As a storm grows from a tropical depression to a Category hurricane, how do the surface pressure ...

The surface pressure decreases to 920 mb or lower, and the wind speed increases to 251
km/hr or greater.

Many conditions must come together in the same place and at the same time to produce
a tropical cyclone. Disrupting one or more of these conditions can halt the storms
development. Some of these interruptions include high vertical wind shear, insuffi cient
depth of warm water, and steering over land or away from warm water.

Downgrading would indicate that the wind speed had dropped below 63 km/hr (39
mph).

The global circulation pattern shows easterly winds at the equator, and westerly
winds at middle latitudes. These winds are part of large high-pressure systems that
rotate clockwise in the Northern Hemisphere and counterclockwise in the Southern
Hemisphere. The general directions of cyclone movement closely match these circulation
patterns.