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.

Valley Breeze - A breeze that flows up a mountainside during the day. This upsloping breeze occurs due to the fact that the valley walls warm up faster than the higher terrain in the mountains. This makes the air in the valleys lighter than the air along the higher terrain of the mountains, making the air near the valley buoyant. The result is the upward breeze along the mountainsides during the day.