Primer and Review on Thunderstorm Development
Since we are now entering the season when thunderstorms occur more often in southern New England, some of you may be interested in some of the causative factors (although already discussed to some extent above). This is a fairly lengthy discourse but may be of interest to a few of the curious.
Atmospheric Convection
Let’s first review a few basics. Thunderstorms constitute convective weather strong enough to produce the charge separation needed for lightning. Atmospheric convection is the upward motion induced by a column of warmer air (being lighter than cooler air) rising aloft. An example of convection is rising smoke from a campfire. For deep convection to occur, the rising air must remain warmer (and lighter) than the surrounding air for many thousands of feet. Now, the rising air itself cools because it is going from higher pressure near the earth’s surface to lower pressure aloft. If the rising air is moist enough, condensation (water vapor to the liquid water droplets of clouds) will take place. This is due to the fact that cooler air cannot hold as much moisture in water vapor form and eventually becomes saturated as cooling continues. One witnesses condensation on the surface of a cold glass of water on a humid day. Another example can be the formation of dew on the grass due to overnight cooling. An aspect of condensation from the laws of thermodynamics is that heat is released from water vapor when it condenses into liquid water. Note that the opposite happens (i.e., cooling) when liquid water evaporates into water vapor as exemplified by the cooling we feel when coming out of the ocean or swimming pool as the water on our skin evaporates. The connection to thunderstorms is that when sufficiently moist air reaches the condensation level, the release of that heat from condensation (“latent heat of condensation”) partially offsets the cooling of the rising air and causes that air column to be more buoyant.
When we say that the atmosphere is unstable on a particular day, that means conditions favor the rising air to keep rising to great heights. But how do we know that? The factor determining the stability of the atmosphere is the difference in temperature in the lower levels versus the upper levels of the atmosphere. That vertical difference in temperature is typically referred to as the “lapse rate.” Meteorologists refer to a large vertical temperature difference as a steep lapse rate. Anyone who has hiked to a higher elevation or looked at the temperature indicated outside of an aircraft is likely aware of the fact that the temperature typically cools with height (through the troposphere). How quickly the temperature cools with height (i.e., the steepness of the lapse rate) changes from day to day or even from one part of the day to the next (e.g. daytime heating steepens that lapse rate or vertical temperature difference). Now putting this together, we know that rising air cools, but it will continue to rise if the surrounding air (determined by that lapse rate) is cooler still. In other words, rising air will continue to rise as it cools as long as it remains warmer than the surrounding air. All of this can be quantified because the rate of cooling of rising air is set by thermodynamics (one rate before saturation and another lower rate of cooling after saturation) and the lapse rate can be determined by upper air measurements (from the usually twice daily release of balloons with radiosonde equipment attached).
I realize this is a lot of information to go through to just explain the concept of atmospheric instability, a driver of the convection process associated with thunderstorms. Hopefully, for those interested, this gives one a little clearer understanding of the basis for convective weather. One of course can find a lot of material on the internet and textbooks that provide greater information and helpful illustrations with respect to atmospheric instability/convection.
Severe Thunderstorms
What about severe versus non-severe thunderstorms? Sufficient moisture and instability (described above) are two main factors for thunderstorms. Since the instability is a function of how steep the lapse rate is, most thunderstorms (albeit there are exceptions) develop during the afternoon when the lapse rate is steepest (i.e., greater rate of cooling in the surrounding environment with height). In some circumstances (especially with stronger winds aloft), thunderstorms will continue to redevelop in the evening as outflow from one set of thunderstorms helps initiate another set of thunderstorm cells downstream. But what determines severity as defined by damaging wind gusts and/or large hail?
In a very unstable (and sufficiently moist) environment, strong updraft motions in single thunderstorm cells may lift huge quantities of supercooled (below freezing) liquid water (or combination of liquid water and ice) to great heights before the weight of the water brought aloft falls through the updraft and ends the life of that particular cell. However, that falling weight of the water aloft can produce a very localized area of damage when it reaches the ground in what is referred to as a “wet microburst.” We usually experience a number of these very localized wet microbursts in southern New England each warm season. Less frequently in southern New England but more frequently in the more arid western United States, “dry microbursts” occur when that falling water evaporates and causes evaporational cooling of the falling air, which becomes much heavier than the surrounding air and accelerates toward the ground.
Most of the severe thunderstorm events in southern New England involve another ingredient besides instability and moisture: “wind shear.” Vertical wind shear is essentially the difference in wind speed near the surface and aloft. Directional wind shear can be a factor as well. Vertical wind shear will tend to tilt the updrafts of thunderstorms and prevent the falling water from aloft “killing” the updraft. Instead, that downward wet air falls outside of the updraft, which can enable the storm to persist longer or at least long enough to generate subsequent thunderstorm cells from the initiation of new updrafts by the outflow. Still another and often important factor is that when winds are relatively strong aloft, the downdrafts will carry the momentum of those stronger winds to the surface. This can be a source of straight-line wind damage with the direction of the damage consistent with the storm motion and the direction of the stronger winds aloft.
Most commonly, our severe events in southern New England involve a combination of unstable air (steep lapse rates), a triggering uplift mechanism such as an approaching cold front, and sufficiently strong vertical wind shear. Typically, many of our severe events involve thunderstorm clusters or lines of thunderstorms that propagate to the east or southeast. On occasion, however, the thunderstorms will weaken as they approach the coast and ingest the cooler maritime air into the updrafts.
When the air mass is particularly unstable, especially strong updrafts may result in large hail. Large hail forms when layers of ice form on particles held high in the colder part of the storm due to an exceptionally strong updraft. However, if the freezing level is especially high, the hail may melt before reaching the ground.
Finally, high instability combined with strong vertical and directional wind shear can result in rotating supercells. These storms can be especially prolific in producing damage from straight-line winds, tornadoes, and/or large hail. That directional turning of the wind with height is referred to as helicity. A rough example would be southerly flow near the surface with stronger more westerly flow aloft, which causes a twisting about the vertical axis as air is ingested into the updraft. This can be a little challenging to visualize. When speaking in person, I will sometimes use a paper towel roll pointed to me (which I say is the storm with the updraft) with a wind toward me at the bottom of the roll (say from the south) and a perpendicular wind (say from the west) at the top of the roll. As that paper towel roll approaches me (the storm) it is lifted up with the rotation initially about a horizontal axis become about a vertical axis. That description is somewhat of an oversimplification and probably a little hard to grasp reading words but perhaps might make sense to one or two of you! Supercells account for a disproportionate amount of damage and can have relatively long life spans. The EF3 Springfield Tornado on June 1, 2011 and the F4 Worcester Tornado on June 9, 1953 and two examples of impressive supercells that produced violent tornadoes in southern New England.
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Message issued June 8, 2026 by:
Bob Thompson
Retired National Weather Service Meteorologist
Blue Hill Observatory and Science Center Board member
