What Causes Hurricanes to Form, Strengthen and Weaken?
10/10/26 – After a life measured in days, Hurricane Isaias went from an area of low pressure in the Bay of Campeche to a killer, Cat 3 hurricane to a post-tropical storm now causing flash flooding in the Appalachians? So, what are the forces that cause hurricanes to form, strengthen and weaken?
I dedicate this post to curious readers and budding scientists still in school. Please share it with your students, children and grandchildren.
What Causes Hurricane Formation?
The forces listed below support formation of an organized tropical cyclone.
| Key force or condition | What happens physically | Effect on the developing storm |
|---|---|---|
| An existing weather disturbance | Thunderstorms gather around a low-pressure area, such as a tropical wave. | Provides the starting circulation around which a storm can organize. |
| Warm ocean water | The ocean supplies heat and water vapor to air above it. Water temperatures around 80°F (27°C) generally favor development. | Supplies energy for sustained thunderstorms. |
| Moist air | Humidity limits evaporation and cooling within developing thunderstorms. | Helps thunderstorms persist and organize. |
| An atmosphere that supports rising air | Warm, moist air rises, cools, and condenses, releasing heat. | Builds deep thunderstorms that help strengthen the circulation. |
| Low vertical wind shear | Winds change relatively little with height, allowing the lower and upper circulation to remain aligned. | Helps thunderstorms organize around the surface center. |
| Sufficient distance from the equator | Earth’s rotation deflects moving air through the Coriolis effect. | Helps organize a rotating circulation; tropical cyclones rarely form within about 5 degrees of the equator. |
These conditions favor formation, but their presence does not guarantee that a hurricane will develop. Sources: NOAA’s hurricane overview and formation explanation. www.noaa.gov.
What Causes Hurricanes to Strengthen?
Once a tropical storm develops, alignment of the conditions below can intensify it into a hurricane (sustained winds 74 mph or greater).
| Key force or condition | What happens physically | Effect on the hurricane |
|---|---|---|
| A continuing supply of ocean heat and moisture | Surface air picks up heat and water vapor, then carries that energy into the eyewall. | Fuels stronger thunderstorms and circulation. |
| Warm water extending deep below the surface | Wind-driven mixing brings up relatively warm water instead of quickly exposing cold water. | Sustains the energy supply as the hurricane churns the ocean. |
| Concentrated heating in the eyewall | Rising water vapor condenses and releases heat close to the center. | Strengthens the warm core and helps central pressure fall, increasing winds. |
| Low or no vertical wind shear | The circulation remains more vertically aligned. | Allows eyewall thunderstorms to organize and heat the core efficiently. |
| Moist surrounding air | Less dry air enters and disrupts eyewall updrafts. | Helps maintain vigorous thunderstorms around the center. |
| Effective outflow aloft | Air rising through the eyewall spreads outward near the top of the storm. | Supports the continuing flow of air inward below and upward through the eyewall. |
| An increasingly organized inner core | Thunderstorms form a more continuous eyewall; the eyewall may contract. | Concentrates the strongest circulation closer to the center and can accelerate strengthening. |
Sources: NOAA’s ocean conditions research and hurricane modeling explanation.
What Causes Hurricanes to Weaken?
These conditions cause hurricanes to weaken as they move.
| Key force or condition | What happens physically | Effect on the hurricane |
|---|---|---|
| Strong vertical wind shear | Winds at different heights tilt the circulation and displace thunderstorms from the surface center. | Disrupts the organized eyewall and reduces heating of the core. |
| Dry air entering the inner core | Evaporation cools air within thunderstorms, encouraging downdrafts and interrupting moist updrafts. | Erodes eyewall thunderstorms and can open gaps in the eyewall. |
| Cooler ocean water | Less heat and water vapor transfer from the ocean to the atmosphere. | Reduces the energy available to sustain the hurricane. |
| Cold water mixed upward by the storm | Strong winds churn the ocean and can bring colder water to the surface. | Allows the hurricane to weaken its own energy supply, especially over a shallow warm layer. |
| Landfall | Access to ocean heat and moisture diminishes; surface friction increases, and terrain can disrupt circulation. | Slows winds and breaks down the eyewall, although heavy rain can persist. |
| An eyewall replacement cycle | An outer eyewall develops and intercepts inflow feeding the original eyewall, which weakens and disappears. | Often temporarily lowers maximum winds while expanding the wind field; strengthening may resume. |
Isaias encountered wind sheer shortly before reaching land. That lowered its intensity at landfall from Cat 3 to Cat 2.
Sources: NOAA’s satellite interpretation guide, hurricane FAQ, and eyewall research.
Effect on Flooding
Note that “strengthening” and “weakening” usually describe changes in maximum sustained winds. A weakening hurricane can still produce destructive storm surge and prolonged, intense rainfall. Hurricane Harvey is an excellent example.
In 2017, Harvey’s winds weakened dramatically before much of Houston’s catastrophic flooding occurred. The National Weather Service documents the sequence below.
| Stage | What happened |
|---|---|
| Landfall, August 25 | Harvey struck near Rockport as a Category 4 hurricane with 130 mph sustained winds. |
| Weakening, August 26 | It weakened to a tropical storm within approximately 12 hours, then maintained winds around 40 mph for the next couple of days. |
| Prolonged flooding | Weak steering currents caused Harvey to stall near the coast. Part of its circulation remained over the Gulf, sustaining its moisture supply and repeated torrential rainfall. |
The results were extraordinary:
The key point: Harvey weakened as a windstorm, but remained a catastrophic rainstorm.
A lower hurricane category does not necessarily mean a lower flood threat.
In fact, Harvey produced the highest recorded rainfall total ever from a single tropical cyclone in the continental United States: 60.58 inches near Nederland, Texas, according to the National Weather Service.
Posted by Bob Rehak on 10/10/2026
3329 Days since Hurricane Harvey
