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.

Rain Gauge - An instrument used to measure the amount of rainfall that occurs over a specific site. A standard rain gauge can measure up to 8 inches. The first known device that was used for measuring rainfall was invented in Korea in 1441.

La Nina - La Nina is the cool phase of the El Nino-Southern Oscillation cycle. During a La Nina event, the east-central Equatorial Pacific Ocean becomes abnormally cool due to stronger than normal trade winds. La Nina events are a boon to the Peruvian fishing industry due to the nutrient rich waters that develop off the coast of western South America. Global weather patterns can also be influenced by the presence of a La Nina.

Seiches - Standing waves that oscillate back and forth over an open body of water. They are most common in large bodies of water such as bays or lakes. The impetus for a seiche to occur is persistent strong winds that push water from one end of the body of water to the other. When the winds let up, the water that has been pushed to one side of the bay or lake starts to oscillate to the opposite shore and then back again. Anyone who sloshes water from one side of a bathtub to the other side has witnessed a small-scale seiche. The Great Lakes occasionally see seiche events that are noted by a rhythmic rise and fall of water levels.

Arctic Front - The semi-permanent, semi-continuous boundary that separates the deep, cold arctic air from the shallower, less frigid polar air of the northern latitudes. Major shifts in the polar jet stream can drive arctic fronts southward into the mid-latitudes during the winter season.

Absolute Zero - A temperature of 0 on the Kelvin scale equates to -273 degrees C or -460 degrees F. This is theoretically the lowest possible temperature as there is no molecular motion at this temperature. Researchers at NASA have cooled sodium gas to just above absolute zero in a vacuum chamber.