A volume of space that is essentially empty of matter, such that its gaseous pressure is much lower than atmospheric pressure.

The condition produced by a change in wind velocity (speed and/or direction) with height. Vertical wind shear can weaken or destroy a tropical cyclone by interfering with the symmetric nature and organization of the deep convection around the cyclone centre. Source: Canadian Hurricane Centre

The greatest distance an observer can see and identify prominent objects. Source: NOAA-NWS SRH

This type of evacuation is targeted toward people most vulnerable to the threat, including offshore workers, persons on coastal islands and other flood prone areas, and other special populations having particularly long lead-time evacuation requirements. This also includes people in harm’s way from other events. No special traffic control or transportation measures are usually taken during voluntary evacuations, and people may remain if they so choose. Source: US DOT

An atmospheric feature that tends to rotate. It has vorticity and usually has closed streamlines. Source: NOAA-NWS ERH

The location of the center of a tropical or subtropical cyclone obtained by reconnaissance aircraft penetration, satellite, radar, or synoptic data. Source NOAA-NHC.

A measure of the local rotation in a fluid flow. In weather analysis and forecasting, it usually refers to the vertical component of rotation (i.e., rotation about a vertical axis) and is used most often in reference to synoptic scale or mesoscale weather systems. By convention, positive values indicate cyclonic rotation. Source: NOAA-NWS ERH