揭秘“暴风雨前的宁静”

Have you ever spent an afternoon in the backyard, maybe grilling or enjoying a basketball game, when suddenly you notice that everything goes quiet? The air seems still and calm—even the birds stop singing and quickly return to their nests.

After a few minutes, you feel a change in the air, and suddenly a line of clouds ominously appears on the horizon—clouds with a look that tells you they aren’t fooling around. You quickly dash in the house and narrowly miss the first fat raindrops that fall right before the downpour. At this moment, you might stop and ask yourself, “Why was it so calm and peaceful right before the storm hit?”

It’s an intriguing phenomenon that people have recognized for centuries, but what on earth causes this calm? And there is an old phrase “calm before the storm”, often used in its figurative meaning—a quiet period just before a great activity or excitement. According to our own experience, we know there is literally calm before the storm. But what causes this calm? And is it always calm before the storm? Let’s hear what scientists have to say.

The answer is sometimes there is; sometimes there isn’t. Under the right conditions, an eerie or peaceful calm will befall your picnic before a storm moves in. Other storms skip the calm and proudly announce their presence by instantly unleashing bad weather.

A period of calm happens in a particular kind of storm, the simplest kind of storm—a single-cell thunderstorm[1]. In this type of thunderstorm, there is usually only one main updraft, which is warm, humid air and drawn from places near the ground. Storms need warm and moist air as fuel, so they typically draw that air in from surrounding environment. Storms can draw in the air that fit their need from all directions—even from the direction in which the storm is traveling.

As the warm, moist air is pulled into a storm system, it leaves a low-pressure vacuum[2] in its wake. The rising air meets the cold dry air that has already existed in the storm clouds, thus the temperature of the warm air drops, and the water vapor in it condenses[3] into tiny droplets that are a precondition of rain. These droplets clump up and build on larger particles like dust, until they grow large enough to form raindrops.

This warm, moist air keeps moving upwards, but it becomes colder and drier during its trip through cloud which involves cooling and condensation. When it reaches the top of the cloud mass, the air gets spit out at the top. This air is sent rolling out over the big, anvil[4]-shaped head of the thunderclouds. From there, the air descends—drawn back toward lower altitudes by the very vacuum its departure created in the first place. Warm and dry air is relatively stable, and once it blankets a region, it stabilizes that air in turn. This causes the calm before a storm.

Most thunderstorms, though, don’t start with a calm. That’s because most are actually clusters of storms with complex wind patterns. There’s so much air moving up and down in the vicinity[5] of these storm clusters that the calm before the storm never happens. And instead, before the storm, it might be really windy!