What Happens to Barometric Pressure When It Rains?

Barometric pressure almost always falls before and during rainfall, then climbs back up once the rain passes. This happens because most rain is produced by low-pressure weather systems, where air rises, cools, and releases its moisture as precipitation. The drop can be modest, just a few millibars over many hours for a gentle frontal rain, or steep and sudden ahead of a severe thunderstorm. But the basic pattern is consistent enough that a falling barometer has been one of the oldest and most reliable informal weather-forecasting tools for centuries.

Why Pressure Falls When Rain Is on the Way

To understand why rain and low pressure travel together, think about what a low-pressure system actually is. It is a region of the atmosphere where air is rising. As air rises, it leaves less mass pressing down on the surface, so the pressure reading at ground level drops. That rising air also cools as it gains altitude, and cooler air holds less moisture. Eventually the water vapor condenses into cloud droplets and, if there is enough moisture and enough lift, those droplets grow heavy enough to fall as rain.

So rain does not cause low pressure, and low pressure does not exactly cause rain. They are both consequences of the same process: large-scale upward motion in the atmosphere. A mid-latitude cyclone, the kind of sprawling storm system that sweeps across much of North America and Europe, is the textbook example. Warm, moist air from the south rides up over colder air from the north along frontal boundaries. The lifting triggers condensation, cloud formation, and precipitation. Meanwhile, the removal of air from the surface layer means your barometer reads lower and lower as the storm center approaches.

This is why the barometer starts falling well before the first raindrops arrive. The pressure drop reflects the approaching low-pressure center, whose influence stretches hundreds of kilometers ahead of the actual precipitation. By the time rain reaches you, the pressure may have been dropping for twelve hours or more.

How Much Does Pressure Actually Change?

On a calm, clear day, sea-level barometric pressure hovers around 1013 millibars (also written as hectopascals, or hPa, which are the same unit). A typical frontal rain event might pull the pressure down by 10 to 20 millibars over the course of a day or two. A strong mid-latitude cyclone can push it below 980 millibars. Hurricanes and typhoons go further still; the most intense tropical cyclones on record have had central pressures below 900 millibars.

For everyday rain, though, the changes are less dramatic. You might see a drop of 5 to 15 millibars from fair-weather baseline to the lowest reading during a rainy period. That does not sound like much in absolute terms, but it represents a measurable shift in the weight of the entire column of air above you, and it is more than enough to show up clearly on any decent barometer or weather app.

The rate of the drop matters as much as the total size. A slow, steady decline of a few millibars over many hours usually means a broad, gentle low-pressure system is approaching, likely bringing steady, prolonged rain. A fast drop, several millibars in just an hour or two, signals something more intense: a rapidly deepening storm, possibly with strong winds and heavy downpours.

What Happens During and After the Rain

The lowest pressure reading generally occurs around the time the center of the low-pressure system passes closest to your location. In a classic frontal system, this timing roughly coincides with the transition between the warm front’s steady rain and the cold front’s heavier, more showery precipitation. Once the storm center moves past you, the barometer begins to rise. Cold, dense air rushes in behind the system, pressing down on the surface and pushing the pressure back up.

This post-storm pressure rise often happens faster than the pre-storm drop. You can sometimes watch the barometer climb several millibars in just a few hours after the rain ends and the skies start to clear. Weather enthusiasts call this pattern “falling, bottoming out, rising” and it is remarkably consistent across most rain events driven by mid-latitude cyclones.

The rain itself does not stop because the pressure rises. Rather, the rising pressure reflects the arrival of a different air mass, one that is cooler, drier, and sinking rather than rising. Since sinking air suppresses cloud formation and precipitation, the rain fades as the higher-pressure air mass takes over.

When Rain Arrives Without a Big Pressure Drop

The “low pressure equals rain” rule has exceptions, and they are common enough to be worth knowing about. Summer afternoon thunderstorms are the most familiar one. On a hot, humid day, the sun heats the ground, the ground heats the air above it, and that warm air rises in strong, localized updrafts. Towering cumulonimbus clouds build quickly and dump heavy rain, sometimes with hail and lightning, in a burst that lasts thirty minutes to an hour. The barometric pressure may dip only slightly during these storms, and the dip is localized and brief because there is no large-scale low-pressure system driving the event. Your barometer a few kilometers away might not budge at all.

Orographic rain is another exception. When moist air is forced upward by a mountain range, it cools and drops its moisture on the windward slope. This can produce persistent, heavy rainfall without any significant drop in barometric pressure at sea level, because the lifting is mechanical (the mountain pushes the air up) rather than driven by a low-pressure circulation.

Tropical rain showers in equatorial regions also sometimes break the pattern. Near the equator, daily convective cycles driven by intense solar heating can produce rain with minimal surface-pressure variation. The pressure oscillations that do occur are often dominated by a regular twice-daily tide driven by solar heating of the upper atmosphere, not by passing storm systems.

How Latent Heat Deepens the Storm

There is a feedback loop at work in rain-producing low-pressure systems that makes them self-reinforcing, at least up to a point. When water vapor condenses into cloud droplets, it releases latent heat, the energy that was absorbed when the water originally evaporated from a lake, ocean, or wet surface. That released heat warms the surrounding air, which makes it more buoyant, which causes it to rise faster, which lowers the surface pressure further, which draws in more moist air from the surrounding area, which leads to more condensation, more heat release, and even lower pressure.

Research on how moisture interacts with storm development has shown that condensation and latent heat release significantly accelerate the growth of mid-latitude cyclones compared to what dry models would predict. The condensation occurs along frontal boundaries and wraps around the developing cyclone, consistent with what meteorologists observe in real storms. This process also creates an asymmetry: low-pressure systems become more intense than their high-pressure counterparts because they benefit from this extra energy source, while high-pressure systems, dominated by sinking dry air, do not.¹

This feedback is the reason why storms over warm oceans can intensify so rapidly. Tropical cyclones are the extreme case: they are essentially heat engines powered almost entirely by latent heat release from condensation. But even ordinary mid-latitude rain events benefit from this mechanism. A storm system moving over the Gulf of Mexico, for instance, picks up moisture and deepens more than an identical system moving over dry land, simply because there is more water vapor available to condense and release energy.

What Your Body Might Notice

Many people report that they can “feel” a storm coming, often describing headaches, joint aches, or a general sense of heaviness before it rains. For a long time, this was dismissed as folklore, but there is now real evidence connecting barometric pressure changes to certain kinds of pain.

A study examining atmospheric pressure fluctuations and migraine occurrence found that migraines occurred most frequently when atmospheric pressure dropped by 6 to 10 hPa relative to the standard atmospheric pressure.² That range of pressure change is well within what a typical frontal rain system produces, which means the common experience of getting a headache before it rains has a plausible physiological basis. The mechanism is not fully settled, but one leading explanation is that falling external pressure allows slight expansion of blood vessels or sinus tissues, which can trigger pain in susceptible people.

The connection extends beyond migraines. Research on fibromyalgia patients found that lower barometric pressure and increased humidity were both significantly associated with greater pain intensity and pain unpleasantness.³ Since rainy weather delivers both of these conditions simultaneously, people with fibromyalgia may experience a double hit. The same study found that barometric pressure was also associated with stress levels, independently of humidity.

If you are someone who swears your knees or your head tell you when rain is coming, you are probably not imagining it. The pressure changes involved are small in absolute terms, but for people whose pain sensitivity is already heightened, those few millibars of difference appear to be enough to cross a threshold.

How Animals Read the Barometer

Humans are not the only ones who respond to falling pressure before a storm. Animals across a wide range of species seem to detect and react to barometric pressure changes, often in ways that improve their chances of surviving bad weather.

In laboratory experiments, white-crowned sparrows exposed to declining barometric pressure responded by eating more, even though nothing else about their environment changed. The pressure drop did not affect their metabolic rate or stress hormones; it specifically triggered increased food intake, as if the birds were stocking up calories before a storm that would make foraging difficult or dangerous.⁴ The researchers proposed that pressure-sensing ability is likely common in wild vertebrates, especially small-bodied animals for whom a single storm can be a life-threatening event.

Marine animals show similar responses. A group of juvenile blacktip sharks being tracked in a shallow coastal nursery area moved to deeper water as barometric pressure fell with the approach of Tropical Storm Gabrielle. The behavior was consistent across all the sharks being monitored, and every one of them returned to the shallow nursery after the storm passed.⁵ The researchers concluded this was innate behavior, not a learned response, since the sharks were less than a year old.

The exact sensory mechanisms involved vary by species and are still being studied. Fish and sharks may sense pressure changes through their swim bladders or lateral line systems. Birds may detect pressure through specialized receptors in the middle ear or through air sacs connected to their respiratory system. The common thread is that a falling barometer is such a reliable predictor of dangerous weather that multiple evolutionary lineages have independently developed the ability to detect it and respond preemptively.

Reading a Barometer for Weather

If you have a barometer at home or a weather app that shows barometric pressure, the readings are more useful than you might think, and the interpretation is simpler than most people assume. A falling reading means the atmosphere above you is becoming less dense, usually because a low-pressure system is approaching. If the reading has been falling steadily for several hours and the forecast mentions rain, trust both of them. If the barometer is rising, fair weather is usually on the way or already arrived.

The absolute number matters less than the trend. A barometer reading of 1005 millibars that has been steady for two days means something very different from a reading of 1005 millibars that was 1020 yesterday. The first scenario might just be your local average. The second scenario means a significant low-pressure system has moved in, and rain is likely either falling already or imminent.

Sharp drops deserve attention. If your barometer drops more than about 5 millibars in three hours, that is a fast-moving or rapidly intensifying storm. This does not mean you need to head for a shelter, but it does mean the coming weather is likely to include strong winds and heavy rain rather than a gentle drizzle. Fishers and sailors have relied on this rule of thumb for generations, and it remains as valid as ever in the age of satellite forecasting.

Why Humidity and Pressure Are Not the Same Thing

A common misconception is that humid air is heavier than dry air, so rainy days should have higher pressure, not lower. The opposite is actually true. Water vapor molecules are lighter than the nitrogen and oxygen molecules that make up most of the atmosphere. When water vapor replaces some of the nitrogen and oxygen in an air parcel, the parcel becomes slightly less dense. So very humid air at the same temperature and altitude actually exerts slightly less pressure than dry air.

This effect is small on its own, nowhere near enough to explain the pressure drops associated with rain. The dominant factor is still the large-scale rising motion in a low-pressure system. But the humidity misconception trips people up because it seems so intuitive that moisture-laden air should be “heavier.” If you have ever walked outside on a humid summer day and felt like the air was oppressive and heavy, that feeling comes from your body’s inability to cool itself through evaporation, not from any actual increase in air pressure. In fact, the barometer is often slightly lower on those muggy days than on crisp, dry ones.

Understanding this distinction helps clear up another related confusion. People sometimes see a rising barometer paired with rain and think the rule is broken. What is usually happening is that the rain is trailing behind the cold front, falling from clouds that were generated earlier in the warm, moist air mass, while the cold, dry air mass that is now raising the barometer has already arrived at the surface underneath. Weather is three-dimensional, and what is happening at cloud level can lag behind what is happening at the ground by hours.

Pressure Patterns in Different Types of Storms

Not all rain-producing systems create the same barometric signature. Mid-latitude cyclones, as discussed above, produce a broad, gradual pressure drop followed by a broad rise. But other storm types have their own characteristic patterns.

Tropical cyclones produce the most dramatic pressure drops in nature. A hurricane making landfall can drop local pressure by 50 millibars or more over just a few hours as the eye wall approaches. If the eye passes directly overhead, the pressure drops rapidly, hits a minimum during the calm of the eye, and then rises sharply as the back side of the eye wall arrives with renewed heavy rain and wind. People who have been through a hurricane’s eye often describe an eerie quiet, followed by a barometric reading lower than their instrument has ever shown.

Squall lines and mesoscale convective systems, the organized bands of thunderstorms that sweep across the Great Plains and Midwest, produce a characteristic pressure pattern called a “mesohigh.” As the cold downdraft from the thunderstorm hits the ground and spreads out, it actually causes a brief, sharp rise in pressure at the surface directly under the storm. So during the heaviest rain from a squall line, the pressure may momentarily spike upward before dropping again as the system passes. This is one of the situations where the simple “low pressure equals rain” rule genuinely breaks down at short time scales, even though the broader system driving the squall line is still a low-pressure trough.

Monsoon rains present yet another pattern. In regions affected by seasonal monsoons, the pressure shift happens on a timescale of weeks to months rather than hours. The Asian summer monsoon, for example, is associated with a persistent thermal low over the continent that draws moist ocean air inland. Barometric pressure across the affected region stays generally lower throughout the wet season, with individual rain events superimposed as smaller fluctuations on top of that seasonal baseline. A barometer in Mumbai reads lower in July than in January by a wide margin, and that difference is inseparable from the fact that July is the peak of the monsoon rains.

Smartphones and Modern Pressure Sensing

Most modern smartphones contain a barometric pressure sensor. It was originally included primarily to improve altitude estimates for GPS navigation, since pressure decreases with altitude in a predictable way. But weather apps now expose the raw pressure data to users, which means millions of people have a surprisingly accurate barometer in their pocket without knowing it.

The resolution of these phone sensors is good enough to detect the pressure changes associated with frontal weather systems. If you track your phone’s barometric readings over a day when rain is forecast, you can watch the slow decline for yourself, often beginning 12 to 24 hours before precipitation starts. Some weather apps plot this trend automatically and flag rapid drops as storm indicators.

Crowd-sourced pressure data from phones is also starting to improve weather forecasting itself. Traditional weather stations are spaced relatively far apart, especially over oceans and in developing countries. But smartphones in the pockets of thousands of people provide a much denser pressure-observation network. Several research groups and weather services have begun incorporating this data into forecasting models, which could improve predictions of local pressure changes and, by extension, rainfall timing.