Geography 1700
Chapter 9
Part 1 of 4
in support of Quiz #3, Autumn 2013
U.S. Storms
•
Tornadoes in 2011 killed more than 550 Americans.
• At least one tornado in 2013 was among the largest ever recorded – more
than two miles wide on the ground and exhibiting wind speeds above 300
miles per hour.
• The 1925 “Tri-State Tornado” may have been much larger than any tornado
ever ‘recorded.’ This one cyclone lasted for hours – killing more than 700
people. <What would happen now if that same ‘twister’ occurred again?>
• Cyclones can occur anywhere cold/dry and warm/wet air meet, usually far
north or south of the Equator, usually in mid-latitudes where large
continents create extremes and where coriolis force is active.
Severe Weather and World-Class Storms
• The United States is regarded as having the greatest variety of bad weather.
Why? <Large continents have interiors far from mild ocean influence, cold-dry air
collides with warm-wet air and coriolis force causes spin acceleration -- faster
wind.>
• Typhoons in the Pacific Ocean and ‘cyclones’ in the Indian Ocean have killed as
many as 500,000 people in one storm alone. Even so, the USA can be worse due
to having more cold-dry air than Bangladesh, which collides with wet-warm.
• Why does it matter that Americans tend to be some of the world’s best
gamblers? <Americans can run fast or hide quicker than people in most poor
countries.>
• More than 90% of the world’s tornadoes occur in the USA.
Summary of World and U.S. Climates
A = tropical, equatorial, warm, wet, humid
B = dry, mostly warm to hot, but some cold
C = mild, lower mid-latitude, but not too dry:
either continental, Mediterranean, or maritime
(ocean influence).
D = higher latitude, more severe, dry or wet,
tending to colder, but high seasonal variability.
E = polar, frigid, dry due to cold air
H = highland, mountains, severe
Air Masses – personality types
- continental
- tropical/tropical
- polar
-maritime
Mr. Allred’s Climate Rules
•
•
•
•
Distance from ocean or large body of water
Distance from sea level
Distance from equator
Are you downwind or upwind?
– Being downwind of a lake or ocean means that
milder conditions that prevail over water will be
blown toward you.
– Being upwind from a lake or ocean won’t
moderate your weather or climate very much.
The Coriolis Force
The earth is a sphere that is rotating.
So, all objects moving through the air are
deflected to the right.
Curving motion is a form of acceleration,
contributing to the ground speed of wind.
Coriolis force is stronger with latitude, so midlatitude storms can be more violent than at the
tropics.
The United States has all of the climate zones
- Most humans live in mid-latitudes, which have no permanent
air mass of their own, so they are always being invaded by air
masses from elsewhere. Mid-latitudes are a zone of conflict
and variation.
- Mid-latitude weather is ‘temperate’ rather than torrid or
frigid. We get variety – four seasons.
.
- USA in particular has a unique combination of exposure to
BOTH trade winds (warm and wet) and continental severity
(cool and dry or cool and wet) – “fight club” or “tornado alley.”
- Coriolis force is stronger with higher latitude. At the equator
there is little or no Coriolis effect.
Anatomy of ‘bad’ weather
The “Big Six” Attributes of the Atmosphere
-
Differential heating of the earth causes air pressure
-
air moves from higher pressure to lower pressure as wind.
-
Moving air and heat evaporate water – energy becomes latent <hidden> in
water vapor, called humidity.
-
Wind carries water vapor and heat to other places.
-
When that air cools, latent heat and liquid water return as warm clouds.
-
Precipitation occurs when liquid or frozen water falls from clouds.
How a rainy day can turn violent.
“Sometimes Mikey likes it”
Any time air becomes laden with moisture it will tend to be more buoyant than drier air
elsewhere. When that air also becomes warm it can become even more buoyant. It
could start rising on its own (convective rising).
In dry Utah, when wind moves air upward over a mountain, the cooling effect can cause
condensation, releasing heat and producing clouds.
If air is wet enough, a simple rainy episode on the mountainside could turn into a
genuine ‘convective’ uplift, leading to more substantial precipitation, stronger wind and
so on.
So, humidity that moves through Utah might not have turned into a genuine stormy day
were it not for mountains that started the air moving upward. Afterward, the humid took
over and caused its own rising and more storminess.
A few times each year, there is enough water in Utah air to cause a strong convective
uplift that could produce a tornado.
This photo of Tooele County illustrates how Utah can have severe weather anytime
there is enough moisture in the air.
(Humidity is water vapor that contains latent (hidden) heat.
Utah is mid-latitude, interior
lacking ocean influence,
and subject to all four types of
precipitation (storminess)
Severe storms are caused by heat stored as water vapor
in the atmosphere.
• Modern science is now able to provide 10-11 minutes of warning ahead of
cyclones. <what does ‘warning’ mean?>
• Severe storms are part of the normal water cycle. Without hurricanes, Florida
would have persistent drought. Dangerous cyclones in India, Myanmar and
Bangladesh are part of the vital monsoon season that helps prevent famine and
starvation.
• Understand adjustments that can minimize damage and personal injury from
coastal cyclones
• Know the prudent actions to take for hurricane or extra-tropical cyclone watches
and warnings
Cyclones
• An area or center of low pressure with rotating winds
– Counter-clockwise in Northern Hemisphere
– Clockwise in Southern Hemisphere
• Tropical or extra-tropical
– Based on origin and core temperature
– Tropical storms may become more intense as they move out of the
tropics and into the sub-tropics and mid-latitudes
Tropical and Extra-tropical Cyclones
• Tropical cyclones
–
–
–
–
Form over warm tropical or subtropical ocean water (50–200)
Have warm central cores
Tropical depressions, tropical storms, hurricanes
High winds, heavy rain, surges, and tornadoes
• Extra-tropical cyclones
– Form over land or water in temperate regions (30o–700)
– Associated with fronts
– Strong windstorms, heavy rains, surges, snowstorms, blizzards
Is it true that even a blizzard is “powered” by
substantial heat?
Consider absolute zero (-458°F)
to a -10°F blizzard.
Even the coldest winter storm is still a
comparative “heat engine”.
When cold meets a cooling tropical storm, there
is new reason for lifting air and re-heating.
Some major regional storm categories
• Nor’easter - Extra-tropical cyclone that moves northward along East Coast U.S.
• Hurricanes - Tropical cyclones in Atlantic and eastern Pacific Oceans.
• Typhoon - Tropical and extra-tropical cyclones in Pacific Ocean west of
International Dateline and north of the equator.
• Cyclones – same kind of storm, but in the Indian Ocean.
• Monsoon – a seasonal shift in wind, but often brings either too much rain or not
enough.
Scale of precipitation weather events
Air mass thunderstorms – 30 minutes – “city size” <micro>
Meso-cell complex - longer than an hour – “county”
“super cells” and squall lines – up to a day or more –
“state or regional effect”
Macro-scale – hundreds of miles in size, may last a week
There is a positive correlation between size and duration
and an inverse relationship between size, duration and frequency.
Tropical Depressions and Tropical Storms
• Tropical depressions
– Tropical disturbance wind speeds increase and begin to spin.
– A low pressure center is formed.
• Tropical storm – sometimes leading to a genuine cyclone
(also called hurricane or typhoon)
• Storm may increase by passing over warm water (absorbing
more heat in evaporation)
• Encountering a cold air mass can assist the rise of warm/wet
air, accelerating wind speed and damage.
Hurricanes, cont.
• Rain bands
– Clouds that spiral inward around center
• Counter-clockwise in Northern Hemisphere
– Increase in intensity towards the center of the hurricane
• Eyewall
– Innermost band of clouds
– Contain the greatest winds and rainfall
• Eye
– Area of calm at center of the hurricane
– Narrow at surface and wider at top
Hurricane Structure
• Warm, moist air spirals upward around eyewall.
• Air rises and cools, then condenses out clouds and hidden heat
• Release of heat and moisture promotes more rising air, sometimes with
increasing speed
• Upward rotation draws air from eye, causing dry air to sink back into
center.
• Upward rotation also causes air to flow out the top of the storm
concentrated in exhaust jets.
• Allows additional warm, wet air to feed in at the base of the storm
• Storm will continue until warm, wet air supply ends at the base
High-pressure,
clockwise outflow:
opposite of cyclonic
Cross-Section of a meso-scale cyclone
Some references call it “macro” – no storms are larger
A brief Utah ‘air mass’
thunderstorm by comparison.
Hurricanes
• Necessary conditions
– Thick layer of warm water at the ocean surface
– Atmosphere that allows warm, moist air to rise upward to top of
troposphere
– Weak upper level winds
• Hurricane description
– Winds > 119 km (74 mi) ph
– Average diameter 500 km (310 mi)
**Super Storm Sandy reached 800 mile width
The Big Five Hurricane Effects
•
•
•
•
Low air pressure allows sea level to rise (3 feet)
Low air pressure provokes wind damage
High wind pushes waves higher & further (10 feet)
Heavy rainfall causes downstream flooding even as
ocean is going upstream. (up to several feet)
• If a high tide (or even a ‘spring’ tide) coincides, total
storm surge can be much higher (3-6 feet)
• (one student reported hearing that ‘Super Storm Sandy’ surge reached 37 feet
– I understand that it was up to 17 feet.
Aside from the storm, consider local
conditions:
• Shape and slope of the shoreline
• Human position, inland or on-shore
• Time of day
For years forecasters have expected a storm that
lines up with the Hudson river in New York.
Surge had an easy channel inland.
What sustains “extra-tropical” storms?
– A cyclone will run out of energy when the difference in moisture and
temperature inside the storm is not much greater than the drier/cooler
air outside. Buoyancy is lost, so no rising air, no wind, no cycle of
rewarming for more rising.
– Passing over dry land or over cool water will tend to drain a cyclone of
energy.
– However, an extra-tropical storm that collides with a blizzard or
“Nor’easter” can rebuild temporarily by having something cooler and
drier to use for ‘pushing-off’ or rising again for a while.