Derechos and Squall Lines: Sudden Pressure Signatures

A squall line is a long, organised band of thunderstorms, often hundreds of miles from end to end, that moves as a single unit ahead of or along a cold front. When one passes overhead, the barometer does something it does at no other time: it jumps. A rise of three to six hPa inside ten to fifteen minutes is typical, sometimes more, produced by a dome of cold air spreading out beneath the storms. A derecho is the extreme version — a squall line that travels at least 400 miles producing widespread damaging wind, and whose pressure jump can exceed ten hPa. These are the fastest barometric changes in ordinary weather, and they are almost entirely invisible in a daily record.

If you have ever had a headache arrive with a summer storm and found nothing in your pressure log to explain it, this is the most likely reason.

What produces the jump

The mechanism is called the mesohigh, and it is genuinely different from the way a frontal low or a hurricane moves the barometer.

Inside a mature thunderstorm, precipitation falls through the storm and drags air down with it. That air cools further as rain evaporates into it, becoming denser than its surroundings, and it accelerates downward. On reaching the ground it spreads outward in all directions as a cold pool. When dozens of storms line up, their individual cold pools merge into a single continuous dome of dense air beneath the line, sometimes hundreds of miles long.

Dense air weighs more. A column of air with a layer of cold, heavy air at the bottom exerts more pressure on the surface than the same column did an hour earlier. The result is a shallow but intense area of high pressure moving along with the line — the mesohigh.

The leading edge of that cold pool is the gust front, and it is what you feel: a sudden wind shift, a temperature drop of ten to twenty degrees, a shelf cloud overhead if the geometry is right, and the pressure jump. All of it happens within a few minutes.

Behind the line, as the cold pool warms and the storms weaken, pressure often falls back — sometimes into a small area of low pressure called the wake low, which can produce strong wind of its own with no rain at all. The complete signature, from the pre-storm sag through the jump to the wake low, can be traced on a chart in a way that mirrors nothing else in the weather.

Derechos: the severe end

The term derecho was coined in 1888 by Gustavus Hinrichs, a physicist at the University of Iowa, from the Spanish word for straight — intended to contrast with tornado, from the Spanish for turned. The distinction he was drawing was between rotating wind damage and straight-line wind damage.

The current working definition requires a swath of wind damage extending at least 400 miles with wind gusts of at least 58 miles per hour along most of its length, plus several stronger gusts. In practice, a derecho is a squall line that becomes self-sustaining: the cold pool at the surface and the wind shear in the environment reach a balance in which the line continuously regenerates storms at its leading edge, and the whole system can travel for many hours across multiple states.

The pressure signature scales up accordingly. Where an ordinary squall line gives a three to five hPa jump, a strong derecho can produce eight to twelve, arriving in a matter of minutes, along with wind that does damage comparable to a moderate tornado but over a vastly larger area.

Where they happen

The corridor is well defined and largely continental.

The southern and central Plains are where many of these systems originate, typically forming in the afternoon along a dryline or a frontal boundary. Around Nixa, Missouri and across the Ozarks, the classic pattern is a line that develops upstream in the late afternoon and crosses after dark, which is when these events are most dangerous because people are asleep.

The Corn Belt and upper Midwest see the highest derecho frequency in the world. The August 2020 event that crossed Iowa in a few hours is the reference case in recent memory. Locations like Morris, Illinois and New Berlin, Wisconsin sit squarely in the corridor.

The Ohio Valley and Mid-Atlantic are the downstream end. Systems that form over the Midwest can survive the entire journey east, and the June 2012 derecho that ran from Iowa to the Atlantic coast left millions without power across the Washington region. Places like North Bethesda, Maryland experience these as an evening event arriving with very little warning after a day of extreme heat.

The Southeast gets squall lines in a different season. In central Georgia, around Milledgeville, the characteristic pattern is a late-winter or spring line racing ahead of a strong Gulf cold front, often at forty to fifty miles per hour.

Squall lines of the ordinary kind occur almost everywhere east of the Rockies. Derechos are concentrated in a belt from Oklahoma through the Great Lakes, with a secondary corridor across the southern states.

Why this matters for tracking

Here is the practical problem. Most people who track barometric pressure alongside their symptoms do it with a daily reading, or with weather-app data sampled hourly, or by looking back at a station record after the fact.

A squall line passage is invisible in all three. The pressure at nine in the morning and the pressure at nine at night might be nearly identical, with a six hPa spike sitting between them that lasted twenty minutes. Hourly data smooths it substantially. Even a station that records every five minutes will show the jump, but it will not show it if you are reading a daily summary.

So a person whose symptoms are triggered by rapid pressure change may go years concluding that pressure is not their trigger, because the events that actually affect them never appear in the data they are looking at. Their log shows storms without pressure changes, and they reasonably conclude that something else about the storm — humidity, ozone, light, noise — is responsible.

This is the specific case where continuous sampling changes the answer rather than merely refining it. A migraine tracker app recording every few minutes captures the jump; nothing coarser does.

What else arrives with the line

It would be a mistake to attribute everything about a squall line to the barometer. A gust front passage delivers several changes simultaneously:

• Pressure rises three to six hPa in minutes.
• Temperature drops ten to twenty degrees in the same window.
• Humidity rises sharply as the cold pool arrives.
• Wind shifts direction abruptly and gusts to damaging levels.
• Light falls dramatically under the shelf cloud, then the sky flickers with lightning.
• Sound — near-continuous thunder in a strong line.

Any of those could plausibly matter, and they are nearly impossible to separate from one another because they always arrive together. What a pressure log can do is establish the timing precisely — whether the symptom began before the line arrived, at the moment it crossed, or an hour later — and that timing is genuinely informative about which of the candidates is worth investigating.

Practical steps

The forecast lead time for squall lines is much shorter than for a winter storm. Convective outlooks identify the risk area a day or two ahead with reasonable skill, but the specific timing and track of a line often only become clear a few hours out.

What that supports is a different kind of planning. Rather than rescheduling a week, the useful move is to know that a given afternoon or evening carries a severe risk, and to be somewhere sensible when it arrives with medication at hand rather than in the car.

Checking the barometric pressure forecast alongside the day's convective outlook gives a fuller picture than either alone. Track it with Pressure Pal.

FAQ

How much does barometric pressure change when a squall line passes?

A rise of three to six hPa within ten to fifteen minutes is typical for an ordinary squall line. A strong derecho can produce eight to twelve hPa. This is a rise, not a fall, which surprises people who associate storms with falling pressure — the mesohigh beneath the storms is genuinely an area of high pressure.

What is the difference between a derecho and a squall line?

Scale and persistence. A derecho is a squall line that meets specific criteria: a damaging wind swath of at least 400 miles, with widespread gusts above 58 miles per hour. Most squall lines are not derechos. Every derecho is, structurally, a squall line.

Is a derecho more dangerous than a tornado?

They are different risks. A tornado is far more intense at any given point but affects a narrow path. A derecho produces weaker wind over an enormously larger area, so total damage and the number of people affected can be greater, and power outages can last days across several states.

Why does pressure rise rather than fall in a thunderstorm?

Because the dominant effect is the cold pool — dense air from downdrafts spreading out under the storm and increasing the weight of the air column. There is often a small pressure fall just ahead of the line as warm air is drawn in, and sometimes a wake low behind it, but the passage itself is a rise.

Can I see this in my weather app?

Usually not. Most consumer weather apps display hourly or three-hourly pressure, which smooths a fifteen-minute jump into near-invisibility. You need continuous logging, either from a personal barometer or from an app sampling at intervals of a few minutes, to see the actual shape.

When is derecho season?

Late spring through summer, with the peak from late May through early August. Derechos require the combination of instability and wind shear that is most common in the warm season across the central United States. Squall lines, more broadly, occur year-round, with a distinct late-winter and spring maximum in the Southeast.

I get headaches with summer storms but my pressure log shows nothing. What should I do?

Switch to continuous sampling before concluding pressure is not involved. A daily or hourly record will not show a squall line passage. If continuous logging still shows nothing lining up with your symptoms, then the other things that travel with a storm — humidity, light, noise, disrupted sleep, and the anxiety that can accompany severe weather warnings — become the more likely candidates, and they are worth tracking in their own right.