As we move into to the spring and summer storm season, we thought it would be a good time to share with everyone some basic information on how thunderstorms form and why they are most common during this time of the year.  So grab a snack and a beverage, find a comfy chair, and let’s learn!

Thunderstorms are “convective storms” that form with rising air and contain at least some amount of lightning.  To understand how thunderstorms form, you’ll first need to know that warm air is less dense and more buoyant than cold air.   Warm air wants to rise and will keep rising as long as it is warmer than the air surrounding it.   This rising air, along with a few other key ingredients, is what causes thunderstorms to form.  The United States experiences about 100,000 thunderstorms each year with only about 10% of those (10,000) classified as Severe.  To be classified as a Severe Thunderstorm, the storm must have wind gusts measured at 58 mph or more, or have 1 inch (or large) hail, or produce a tornado.

Thunderstorms need just three key ingredients to form:

• Moisture (water vapor)
• Rapidly rising warm air (lift)
• Unstable atmosphere

The first key ingredient, moisture, is pretty easy to come by in the spring and summer here in Texas.  Southerly winds carry moisture up from the Gulf of Mexico, all the way through Texas and up into the plains.   The second ingredient, rapidly rising warm air, is also pretty easy to come by during spring and summer as the sun is able to heat the earth’s surface for an increasing amount of time each day.  This surface heating causes air to rise, expand and cool, then condense to forms clouds.  The last key ingredient, an unstable atmosphere, is a condition that exists when the lower layer of the atmosphere is warmer than the air above.

Once all the ingredients are in place, pockets of warm moist air rise up from the earth’s surface, expand, cool and condense into clouds.   Recall that warm air wants to rise and will keep rising as long as it’s warmer than the air around it.   As the cloud builds, the transformation of water vapor into liquid or solid cloud droplets creates heat (latent heat is the technical term), which makes the inside of the cloud even warmer than the air around it, allowing it to continue rising.   The rapidly rising air inside a thunderstorm, called an updraft, can be quite robust reaching speeds of over 100 mph in very severe storms.  The end result of this process can be a thunderstorm with cloud tops as high as 40,000 to 60,000 feet (8 to 12 miles).

While thunderstorms can occur at any time of the year, they are more prevalent during the spring and summer due to the increased amount of frontal systems moving down from Canada and through the plains.  These frontal boundaries trigger the lifting mechanisms in the atmosphere when cool air from the north collides with warm air from the south, thus kicking convection into action.  There also tends to be a greater temperature difference between air at ground level and air higher in the atmosphere, which gives the warm air lifting from the ground a lot more room to roam and potentially grow into a powerful thunderstorm.

The four largest threats from thunderstorms are lightning, hail, damaging winds (including tornadoes) and flash flooding.  According to the National Weather Service, lightning has killed an average of 39 people each year between  2001-2010.  Floods killed an average of 71 people each year, and tornadoes killed an average of 56 people each year for that same 2001-2010 time period.   According to the National Severe Storms Laboratory, damage from hail averages close to 1 Billion dollars each year, and damaging winds (classified as winds  exceeding 50-60mph) account for over half of the severe weather damage reports each year, even more frequently than tornadoes.

Thunderstorms can also be classified into several types, most of which you should be familiar with from listening to your local TV meteorologist.

•  Ordinary – These are the smaller, less severe storms that tend to pop up during the heat of the day, especially along the coast.  Growing up in Houston, I recall these always popping up during the summer at about 2pm, lasting for about 30 minutes, then dying out and leaving the air about a billion times more humid.
• Supercell – These are defined as severe, rotating storm systems lasting more than one hour.  These storms rotate counterclockwise due to wind shear which is wind flowing into the storm from different angles at increasing elevations.  This is the classification of thunderstorm most often associated with tornadoes and large damaging hail.
• Squall Line – a line of thunderstorms (ordinary or supercell) that extend for many, sometimes hundreds, of miles.   Sometimes on radar, these will appear to take on the shape of a bow and will be referred to as a “Bow Echo”.  This type of thunderstorm system is most often associated with wind damage, although all types of thunderstorms can cause damaging winds.
• Mesoscale Convective System (MCS) – a large circular complex of thunderstorms that can exceed hundreds of miles in diameter.  My personal, non-scientific, term for these are Messyscale Convective Systems because of the amount of rain they tend to produce.  Flash flooding is nearly always a major concern with these.

We hope you’ve enjoyed this overview of Thunderstorms and learned something new.  We are also planning in the near future to provide an overview of Tornadoes, hail and maybe an overview Hurricanes as we get closer to that portion of our summer season.