Psychrometer - An instrument used to measure the water vapor content of the air. The first pantented instrument that was coined a "psychrometer" was invented in 1818 by a German inventor named Ernst Ferdinand August.

Millibar - A millibar is a measure of atmospheric pressure taken by a barometer. The standard pressure measured at the surface is 1,013 millibars. Other units that are used for pressure measurements are inches of mercury and pascals. The lowest pressure readings that have occurred at the surface have been measured underneath intense hurricanes.

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.

Flood - A condition that occurs when water overflows the natural or artificial confines of a stream, river, or drainage ditch. Any ponding of water on what is normally dry land is also considered a flood. Floods can occur within minutes or hours of an excessive rainfall.

Saturation - An atmospheric condition whereby the level of water vapor is the maximum possible at the existing temperature and pressure. Air has a greater capacity for water vapor as the temperature increases. This is because air molecules become less densely packed when the temperature rises, thereby giving more room for water vapor to occupy. The air reaches saturation any time that the dew point and temperature are the same. This can lead to the formation of fog.

Deposition - The process where water turns directly from vapor to ice, without first becoming liquid. A common instance of deposition is when frost forms on the grass during cold mornings.

Climate - The average weather conditions for a specific location during a year or for many years. This can be thought of as the prevailing weather conditions that occur for a certain location in general, or over a long period of time.