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.

Radiation Fog - This type of fog forms at night under clear skies with calm winds. Under these conditions heat that was absorbed by the earth's surface during the day is allowed to radiate back into space. As the earth's surface continues to cool, provided a deep enough layer of moist air is present near the ground, the humidity will reach 100% and fog will form. Radiation fog varies in depth from 3 feet to about 1,000 feet and is always found at ground level and usually remains stationary. This type of fog can reduce visibility to near zero at times and make driving very hazardous.

Advection - The transfer of an atmospheric property by wind. The most common entities that meteorologists look at when diagnosing areas of advection are temperature and moisture.

Eddy viscosity - The turbulent transfer of momentum by eddies giving rise to internal fluid friction. Eddy viscosity is an important concept in turbulence theory.

Hydrosphere - The hydrosphere is composed of the sum of all the water on Earth. This includes all the Earth's surface waters such as oceans and lakes, as well as all sources of frozen water. Water vapor and therefore clouds are also included in the hydrosphere.

Overshooting top - A dome-like protrusion above a thunderstorm anvil, representing a very strong updraft and hence a higher potential for severe weather with that storm. A persistent and/or large overshooting top is typically seen in a type of thunderstorm known as a supercell. These types of thunderstorms have a longer lifespan and can produce high winds, hail, and isolated tornadoes. A storm with a short-lived overshooting top, or one that forms and dissipates in cycles, may indicate the presence of a pulse storm. Pulse storms are known for quick intensification and weakening cycles during their lifetime.

Pluie - This is the french word for rain. To say, "it is raining today" one would say, "Il pleut aujourd'hui."