Shelf Cloud - Also known as a roll cloud. A low, horizontal, sometimes wedge-shaped cloud formation associated with the leading edge of a thunderstorm's outflow winds or gust front. Shelf clouds normally stretch across the entire horizon as an organized line of storms approaches. When a shelf cloud is observed, prepare for a quick increase in winds. A severe line of storms can produce shelf clouds that indicate incoming straight-line winds of over 60 mph.

Rain Gauge - An instrument used to measure the amount of rainfall that occurs over a specific site. A standard rain gauge can measure up to 8 inches. The first known device that was used for measuring rainfall was invented in Korea in 1441.

Condensation funnel - A condensation funnel is composed of water droplets and rotates while it extends down from the base of the thunderstorm. When it reaches the ground, it is called a tornado. If it stays above the ground, it remains a funnel cloud.

Planetary Boundary Layer - Also known as the Atmospheric Boundary Layer, it is the lowest part of the atmosphere and its behavior is directly influenced by its contact with the surface. The depth of the planetary boundary layer changes from day to night. During the day, the sun heats up the surface resulting in rising air currents that raise the height of the boundary layer. At night, the height of the boundary layer falls very close to the surface.

Fujita Scale or F-Rating - A scale that uses visual damage to determine the strength of a tornado. The Fujita scale was developed by Dr. Fujita in 1971 while he was conducting research at the University of Chicago. The scale goes from 0 to 5, with 0 being weak and 5 being violent. The Fujita scale was decommissioned in the US in 2007 because it didn't fully account for different structural types. The enhanced Fujita scale is now being used to rate tornadoes.

Multi-Vortex Tornado - A tornado made up of several smaller funnels. Most, but not all, violent tornadoes are of this variety. The winds in subvortices can be over 100 mph stronger than the winds in the main tornado circulation. This is likely the reason that damage from tornadoes can vary greatly along its path.

Forecast - A prediction of coming events or conditions. Forecasts are commonly used in business and weather.