Chapter 10 Climate and Weather Related to Hazards
Chapter 15 Thunderstorms and Tornadoes
Chapter 14 Hurricanes and Nor’easters: The Big Winds
Days: Rotation of the Earth on its axis.
Seasons: Tilt of the Earth’s axis with respect to its annual orbit around the sun.
Greenhouse gases trap heat in the Earth’s atmosphere much as the glass in a greenhouse permits
the sun to shine in but prevents most heat from escaping.
Carbon dioxide, methane, nitrous oxide, and chlorofluorocarbons (CFC's) are some greenhouse
gases of anthropogenic origin.
Weather is the condition of the atmosphere at a particular time and place. It refers to such conditions
of the local atmosphere as
• atmospheric pressure,
• humidity (the amount of water contained in the atmosphere),
• precipitation (rain, snow, sleet, & hail),
• wind velocity.
Vertical Structure of the Atmosphere
• General trends with increasing altitude:
• Air pressure decreases. At any given altitude, the air pressure is caused by the weight of air
• Constituent gases decrease in density. Because air pressure decreases with altitude, the amount
of air per unit volume (density) also decreases with altitude.
• Temperature decreases in the troposphere where weather occurs.
• Water vapor decreases dramatically
Water Vapor in the Atmosphere
Water can exist in all 3 states at the normal range of earth temperature and pressure.
Whenever matter changes from one state to another, energy is either absorbed or released.
• From liquid to gas - evaporation - heat energy is absorbed
• From gas to liquid - condensation - heat energy is released
The Hydrologic Cycle
Water continuously evaporates from oceans and other water bodies, falls as rain or snow, is transpired by
plants, and flows through streams and groundwater back to the oceans.
Saturation vapor pressure = Maximum amount of moisture air can hold
• cannot be exceeded.
• evaporation = condensation
• temperature dependent.
Relative Humidity = The percentage of moisture in air relative to the maximum amount it can hold
(Saturation Vapor Pressure) under its given temperature and pressure.
Therefore, relative humidity can be changed by...
• Changing the water vapor content.
– Add water, increase relative humidity.
– subtract water, decrease relative humidity.
• Changing the temperature.
– Increase temperature, decrease relative humidity.
– Decrease temperature, increase relative humidity.
Processes that occur without the addition or subtraction of heat from an external source.
Because air pressure decreases with increasing altitude, rising air expands and sinking air is compressed.
Compressional warming - when air is compressed, the temperature rises.
Expansional Cooling - when air expands, the temperature decreases.
The adiabatic lapse rate - the way temperature changes with altitude in rising or falling air.
Lifting condensation level = altitude at which the rising parcel reaches saturation temperature and cloud
Upward movement of air results from:
• Convergence lifting - when flowing air masses of equal density converge and are forced upward.
• Convective (Density) lifting - When warm, low-density air rises convectively and displaces cooler,
• Orographic lifting - When flowing air is forced upward over a mountain range.
• Frontal lifting - when two flowing air masses of different density meet.
Warm Front: Warm air mass advances rapidly.
Cold Front: Cold air mass advances rapidly.
Atmospheric stability is a property of air that describes its tendency to remain in its original position or
sink (stable) or to rise (unstable) once the initial lifting force ceases.
– Lifting processes force air upward.
– Rising air does not mix substantially with the surrounding atmosphere.
A parcel of air forced to rise will expand and cool adiabatically.
Stable air - if an air parcel that is forced aloft cools faster than the surrounding environment. If the
lifting forced ceased, the parcel would have the density to sink.
High pressure system – an area characterized by descending cooler dry air and clear skies.
Cloud formation may occur at an altitude where the saturation temperature is reached (LCL), but clouds
would be layered without much vertical development - fair weather clouds.
Unstable air - if an air parcel that is forced aloft cools slower than the surrounding environment. If the
lifting force ceased, the parcel will continue to rise because it is warmer and more buoyant than its
Low pressure system – An area characterized by rising warmer and humid air and cloudy skies.
If the air parcel rises to an altitude where the saturation temperature is reached (LCL), clouds with
vertical development will form as the buoyant air rises on its own. (thunderstorm clouds).
Horizontal differences in air pressure between high and low pressure systems create winds.
Wind results when air flows from a place of high pressure to one of low pressure.
Magnitude is determined from the spacing of the isobars.
Isobars - lines connecting places of equal air pressure on a map (Compare to the slope of a hill).
The spacing of the isobars indicates the amount of pressure change over a given distance = pressure
Due to the rotation of the earth on its axis.
Deflects all free moving objects to the right of their path in the Northern Hemisphere and to the
left in the Southern Hemisphere.
Deflection increases with wind speed.
Deflection is strongest at the poles and weakens equator ward.
Convergent and Divergent Flow
In the Northern Hemisphere
Around a low pressure cell (rising air), an inward counterclockwise flow develops near the ground
surface; Centers of low pressure are called cyclones = convergent flow
Around a high pressure cell (sinking air), an outward clockwise flow develops near the ground surface.
Centers of high pressure are called anticyclones = divergent flow.
Right Hand Rule for Rising or Falling Air in the Northern Hemisphere
(Left Hand Rule for the Southern Hemisphere)
Centers of high pressure: Net downward movement of air and fair weather. Around a surface high
air is spiraling outward, which leads to a downward flow of air at the center of the high and
Centers of low pressure: Net upward movement of air, often resulting in cloud formation and
precipitation. Low pressure center generally related to unstable conditions and stormy weather.
Around a surface low air is spiraling inward, which leads to an upward flow of air at the center and
Violent, local atmospheric disturbance that mostly last only 30 minutes, but can create several dangerous
Torrential rain: usually intense, but short in duration - flash flooding and slope failure in mountainous
terrain is often associated with this type of precipitation.
High wind: Winds at the surface beneath a thunderstorm can reach well over 50 mph (80 kph).
Hail: falling at several meters per second, hail stones can result in extensive damage to crops and
property in just a few minutes and can injure or kill.
Lightning: every year in the U.S., lightning is responsible for an average of almost 100 deaths and 300
injuries. Lightning frequently starts fires which threaten homes, businesses, and lives. Power and
communication outages caused by lightning (as well as wind) can result in large scale disruption of
Thunderstorms: Ingredients for Cooking up the Storm
All thunderstorms require three ingredients for their formation:
Sources of moisture: It is estimated that there are as many as 40,000 thunderstorm occurrences
each day world-wide, most occurring in latitudes near the equator. Late afternoon hours in Spring and
summer bring the greatest risk of thunderstorms to most of North America.
A Lifting Mechanism: Cold air pushing under warm air along a cold front is a common lifting
Life Cycle of A Thunderstorm
Cumulus Stage: A cumulus cloud begins to grow vertically. Air within the cloud is dominated by
Mature Stage: The storm is at its most intense stage of development and is now a cumulonimbus
cloud. The top of the cloud reaches the tropopause where upward motion is hindered and the cloud
spreads out horizontally. Strong updrafts and downdrafts coexist. The mature air mass
thunderstorm contains heavy rain, thunder, lightning, and produces wind gusts at the surface.
Dissipating Stage: The thunderstorm enters the dissipating stage as the supply of warm moist air is
depleted. Air currents within the convective storm are now mainly downdrafts. Light rain and weak
outflow winds may remain for a while during this stage, before leaving behind just a remnant anvil top.
Lightning occurs when thunderstorms concentrate positive electrical charges in the upper part of
cumulonimbus clouds and negative charges in the lower part. When the difference in the charge become
great enough to overcome air resistance, a sudden and visible violent electrical discharge occurs in the
form of a lightning stroke. The electrical discharge can be within clouds and/or between the cloud and
The lightning heats the surrounding air to about 30, 000 degrees Celsius, which expands at supersonic
speeds, creating the mighty crashes we recognize as thunder.
Lightning and thunder happen at exactly the same time, but you see the lightning before you hear the
thunder because light travels faster than sound.
Hailstones appear when warm humid air in a thunderstorm rises rapidly into the upper atmosphere and
freezes forming tiny ice crystals. If the updraft is strong enough, the ice crystals can rise and fall many
times, with the hail getting new coats of water which then freeze, making the hail grow. When the hail is
too heavy to be sustained in the air by the updraft, it falls to earth.
A short lived upstream flood characterized by rapidly rising, high velocity water.
A tornado is a violently rotating funnel of air spawned from supercell thunderstorms.
3 main atmospheric conditions must occur simultaneously to produce a tornado
1. a northerly flow of warm, moist air from the Gulf of Mexico
2. a cold, dry air mass rapidly moving southward from Canada or the Rocky Mountains
3. strong easterly jet stream
These three air masses moving in different directions produce shearing conditions that give
thunderstorm clouds a "spin".
The rotational cell sags below the cloud base to form a distinctive slowly rotating wall cloud. Strong
tornadoes form within and then descend from the wall cloud. The momentum of the mesocyclone is
what generates sufficient strength to extend a funnel below the cloud base to the ground.
When viewed from the top on weather radar screens, the counter-clockwise rotation of the
mesocyclone gives the supercell a characteristic "hook echo".
The descending funnel is generally white or clear, made visible because water vapor within it
condenses into liquid droplets. After the funnel touches the ground and becomes a tornado, the color
of the funnel will change, often depending upon the type of dirt and debris.
As a tornado matures, it becomes wider and more intense.
As the tornado gradually loses intensity, the funnel decreases in size and takes on a contorted, rope-
like appearance before it completely dissipates.
About 70% of all tornadoes on Earth occur in the central and southern US.
One section of the nation is best at producing tornadoes. This area is called "Tornado Alley," with TX
and OK having the most.
Occur mostly in late spring - early summer when conditions are best for tornado formation; but can
occur anytime. As the season progresses, tornado formation migrates northward.
Most tornadoes travel northeast. They can move across the ground at speeds up to ~60 mph and have
max wind speeds of >300 mph.
Tornadoes can last from several seconds to more than an hour, but most last less than 10 minutes.
The Fujita Scale
The severity of a tornado is categorized based on wind speed estimated from the damage that they
cause on the Fujita Scale.
Tornado width is estimated by the path of destruction -- which can be up to one mile.
The size and/or shape of a tornado is no measure of its strength. Wind speed increases along with the
difference between atmospheric pressure inside the funnel, and the pressure outside it.
Extreme winds and flying debris cause almost all of the destruction.
A strong tornado will completely demolish houses and everything in them.
A tornado outbreak is a series of tornadoes spawned by a group of storms. Ex. Super Outbreak of 1974
A hurricane is a massive tropical cyclone with organized convection (i.e. thunderstorm activity) and
counterclockwise (N.H.) cyclonic surface wind circulation exceeding 74mph, blowing around a relatively
calm central area of very low pressure.
Hurricanes in the Atlantic and East Pacific,
Typhoons in the west pacific,
Cyclones in the Indian Ocean and Southern Hemisphere.
The process by which a tropical cyclone forms and subsequently strengthens into a hurricane depends on
1. A pre-existing disturbance with thunderstorms (typically emerging from the west coast of Africa)
2. Warm (at least 80ºF) ocean temperatures.
3. Light upper level winds that do not change much in direction and speed throughout the depth of the
atmosphere (low wind shear)
Tropical systems are classified into four categories according to its degree of organization and maximum
sustained wind speed.
Warm ocean waters add moisture and heat to the air which rises, creating an area of very low pressure.
As the moisture condenses heat is released, contributing additional energy to power the storm.
The winds near the ocean surface spiral into the low pressure area due to Coriolis forces and the storm
begins to take on the familiar spiral appearance.
Structure of a hurricane
Rain bands: bands of heavy convection spiraling inward towards storm’s center
Eye wall: ring of most intense convection surrounding storm center. Strongest winds, thickest
clouds, heaviest rain
Eye: region of calm winds, slowly descending air, relatively clear skies in center of hurricane.
Because the heated, rising air creates very low atmospheric pressures at the surface, the
pressure in the upper atmosphere above the storm is actually higher, causing a downward flow of
air in the center of the storm forming the “eye”.
Once formed, they are pushed west with the trade winds. The Coriolis force causes storms north of the
equator to travel in right-curving paths.
The Hurricane’s End
• Strong vertical wind shear tears the hurricane apart.
• Moving over cooler water can lead to weakening.
• Moving over land shuts off the moisture source and reduces surface circulation due to friction.
The Saffir-Simpson Hurricane Scale
The Saffir-Simpson Hurricane Scale is based on barometric pressure and average wind speed.
Storm Surge is the rapid sea level rise caused by both the low atmospheric pressure of a major storm and
the strong winds that accompany the storm and push water forward into a broad mound.
Storm surge locally raises sea level, moves inland as a swiftly flowing current, and raises the height of
The highest surge levels are in the north to northeast quadrant of the hurricane.
Storm surge is higher with greater wind speed.
When the surge comes at high tide, the resulting sea level rises still higher, with greater reach
inland and greater damage.
Surge height is greater in shallow water, bays, inlets, and river channels.
Storm Surge Hazards
• 90% of all deaths in tropical cyclones result from storm surge.
• Surge flooding.
• Coastal Erosion.
• Destruction of homes, buildings, roads, bridges, and piers.
• Contamination of groundwater supplies with saltwater, agricultural and industrial chemicals and
Waves and Wave Damage
Higher waves impact the coast with much more energy and erode the sand from beaches and dunes to a
High waves undermine structures and waves and loose debris batter buildings.
Winds and Wind Damages
Wind damages include blowing in windows, doors, and walls, lifting off roofs, blowing down trees and power
lines, and flying debris.
Most damage is from winds.
Hurricanes frequently produce huge amounts of rain. A typical hurricane brings at least 6 to 12 inches of
rainfall to the area it crosses.
Rain and flooding from hurricanes can be greater with large diameter, slower moving storms.
Weather satellites use different sensors to gather different types of information about hurricanes:
Doppler radar - Precipitation Location, Intensity, Wind Speed and Direction
Visible - clouds, circulation patterns
Infrared - temperature differences, cloud heights
The National Hurricane Warning Center in FL uses various weather models to predict the hurricane's
movement, location and time of arrival, and magnitude to issue hurricane watches and warnings to areas in
the storm's path. Unfortunately the storms can unpredictably weaken, strengthen, or change direction.
• A tropical-storm watch - possible within 36 hours.
• A tropical-storm warning - possible within 24 hours.
• A hurricane watch - possible within 36 hours.
• A hurricane warning - possible within 24 hours.
The National Hurricane Warning Center tries to give twelve hours of warning, but evacuations can take as
many as thirty hours.
Deaths In Hurricanes: Developed Countries
• Many people think they can evacuate quickly and leave too late.
• Many people delay evacuation because of the inconvenience and cost.
• Many people delay evacuation because they spend time installing materials to help protect their homes.
• Many people’s lack of awareness of the level and range of hazards prevents them from evacuating.
• Once the storm arrives bridges and roads become congested with traffic and accidents.
• Once the storm arrives, storm surges, downed trees, debris, and power lines often close roads.
Deaths in Countries: Poor Countries
Thousands of people in poor countries die in floods and landslides triggered by hurricanes.
Poorly constructed buildings and structures and poor land use practices on unstable steep slopes,
floodplains, and low lying deltas.
Hurricane Deaths and Damage
Better forecasting and warning systems have minimized deaths.
Rapidly growing populations, more building in unsuitable locations, and more expensive structures have
increased monetary losses.
Since 1953, Atlantic tropical storms reaching tropical storm strength have been named from lists
originated by the NHC. When first developed, the lists featured only women's names.
Then in 1979 a six year rotating list with alternating male and female names was developed.
The names of devastating storms are retired, and another name is selected by the WMO to replace it.
Nor’easters: Extratropical Cyclones
Nor’easters are intense areas of low pressure that develop off the East Coast during late fall to early
spring, especially February.
Nor’easters behave much like hurricanes and can cause as much damage, dumping heavy amounts of
rain and snow, producing hurricane-force winds, and creating high storm surge that cause severe
beach erosion and coastal flooding.
A Nor’easter begins as a weak area of low pressure forming near the east coast. Nor'easters usually
develop off the coast of North Carolina when contrasting air temperatures meet, forming a low pressure
At the same time over Canada, winds circulate in a clockwise motion around a high pressure system and dip
deeply into the Northeast.
As the low begins to move north up the coast, air rushes counterclockwise around it, bringing winds that
pick up moisture from the ocean. The movement of the winds and precipitation from the Northeast is why
this storm is called a nor'easter.
The combination of warm and cold air causes the moisture to rise producing snow, sleet, freezing rain and
ordinary rain. A slow-moving storm system picks up more moisture and produces more precipitation than a
• If the storm moves inland it will usually push enough warm air inland to bring rain to the coast.
• If the storm moves further east over the Atlantic Ocean, heavy snow can fall along the coast.
THE PERFECT STORM
The Halloween Nor’easter or 91
As told in the book, "The Perfect Storm," by Sebastian Junger and dramatized in the movie by the same
name, two air masses (or fronts) and a hurricane collided over the Atlantic Ocean to form what the
National Weather Service called THE PERFECT STORM.
• 21st named storm, twelfth hurricane, sixth major hurricane and third Category 5 hurricane of the
2005 Atlantic hurricane season.
• At its peak, it was the most intense hurricane ever recorded in the Atlantic basin, with the lowest
atmospheric pressure ever recorded in the Western Hemisphere of 882 millibars.
Wilma helps fuel powerful nor'easter
By Ken Maguire
The Associated Press
A nor'easter that drew energy from the remnants of far-off Hurricane Wilma battered New England
and the mid-Atlantic states with 20-foot waves and winds up to 70 mph Tuesday, brought some inland
areas their first snow of the season and knocked out power to about 200,000 homes and businesses.
The powerful nor'easter reminded fishermen of the deadly "Perfect Storm" of October 1991.