“Hurricane” is a term for intense, tropical atmospheric vortices with winds stronger than 75 mph in the western hemisphere (North Atlantic, Gulf of Mexico, Caribbean, or North Pacific east of the International Dateline). In the North Pacific west of the International Dateline, similar systems are called “Typhoons”. In the Northern Indian Ocean and throughout the southern hemisphere they are called “Tropical Cyclones” or simply “Cyclones”. “Tropical Cyclone” is the generic term that encompasses Hurricanes, Typhoons, Cylones and tropical storms (winds between 40 and 75 mph) regardless of location. Members of the international tropical-cylone in-crowd advertise their membership by using the abbreviation “TC”.

The "tropopause", at 16-18 km altitude in the tropics, is the boundary between the troposphere, where most weather happens and the stratosphere, which is more tranquil. Nearly 90% of the atmospheric mass lies in the tropsphere.

"Thermodynamic equilibrium" means that the temperature and humidity of the air increase to the point that there is no net transfer of heat or water vapor between the air and water.

Hurricane's minimum central pressure can be as much as 10% lower than the that in the undisturbed atmosphere. If there were no rotation, the air around the storm would be drawn inward to fill up the low pressure. In nature, both the rotation of the Earth and the background rotation of the air (vorticity) are concentrated by the inflow so that the air swirls around the low-pressure center. The situation reaches a balance where the tendency of the air to be sucked inward provides the acceleration required for it to move on circular paths around the center. This situation is called "Gradient Balance." It is the dominant balance of forces in hurricanes and other intense small-scale atmospheric vortices. The diagram at the left shows the balance between the pressure-gradient force (blue arrow) and the acceleration (green arrow). In the equation for balanced motion, the left side is the sum of the centripetal acceleration and the Coriolis accleration due to the Earth's roataion; the right side is the pressure gradient divided by the air denisty.

During the night of October 4, 1995, Opal intesified from a strong Category 2 hurricane to Category 4 as it accelerated toward the U.S. Gulf of Mexico coast. Its central pressure actually reached a minimum below 918 hPa; although its winds were never stronger than 155 mph. Fortunately for residents of coastal Mississippi, Alabama, and Florida, Opal weakened in the last hours before landfall the following morning. Had the weakening not occurred, Opal might have had the somber distinction of being the first U.S. hurricane since Agnes in 1971 to kill more than 100 people. The official term for similar sudden intensification (by more than 30 kts in 24 hours) is "Rapid Deepening". Nearly all major hurricane (Saffir Simpson Cat 3, 4 & 5) experience Rapid Deepening. Forecasters recognize that the difficulty in forecasting rapid deepening will almost certainly be a key element of the next hurricane disaster.

The Tropical Prediction Center records of official track forecast errors since 1954 when they began keeping track at the 24-h time horizon (above). Although the improvements have averaged only 1-2% annually, the accumulated increases in accuracy over a half centrury are sigificant.

Track forecast errors are defiend as the great-circle distance between the forecast and actual position of the hurricane 12, 24, 48 and 72 h in the future (below). Starting with the 2003 Hurriane Season, TPC instituted forecasts for 96 and 120 h as well.

Decreasing forecast errors both provide a quantitative indication of the forecasting enterprise's performance and have real economic value. In the situation shown here, warnings were raised on several hundred miles of shoreline, more or less centered on the predicted landfall. Since the hurricane remained offshore, these warnings resulted in expenditure of hundreds of millions on unneccessary preparations.