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.

Jet Stream - Intense winds that are concentrated in a relatively narrow stream in the upper atmosphere. Most jet streams are found at around 30,000 ft up in the atmosphere. The two jet streams that impact the US are the polar and subtropical. Both act as highways for weather systems to travel along.

Air mass - A generic term for describing areas of the atmosphere that have similar origins, along with similar temperature and moisture qualities. The two main types of air masses are tropical and polar. Polar air masses are typically cold and dry, while tropical air masses are inherently warm and humid.

Doppler Effect - The shift in frequency that occurs when an object goes from moving towards an observer to away from an observer. Everyone has heard this when a police car drives by. As the police car is approaching, the siren will have a high pitch. Then when the police car begins moving away from you the pitch of the siren lowers. This effect is what weather radars use to calculate velocity.

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.

Thermosphere - The atmospheric shell extending from the top of the mesosphere to outer space. It is a region of more or less steadily increasing temperature with height, starting at around 50 miles above the surface. This layer is around 320 miles thick and temperatures in this layer can reach 4,500 F. The International Space Station orbits the Earth in this region of the atmosphere!

NCEP - NCEP stands for National Center for Environmental Prediction. This is a branch of the National Weather Service located in College Park, MD. NCEP is responsible for developing, running, and maintaining the main American computer models.