Hygrometer - An instrument designed to measure the water vapor content and relative humidity in the atmosphere. Leonardo da Vinci built the first version of a hygrometer back in the 15th century.

Astronomical Twilight - Is defined to begin in the morning, and to end in the evening when the center of the Sun is geometrically 18 degrees below the horizon. Before the beginning of astronomical twilight in the morning and after the end of astronomical twilight in the evening, scattered light from the Sun is less than that from starlight and other natural sources. Before and after astronomical twilight, the sky is absolutely dark.

Sferics - Radio waves that are produced in the atmosphere by lightning strikes. When lightning strikes, a disturbance in the electric field occurs, which results in a radio wave being emitted. This concept is used in ground-based lightning detection systems.

Microburst - A strong and localized downdraft that occurs beneath a strong to severe thunderstorm. Microburst's occur on a small scale, as most are smaller than a mile wide in diameter. These localized, intense winds can produce major damage and typically last for a matter of seconds to several minutes.

Black ice - A thin coating of clear ice that can form on roadways. It is most common during the winter season and is called black ice because the dark road surface can be seen through it.

Fahrenheit - Temperature scale developed by the German-born scientist Daniel Gabriel Fahrenheit. The Fahrenheit scale bases the boiling point of water at 212 and the freezing point at 32. The U.S. and a few Caribbean countries still use the Fahrenheit scale. All other countries use the Celcius scale.

Supercell Thunderstorm - A relatively long-lived, large and intense storm cell characterized by exceptionally strong updrafts that can exceed 100 mph. Supercell thunderstorms have updrafts that rotate, which gives them the ability to remain intact for long periods of time. Most of the strong to violent tornadoes that develop are associated with supercells.