By international agreement, tropical cyclone is the general term for all cyclonic circulations originating over tropical waters, classified by form and intensity as follows:
Rotary circulation slight or absent at the surface, but sometimes better developed aloft, no closed isobars (lines of equal atmospheric pressure) and no strong winds, a common phenomenon in the tropics.
One or more closed isobars and some rotary circulation at surface, highest wind speed 39 miles per hour (34 knots).
Closed isobars, distinct rotary circulation, highest wind speed of 74 miles per hour (34-63 knots).
Closed isobars, strong and very pronounced rotary circulation, wind speed of 74 miles per hour (64 knots) or more
CONDITION 4 (Green):
This condition shall automatically go into effect upon such occurrences as the beginning of the hurricane season (June 1 of each year, remaining in effect until November 30 of each year); or other general conditions as determined by the City Manager.
CONDITION 3 (Yellow):
This condition exists when the course of the hurricane is established, and the National Weather Service predicts the possibility of landfall within 36 hours and/or is within 100 miles of Houston.
CONDITION 2 (Orange):
This condition shall be declared by the Mayor or the City Manager when the National Weather Service predicts hurricane landfall within 24 hours and is close enough to Houston to cause damaging winds, high tides and storm surge.
CONDITION 1 (Red):
This condition shall be declared by the Mayor or City Manager when hurricane landfall is within 12 hours or less and damaging tides or hurricane force winds are imminent.