Barometric Pressure
Also called “air pressure” or “atmospheric pressure,” this is the weight of the air (actually the number and speed of air molecules) measured by a barometer, usually in inches of mercury or millibars. A raise of barometric pressure indicates increasing pressure and a fall indicates decreasing air pressure. In general, at sea level, thanks to gravity and lots of stacked molecules, the pressure is about 29.92 inches – the starting point for measuring. The higher you go, the less pressure. Air pressure is an important tool in forecasting. Changes can precede winds, clouds or clearing of the sky. A rising pressure often means fair weather while falling pressure means storms on the way. Very low pressure occurs with severe storms. The lowest air pressure recorded was 25.36 inches in 1979 ahead of Typhoon Tip in the western Pacific ocean. Minnesota recorded its lowest pressure – 28.21 inches in Big Fork – in October 2010.
Bombogenesis
This happens when air pressure drops extremely during development of a mid-continent cyclone. Remember the October 2010 storm? That was a bombogenesis-style land bomb.
Derecho
A widespread, long-lived wind storm linked with bow-shaped bands of showers or thunderstorms. Recent local derechos: July 1999 straight-line winds that leveled acres of trees on the Ontario-Minnesota border; 114-mph winds across northern Wisconsin in July 1977 and winds that a Bayfield church roof off in 2006.
Rip Current
Rip currents form when conditions cause waves to disperse unevenly along the beach and water becomes trapped and converges into a narrow, river-like channel moving away from the shore at speeds of 1 to 8 feet per second. It’s like a sudden runoff from shore and generally will not last long. Those caught in it can swim out of it by paralleling the shore or waiting until it subsides, though it may carry a person out from shore.
Seiche
This occurs when atmospheric pressure on one part of the Lake pushes water toward another – like sloshing water in a bowl. It can quickly change the water level by a few inches or feet.
Water Spout
These watery twisters descend from a cumulus cloud and can form in humid conditions when cold air moves over warmer water.