Download Chapter 8: Weather

Chapter 8: Weather
Two Important Terms for This Chapter
• Meteorology is the scientific study of the
atmosphere.
• Weather is the short-term, day-to-day
condition of the atmosphere, and should not
be confused with climate, which is the longrange weather conditions in a region.
Clouds are fundamental indicators of overall atmospheric
conditions: stability, moisture content, and weather.
A cloud is comprised of tiny water droplets
and/or ice crystals, a snowflake is an aggregate
of many ice crystals, and rain is just liquid water.
Cloud formation
Warm, moist air can form advection fog.
The topography of an area determines what type of fog forms.
Convergent lifting is the widespread, broad lifting of an entire
layer of air. Convergence is an atmospheric condition that
exists when there is a horizontal net inflow of air into a region.
When air converges along the earth's surface, it is forced to
rise since it cannot go downward. Large scale convergence can
lift a layer of air hundreds of kilometers across.
Convectional lifting refers primarily to atmospheric motions
in the vertical direction. As the earth is heated by the sun,
bubbles of hot air (called thermals) rise upward from the
warm surface. A thermal cools as it rises and becomes
diluted as it mixes with the surrounding air, losing some of
its buoyancy (its ability to rise).
When air is confronted by a mountain, it is lifted up and over
the mountain, cooling as it rises. If the air cools to its
saturation point, the water vapor condenses and a cloud forms.
These types of clouds are called "orographic clouds", which
develop in response to lifting forced by the topography the
earth.
Frontal lifting occurs along frontal boundaries, which
separate air masses of different density. In the case of a cold
front, a colder, denser air mass lifts the warm, moist air ahead
of it. As the air rises, it cools and its moisture condenses to
produce clouds and precipitation. Due to the steep slope of a
cold front, vigorous rising motion is often produced, leading
to the development of showers and occasionally severe
thunderstorms.
In the case of a warm front, the warm, less dense air rises
up and over the colder air ahead of the front. Again, the air
cools as it rises and its moisture condenses to produce
clouds and precipitation. Warm fronts have a gentler slope
and generally move more slowly than cold fronts, so the
rising motion along warm fronts is much more gradual.
Precipitation that develops in advance of a surface warm
front is typically steady and more widespread than
precipitation associated with a cold front.
Severe thunderstorms often develop in groups of many
individual storms clustered together. When these
clusters occur as elongate bands, they are known as
squall lines.
Two Areas of Squall Line Development
• Warm sector of middle • Can form along a
latitude cyclones
boundary called a
• 60 to 180 miles in
dryline, a narrow zone
advance of cold front
along which there is
• Develop from a
an abrupt change in
combination of warm,
moisture
moist air near the
surface and an active
jet stream aloft
Mid-latitude cyclones cause of most of the stormy weather in
the United States, especially during the winter. They are areas
of low pressure located between 30 degrees and 60 degrees
latitude. Since the continental U.S. is located in this latitude
belt, these cyclones have a major impact on our weather. A
mid-latitude cyclone looks very distinct on a satellite image.
It is often identifiable by a comma-shaped cloud mass.
These are all examples
of some type of
cyclone – they have
some similarities, yet
they are not the same
thing.
Cyclogenesis
Thunderstorms are simply storms that generate
lightning and thunder. They frequently produce gusty
winds, heavy rain, and hail.
Lightning and Thunder
Lightning is caused
when a separation of
charge occurs. Part of
a large cumulonimbus
cloud develops an
excess negative charge,
and part develops an
excess positive charge.
Lightning equalizes
these differences by
producing a negative
flow of current from
one region to the region
with the opposite
excess charge.
Air is a poor conductor of electricity. As a result, the
electrical potential, or charge difference, must be very
high before lightning will occur.
Lightning rarely occurs before the cloud has penetrated the 5
km level, where there is sufficient cooling to generate some
ice crystals.
Some scientists believe that
the charge separation
occurs during the formation
of ice pellets.
The channel of every
lightning flash, at a
temperature of 30,000 to
50,000 degrees F, is
extremely bright white.
The apparent colors of
lightning, then, are
caused by intervening
dust, moisture, and other
particles in the air.
Ice pellets called hail generally form within
cumulonimbus clouds and are often
associated with severe thunderstorms.
Tornadoes
Tornadoes are violent windstorms that take the form of
a rotating column of air, or vortex, that extends
downward from a cumulonimbus cloud
Maximum winds can approach 480 km, or
300 miles, per hour.
The average tornado has a
diameter between 150 and 600
meters.
Tornadoes
can travel
at speeds
close to 45
kilometers
per hour
The vertical wind
profile in a supercell
may cause the updraft to
rotate. This column of
cyclonically rotating air
is called a mesacyclone,
and is where tornadoes
often form.
Tornadoes can form over water, creating
beautiful but deadly waterspouts.
Although April through June is the period of
greatest tornado activity, tornadoes can and do
occur during every month of the year.
Most tornado damage is caused by the
tremendously strong winds associated
with the storm.
The Fujita Scale
F0
40-72 MPH
F1
73-112 MPH
Minor roof damage; breaks
branches off trees; push over
shallow-rooted trees; damage to
sign boards. Frequency 29%
The lower limit is the beginning
of hurricane wind speed; peels
surface off roofs; mobile homes
pushed off foundations,
overturned or torn apart;
moving autos thrown from
roads. Frequency 40%
F2
113-157 MPH
F3
158-205 MPH
Roofs torn off frame houses;
mobile homes demolished;
boxcars pushed over; large trees
snapped or uprooted; lightobject missiles generated.
Frequency 24%
Roofs and some walls torn
off well-constructed
houses; trains overturned;
most trees in forest
uprooted; heavy cars lifted
off ground and thrown.
Frequency 6%
F4
207-260 MPH
F5
261-318 MPH
Well-constructed houses leveled;
structures with weak foundations
blown off some distance; cars
thrown 300 yards or more and
large missiles generated.
Frequency 2%
Strong frame houses lifted off
foundations and carried considerable
distance to disintegrate; automobile
sized missiles fly through the air in
excess of 100 meters(109 yds); trees
debarked; steel reinforced concrete
structures badly damaged; incredible
phenomena will occur. Frequency
less than 1%
The Fujita
scale actually
goes to F10
(the speed of
sound),
however, no
known tornado
has exceeded
F5 – not even
this one.
Tropical cyclones originate entirely within tropical air masses.
Hurricanes
Hurricanes are large tropical storms with heavy winds. By
definition, they contain winds in excess of 74 miles per hour
(119 km per hour) and large areas of rainfall. They also have
the potential to spawn dangerous tornadoes. The strong winds
and excessive rainfall also produce abnormal rises in sea
levels and flooding
Disturbances that can grow into hurricanes begin where air is
converging and being pushed upward. If the air is unstable, it
will continue rising after the initial upward shove. The
disturbance will grow only when winds at all levels of the
atmosphere from the ocean up to 30,000 feet or higher are
blowing at the same speed and from the same direction.
This is a sea surface temperature map for the northern
hemisphere summer. The yellow, orange, and red colors show
water temperatures warm enough to sustain hurricanes (greater
than 26.5°C, or 50.9°F).
A tropical-storm watch is issued when sustained winds
from 39 to 73 mph (54.7 to 117.48 kph) are possible in
your area within 36 hours.
A tropical-storm warning indicates that these
conditions are likely in your area within 24 hours.
A hurricane watch is issued when hurricane conditions
(sustained winds greater than 74 mph/119 kph) are
possible in your area within 36 hours.
A hurricane warning is issued when these conditions
are likely in your area within 24 hours.
As a hurricane moves closer to shore, the
dome of water under the storm moves with
it. The water is piled up even higher by the
storm's strong winds and is topped with
giant waves. This mass of water, called
storm surge, causes the greatest hurricane
damage.
Camille, September, 1969
Aftermath of Floyd in North Carolina
Advances in technology and
the use of space-based
instruments continue to
improve our ability not only to
forecast the weather. , but to
be prepared for whatever
Mother Nature chooses to
send our way.
References and Resources
The National Oceanic and Atmospheric Administration
http://www.noaa.gov
The National Hurricane Center
http://www.nhc.noaa.gov/
USA Today’s “Weather Basics”
http://www.usatoday.com/weather/wgraph0.htm
Webshots
http://www.webshots.com