Advection Fog - Advection fog often looks like radiation fog and is also the result of condensation. However, the condensation in this case is caused not by a reduction in surface temperature, but rather by the horizontal movement of warm moist air over a cold surface. This is very common during the winter when warm moist winds blow over cold snowcover. Advection fog may also form when moist maritime air masses drift over a cold inland area. This usually happens at night when the temperature of the land drops due to radiational cooling.

Arctic Oscillation - A climate phase that looks at the strength of the winds surrounding the arctic region. During a positive phase of the arctic oscillation, the ring of winds around the arctic remain strong, thereby holding the coldest air masses near the pole. During the negative phase these upper level winds weaken, allowing for the bitter cold arctic air masses to head further south across the northern hemisphere. Each switch in phase can last weeks or even months.

Contrail - A contrail is the condensation trail that is left behind by a passing jet plane or rocket. Contrails form when hot humid air from jet exhaust mixes with ambient air that is colder and drier. Contrails are composed of water droplets, and can therefore be thought of as a man-made cloud streak.

VIL - This is an abbreviation for "Vertically Integrated Liquid." This is a quantity that can be shown on a radar display. It is an estimate of the mass of precipitation in the air. It is given in units of kilograms per square meter. High VIL totals tend to indicate large hail or very heavy rainfall.

Civil Twilight - Is defined to begin in the morning, and to end in the evening when the center of the Sun is geometrically 6 degrees below the horizon. This is the limit at which twilight illumination is sufficient, under good weather conditions, for terrestrial objects to be clearly distinguished. During civil twilight artificial lighting is no longer required as long as clear skis are present. Only the brightest stars and planets can be seen by the naked eye during civil twilight.

Greenhouse effect - The term that describes how the Earth receives solar radiation and how it is essentially trapped in the Earth's atmosphere. The incoming solar radiation is absorbed at the surface and then radiates back up to space in the form of infrared radiation. However, the greenhouse gases in the atmosphere absorb some of this outgoing radiation and then re-radiate it back down to the surface. The higher the concentration of greenhouse gases, the more magnified this effect becomes.

Twilight - Before sunrise and again after sunset there are intervals of time during which there is enough natural light provided by the upper atmosphere, which does receive direct sunlight and reflects part of it toward the Earth's surface. Some outdoor activities may be conducted without artificial illumination during these intervals of twilight. It is useful to have some means to set limits beyond which a certain activity should be assisted by artificial lighting.