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Natural Hazards

Clarification Statement: Examples of design solutions to weather-related hazards could include barriers to prevent flooding, wind-resistant roofs, and lightning rods.

This activity builds on the content below.

Natural Hazards

<p>Fig. 1. A tsunami evacuation sign guides people on a safe route to avoid danger in case of a tsunami.</p>Natural processes can cause sudden or gradual changes to Earth’s systems, some of which may adversely affect humans. Through observations and knowledge of historical events, we know where some hazards—such as earthquakes, tsunamis, volcanic eruptions, severe weather, floods, and coastal erosion— are likely to occur. Although humans cannot eliminate natural hazards, we can take steps to reduce hazard impacts (Fig. 1). For example, satellite monitoring of weather patterns, along with measurements from land, sea, and air, can help identify and forecast severe weather. In addition, homes and buildings can be designed and built to be resilient to weather related hazards.


Tropical Cyclones

A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain or squalls over warm waters. There are three types of tropical cyclones, which are characterized by the speed of their sustained wind: tropical depressionstropical storms, and hurricane (see table below). 

Type of Tropical Cyclone Description
Tropical Depression A system of clouds and thunderstorms with a defined circulation and maximum sustained winds of less than 39 mph.
Tropical Storm A system of strong thunderstorms with defined circulation and maximum sustained winds of 39-73 mph.
Hurricane A system of strong thunderstorms with well-defined circulation and maximum sustained winds higher than 73 mph.

Hurricanes

Hurricanes are a type of tropical cyclone that is accompanied by thunderstorms and counterclockwise wind circulation (in the Northern Hemisphere). The winds revolve around an eye (Fig. 2). The processes that cause a tropical cyclone to form and subsequently strengthens into a hurricane depend on at least three conditions:   

  1. A pre-existing disturbance with thunderstorms
  2. Warm (at least 79 degree F) ocean temperatures to a depth of about 150 feet
  3. Light upper level winds that do not change much in direction and/or speed throughout the depth of the atmosphere (low wind shear)

<p>Fig. 2. Hurricane Madeline tracked towards Hawai'i in August 2016, but weakened before making landfall, causing very little damage.&nbsp;</p><br />

Hurricane Formation

In the early stages of hurricane formation, the system appears on satellite images as relatively unorganized clusters of thunderstorms. The winds near the ocean's surface then begin to spiral into the thunderstorms' low pressure area, generating heat and energy. The warm ocean water adds moisture and heat to the cooler air, which then rises. As the moisture condenses into drops, more heat is released, contributing additional energy to drive the storm. Bands of thunderstorms then form, and the storm’s clouds rise higher into the atmosphere. If weather and ocean conditions continue to be favorable, the storm system can strengthen and become a tropical depression (winds less than 39 mph or 33 kt). At this point, the storm begins to take on the familiar spiral appearance caused by the flow of the winds and the rotation of the earth. If the storm continues to strengthen to tropical storm status (winds 39-73 mph), the bands of thunderstorms will contribute even more heat and moisture to the storm. The storm becomes a hurricane when winds reach a minimum of 74 mph. At this time, the cloud-free hurricane eye typically forms. The eye is caused by rapidly sinking air at the center, which dries and warms the area.

<p>Fig. 3. This diagram shows the structure of a hurricane and the dynamics of the rising warm air and falling cool air.&nbsp;</p><br />

Naming of Tropical Cyclones

Tropical cyclones are called different things depending on where they occur in the world:

  • North Atlantic, the Northeast Pacific Ocean east of the dateline, or the South Pacific Ocean east of 160E longitude = hurricanes.

  • Northwest Pacific Ocean west of the dateline = typhoons

  • Southwest Pacific Ocean west of 160E or Southeast Indian Ocean east of 90E = severe tropical cyclone

  • North Indian Ocean = severe cyclonic storm

  • Southwest Indian Ocean = tropical cyclone


Protecting Against Natural Hazards

Understanding natural hazards can help people prepare for and respond to them. In areas where hurricanes form, for example, people may design and engineer buildings and homes to be resilient to strong winds, flooding, and other storm damage.

In Hawai‘i, there are many different types of natural hazards. Some examples include: floods, fire, landslides, earthquakes, coastal erosion, and sea-level rise. High winds and large waves from tsunamis and hurricanes are the most damaging types of hazards in Hawai‘i. The attachment below gives a detailed guide to protecting your home:

Updates to the Handbook to Prepare for Natural Hazards can be found online. The Voice of the Sea episode below features the Homeowners Handbook and discusses how to prepare for natural hazards. 

 

Roof Design

Roofs help to protect and insulate buildings. Roof design is an important element in protecting homes and buildings from natural hazards. Around the world, roofs are built in many shapes and styles. Often, the style depends on culture, exposure to elements (such as rain, snow, wind, and sunlight), cost, and available materials (Fig. 4). The angle of the roof, the number of sides, the shape, and the overhang all affect its performance.

<p>Fig. 4. <em>Clockwise from top left</em>:&nbsp;A living grass roof in Norway.&nbsp;A stone roof in India.&nbsp;A round roof in Taiwan. Steep, tile roofs in Northern Europe.</p><br />

Tying the house roof to the foundation helps to strengthen the structure. The ties stop the roof from flying off during high winds. Tie-downs can be added as the home is being built. Tie-downs can also be used to retro-fit older homes to make them more storm-resistant (Fig. 5). Tie downs are often called hurricane clips or hurricane ties.

<p>Fig. 5. <em>Left to right</em>: Hurricane clips (circled) helped keep the roof on this home during Hurricane Katrina.&nbsp;Hurricane ties are already in place at the top of the wall, even as the roof is being built.</p><br />
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Elevated Homes

Water is responsible for 90% of the deaths caused by hurricanes. Interactive storm surge maps from NOAA help to show areas of flooding vulnerability. Elevated homes have been a useful building design for 1,000s of years to help protect against flooding, water damage, and outdoor pests (Fig. 6). Elevation can also help protect against erosion and unstable ground (Fig. 7).

<p>Fig. 6. <em>From left to right</em>: Summer homes of indigenous Kamchatka Peninsula people. A modern stilt house in cambodia.</p><br />
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<p>Fig. 7. Stilt homes on the water in Indonesia.</p><br />

 

Hurricane Hazard Vocabulary:

  • Climate: long-term average of conditions in the atmosphere in a particular part of the world
  • Cyclone: an area of low atmospheric pressure, characterized by inward spiraling winds that rotate counterclockwise in the Northern Hemisphere.
  • Eye: the roughly circular area of comparatively light winds in the center of a severe tropical cyclone
  • Hurricane: An area of low pressure around which winds blow counterclockwise in the Northern Hemisphere (east of the International Dateline to the Greenwich Meridian); severe tropical storms with heavy rains and high wind speeds in excess of 73 mph (119 km/hr); bring enormous waves and flooding that can damage buildings and beaches.
  • Hurricane clips or hurricane ties: Adaptations to buildigns that help prevent the roof from flying off during high winds
  • Low pressure area: an area where the air pressure is lower than that of surrounding locations. The formation process of a low-pressure area is known as cyclogenesis.
  • Weather: conditions in the atmosphere, including temperature, precipitation, pressure, cloud cover and humidity
  • Saffir-Simpson Scale: classifies hurricanes with a 1-5 rating based on the intensity. Used to give an estimate of potential property damage expected along the coast from a hurricane landfall. Wind speed is the determining factor.
  • Severe cyclonic storm: a hurricane-like sorm that occurs in the North Indian Ocean.
  • Severe tropical cyclone: a hurricane-like storm Southwest Pacific Ocean west of 160E or Southeast Indian Ocean east of 90E
  • Tropical cyclone: a low pressure system in which the central core is warmer than the surrounding atmosphere tropical depression in the Indian Ocean; the maximum sustained surface wind speed (using the U.S. 1-minute average) is 38 mph or 62 km/hr or less.
  • Tropical Depression: A system of clouds and thunderstorms with a defined circulation and maximum sustained winds of less than 39 mph. 
  • Tropical storm: a tropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1-minute average) ranges from 39 mph or 63 km/hr to 73 mph or 118km/hr.
  • Tsunami: a long high sea wave caused by an earthquake, submarine landslide, or other disturbance.
  • Typhoon: The term used for Pacific tropical cyclones north of the Equator and west of the International Dateline.

 

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Representative Image: 
Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.