When tropical weather threatens, agencies from the local to federal levels watch the systems and try to gauge their strength. When a hurricane is approaching land, the National Hurricane Center will begin issuing watches and warnings. Here's how to decipher those:

HURRICANE: The storm has winds of more than 73 mph.

HURRICANE WATCH: Hurricane-force winds of more than 73 mph are possible within 36 hours.

HURRICANE WARNING: Hurricane-force winds are possible within 24 hours.

EYE: This is the hurricane's roughly circular center area where the winds are comparatively lighter.

TROPICAL STORMS: These generate winds of 39 to 73 mph. Watches and warnings also are issued for these.

LANDFALL: The place where the center of a storm intersects with land. Because the storm's strongest winds are not in the center, an area can be greatly affected even if the storm doesn't make landfall there.

ERROR CONE: This shows a broad path that a storm could take. Because of uncertainty in forecasting a storm's path and strength, watches and warnings cover large areas of coastline. Those alerts mean anyone in a watch or warning area could be hit by a storm.

STORM SURGE: The abnormal rise in sea level accompanies a hurricane or other intense storm. This dome of water (often 50 to 100 miles wide) sweeps the coastline near where the hurricane's eye makes landfall.

MEASURING A HURRICANE

Emergency officials use the Saffir-Simpson scale to decide which areas to evacuate. It ranks hurricanes from Category 1 (minimal) to Category 5 (catastrophic).

Category 1

Wind: 74 to 95 mph

Storm surge: 4 to 5 feet

Damage: Minimal

Example: Hurricane Gaston in 2004

Category 2

Wind: 96 to 110 mph

Storm surge: 6 to 8 feet

Damage: Moderate

Example: Hurricane Frances in 2004

Category 3

Wind: 111 to 130 mph

Storm surge: 9 to 12 feet

Damage: Extensive

Example: Hurricanes Jeanne and Ivan in 2004

Category 4

Wind: 131 to 155 mph

Storm surge: 13 to 18 feet

Damage: Extreme

Example: Hurricane Charley in 2004

Category 5

Wind: 156 mph plus

Storm surge: Higher than 18 feet

Damage: Catastrophic

Example: Hurricane Andrew in 1992