Bombogenesis is cyclogenesis taken to the extreme. Bombogenesis is defined as a mid-latitude cyclone that drops in surface barometric pressure by 24 or more millibars in a 24-hour period. The height contours pack around the center of rotation and the number of height contours increases rapidly in the developing stages. The most common time of the year for bombogenesis to occur is in the cool season (October to March) when the temperature gradient is large between the high and mid-latitudes. Bombogenesis typically occurs between a cold continental air mass and warm ocean waters or between a cold polar air mass and a much warmer air mass. Many Nor-easters are the product of bombs. The contrast in temperature between polar air spilling over the eastern U.S. and the warm Gulf Stream waters sets the stage for cyclogenesis on the boundary between these air masses.