---
source: https://iinventedlight.com/library/notes/how-hurricanes-form.md
title: "How hurricanes form and what makes them dangerous"
license: CC-BY-4.0
license_url: https://creativecommons.org/licenses/by/4.0/
source_license: "US government work, public domain; restated in our own words"
attribution: "Summary by iinventedlight (AI-assisted). Sources: NOAA, What is a hurricane?, US government work, public domain; NOAA, Hurricane Safety Tips and Resources, US government work, public domain."
author: "iinventedlight (synthesis of cited sources); text drafted with AI assistance"
date: 2026-11-04
status: published
checked_on: 2026-11-04
as_of: 2026-11-04
confidence: medium
untested: "No observation or experiment was done; only the fetched government pages were read."
category: coverage-gaps
topic_area: "science-explain: weather and atmosphere"
tags: science, weather, hurricanes, tropical-cyclones
sources:
  - id: S1
    title: "What is a hurricane?"
    publisher: NOAA
    url: https://oceanservice.noaa.gov/facts/hurricane.html
    accessed: 2026-11-04
    license: "US government work, public domain"
  - id: S2
    title: "Hurricane Safety Tips and Resources"
    publisher: NOAA
    url: https://www.weather.gov/safety/hurricane
    accessed: 2026-11-04
    license: "US government work, public domain"
---
# How hurricanes form and what makes them dangerous

## Short answer
A hurricane is a tropical cyclone: a rotating low-pressure system with organized thunderstorms and no fronts, forming over tropical or subtropical water. It is a tropical depression below 39 mph, a tropical storm from 39 mph, and a hurricane once sustained winds reach 74 mph. [S1]

## The main hazards
- **Storm surge**, the abnormal rise of water pushed by the wind, historically causes the most hurricane deaths in the U.S. and can reach miles inland. [S2]
- **Inland flooding** from heavy rain is the second leading cause. [S2]

## Claims
- id: C1
  claim: A hurricane is a tropical cyclone forming over tropical or subtropical waters.
  source: S1
  evidence: "a type of storm called a tropical cyclone, which forms over tropical or subtropical waters"
- id: C2
  claim: A storm becomes a hurricane at 74 mph sustained winds.
  source: S1
  evidence: "When a storm's maximum sustained winds reach 74 mph, it is called a hurricane."
- id: C3
  claim: Storm surge is an abnormal rise of water from a storm's winds.
  source: S2
  evidence: "Storm surge is the abnormal rise of water generated by a storm's winds."
- id: C4
  claim: Flooding from heavy rain is the second leading cause of tropical cyclone deaths.
  source: S2
  evidence: "Flooding from heavy rains is the second leading cause of fatalities"

## Sources
- [S1] What is a hurricane?. NOAA. https://oceanservice.noaa.gov/facts/hurricane.html . License: US government work, public domain. Accessed 2026-11-04.
- [S2] Hurricane Safety Tips and Resources. NOAA. https://www.weather.gov/safety/hurricane . License: US government work, public domain. Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).