Sometimes, the weather changes suddenly just before a storm lets loose. You may suddenly feel a weird calm and quiet before a storm hits, even though it may still be warm and sunny. This happens because of changes in the atmosphere as a storm builds up. Storm clouds pull in warm, moist air from all around as the storm system develops. The air travels up through the clouds and then shoots right out over the top of the cloud, and then back down again. On the way down, the air becomes warmer and drier.
When that warm, stable (calm) air spreads over an area, it makes the rest of the air stable. This is why it is sometimes very calm just before a hurricane or thunderstorm. It’s hard to predict whether a storm system coming up will start suddenly and without warning, or whether there will be calm before the storm.