Although not exclusively nautical, this has been attributed to seagoing folk as a result of their constant and intimate interaction with the weather. Although not known at the time, an approaching storm will drop the barometric pressure, creating a low directly ahead of the storm front. If a storm comes from a direction that is opposite to the prevailing winds, the prevailing breezes will eventually be overcome by the storm front. Just before this happens, however, there will be an equalization of wind speed from two opposing directions resulting in an absence of any wind. The meaning is not lost on landlubbers: Before someone explodes in anger, they almost invariably become overly quiet and, in some instances, even tranquil.
Have you ever spent an afternoon in the backyard, maybe grilling or enjoying a game of croquet, 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 why do whip-like winds, dropping temperatures and rumbling thunder sometimes precede storms instead of a peculiar and eerie calm? Do you want a hint at what might be at the root of this old sailors' adage? Think of all the different types of storms you've seen -- one variety of storm can have a different effect on the atmosphere than another. There are brief thunderstorms that rattle through like a couple of rowdy frames at the bowling alley, and there are long, tumultuous downpours that drown the streets. And then there are the strongest of all, like massive, violent hurricanes or spinning, furious tornadoes. All these different manifestations of intense weather happen because the interactions in the atmosphere can unfold in a variety of different ways, to vastly different effects. 
So, if the weather is calm and fair for your backyard barbeque, does that mean you should make backup plans indoors?Answer:
So, is there a calm before the storm? You may have already guessed the answer. Sometime there is; sometime 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.
In medieval England there were nomadic mercenaries who wandered the countryside and would sell their services to the highest bidder. These were hardened fighters who lived solitary lives in the wilderness. They did not have the luxury of servants to polish their armor and it would oxidize to a blackish hew, E and the 18 to be known as black knights. At local town festivals they would have an exhibition jousting matches in which the winner of the fight would win the losers weapons and armor. The local gentry softened by the good life, would lose to these black knights. The nomadic knights didn't have much use for an extra set of armor and would sell it back to them immediately after the fight. The losing nobility would be forced to buy back their armor and this after the market came to be known as the black market.
