科学揭秘“暴风雨前的宁静”_大学英语阅读

科学揭秘“暴风雨前的宁静”

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.