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.

Vorticity - Any clockwise or counterclockwise air circulation that occurs at any layer of the atmosphere. In meteorology, positive vorticity occurs with counterclockwise circulations, while negative vorticity occurs with clockwise circulations. Meteorologists can detect where upward vertical motions in the atmosphere will occur by diagnosing where positive vorticity will propagate.

Dry line - A boundary that separates a moist air mass from a dry air mass. When a dry line propagates through an area the dew point will abruptly fall off, but the temperature will change little. During certain weather patterns, dry lines can be a catalyst for thunderstorm development.

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.