Hoar Frost - A deposit of interlocking ice crystals onto vegetation or any object that has a surface temperature below the dew point. If the surface of the object cools below the dew point, but not below freezing, dew will form instead. When there is ample moisture in the air, hoar frost can be so thick that it can sometimes look like snow.

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.

Wall Cloud - A localized, persistent, often abrupt lowering from a rain-free base of a cumulonimbus cloud. The location of the wall cloud marks the location of a strong updraft feeding the storm. Therefore, many wall clouds exhibit rapid upward motion and sometimes rotation. However, not all wall clouds rotate. If the wall cloud is seen rotating, the formation of a tornado is possible.

Knots - The unit meteorologists use to measure winds speeds. Knots are the common unit pilots and sailors use when gauging wind speeds. A knot is based on nautical miles rather than statute miles. A wind speed of 1 knot is equal to 1.15 mph.

Squall Line - A line of intense thunderstorms that are accompanied by high winds and heavy rain. Squall lines typically move through quickly and oftentimes occur along or ahead of a fast moving cold front. These lines can sometimes extend for hundreds of miles.

Polar Vortex - The polar vortex is an area of low pressure that coincides with the pool of arctic air that encircles the North or South Pole. Each fall, the polar vortex begins to form due to decreasing daylight near the poles. By mid-winter it becomes more influential to the weather patterns in the mid-latitudes.

Wall Cloud - A localized, persistent, often abrupt lowering from a rain-free base of a cumulonimbus cloud. The location of the wall cloud marks the location of a strong updraft feeding the storm. Therefore, many wall clouds exhibit rapid upward motion and sometimes rotation. However, not all wall clouds rotate. If the wall cloud is seen rotating, the formation of a tornado is possible.