decorations, trimming hedges and trees, and keeping your yard free of dead trees, limbs, and other objects that could be thrown around by strong winds. If the weather forecast suggests that hail is possible, consider taking steps to protect your vehicles beforehand. 

General thunderstorm safety tips

Lastly, let's take a look at some general safety tips for thunderstorms. 

The "30-30" rule. The 30-30 rule is a simple way to gage whether or not it's safe to be outside when a thunderstorm is near. If you see lightning, start counting to 30. If you hear a clap of thunder before you get to 30, it is recommended that you stop all outdoor activity and go indoors. This is because generally speaking, you can divide the number of seconds between the lightning and the thunder by 5 to calculate how far away the strike was from you. If you get to 10, that means it's only 2 miles away - and that's far too close. 

Remember, when the thunder roars, go indoors! 

Cease outdoor activity. The best way to avoid the dangers of a thunderstorm is to check the weather beforehand. If a thunderstorm is in the forecast, you should do your best to avoid outdoor activities like sports, camping, boating, swimming, and so on. 

If you are caught outdoors during a thunderstorm, keep moving towards shelter as fast as you safely can. Avoid large, open fields and tall, isolated objects such as a lone tree or flagpole. Seek shelter in a sturdy, enclosed structure and avoid open and isolated structures such as gazebos. If you're near or on the water, get to shore as fast as you safely can.

If you're in a group, stay as far apart from each other as you can whenever possible. If lightning strikes one person in a group, it will likely injure those in close proximity as well. However, standing apart decreases the chances that everyone will be injured, and allows those who were not injured to administer first aid and immediately seek help.  

Avoid the roads. If at all possible, avoid hitting the roads during a thunderstorm. Heavy rain, wind, and hail can all make driving incredibly dangerous, so it's best to wait where you are until it passes. Remember, if you see water on the road or a bridge, turn around, don't drown!

If you are caught on the road during a severe storm, turn on your hazard lights and pull over to the shoulder of the road or pull into a parking lot. When finding a place to wait out the storm off the road, make sure you park far enough away from things like trees or signs in case they are blown by the wind. 

Avoid plugged in electronics. When a bolt of lightning strikes close to your house, it's possible for electricity to surge through the power grid into your home. This means that electronics plugged into that grid are likely to get a huge jolt, which is likely to shock them and anything touching them. 

For this reason, you should never operate chorded electronics when there is a thunderstorm overhead. In fact, it's probably best if you unplug valuable electronics from the wall before the storm arrives - your surge protector won't save them. Surge protectors are meant to protect electronics from minor surges that can occasionally travel through an electrical grid. The surges from a bolt of lightning are far more powerful.

Avoid bathing and showering during a thunderstorm. Similar to electronics, if lightning strikes near your home, it can also travel through pipes. If you're taking a shower or a bath, or even washing the dishes, you could run the risk of being shocked by the immense surge of electricity. 

Stay away from windows. During a thunderstorm, wind-blown debris and even large hail can break glass and cause injuries. That's why it's best to close your blinds or shades and stay a reasonable distance away from windows until the storm passes.

Germania's Spring Storm Series

We've covered a lot of information here today because, well, there is a lot to say when it comes to thunderstorms and severe weather in general! By understanding thunderstorms, we can understand the dangers associated with them and learn to prepare. The same is true for all severe weather threats you may experience here in Texas!

Click on one of our other severe weather blog topics below and get all the info you need to stay weather aware - and prepared!

1. Thunderstorms (you are here!)
2. Hail
3. Lightning 
4. Tornadoes 
5. Microbursts and Downbursts

A thunderstorm in the dissipation stage

For more information about Germania Insurance and our products and services, request a free quote online or reach out to your local Authorized Agent today!

by Geoff Ullrich

About the Author

Geoff Ullrich is a writer and Content Marketing Specialist at Germania Insurance.