Transcript Ch. 13 * The Nature of Storms

Ch. 13 – The Nature of Storms
•Every thunderstorm has characteristics that are similar no matter
how strong the storm is.
•Cumulonimbus clouds produce thunderstorms.
–So, How do cumulonimbus clouds from?
•Video
•A cumulus cloud may grow into a cumulonimbus cloud.
–1. Moisture levels must be high enough in the lower part of
the atmosphere.
–2. The air must be lifted so the air can condense and release
latent heat.
–3. The air must continue to cool with increasing altitude for
the growing cloud to stay warmer than the surrounding air.
•Video
• An air-mass thunderstorm occurs when the air rose
because of unequal heating of Earth’s surface within
one air mass.
• Sea-breeze thunderstorms occur by extreme
temperature differences between the air over land and
the air over water.
• A frontal thunderstorm is produced by advancing cold
air pushing warm air rapidly up the steep cold-front
boundary.
• Thunderstorm Stages
– 1. Cumulus Stage – air starts to rise upward & clouds form.
– 2. Mature Stage – water droplets that formed at high, cool
levels and sinks rapidly to the ground as precipitation.
– 3. Dissipation Stage – the supply of warm, moist air runs out
and the updrafts slows, eventually stops and precipitation
can no longer form.
• Video
Severe Weather
• What makes one thunderstorm more severe than
another?
• One of the main factors involved is the difference in
temperature between the upper and lower parts of the
storm. This difference cause greater updrafts and
downdrafts in the storm.
– Cold front and low-pressure systems are associated with
this.
• Supercells are characterized by the intense, rotating
updrafts.
– These are extremely powerful storms that can last for hours.
• Video
• Lightning is produced in the same fashion as static electricity.
• Static electricity occurs with the friction of an object – Ex. Your
feet rubbing against carpet.
• A lightning bolt is generated by the friction of an updraft with a
downdraft of a cumulonimbus cloud. Electrons are separated
from atoms because of this.
– Positive & Negative Ions (+ high & - low in clouds).
– To relieve the electrical imbalance, an invisible channel of electrons
(stepped leader), moves from the cloud toward the ground.
– When this nears the ground a channel of positively charged ions (return
stroke) rushes upward to meet it.
• 100 million volts of electricity.
• Heats the air to about 30,000 °C.
• The thunder is the sound produced from the superheated air rapidly
expanding and contracting.
• Video
• Video
• Video
• Downbursts occur when violent downdrafts are
concentrated in a local area.
– Macrobursts can cause a path of destruction up to 5 km
wide and wind over 200 km/h.
– Microbursts are up to 3 km wide and winds over 250 km/h.
– They are hard to detect and prepare for thus they produce
lots of damage.
• Video
• Hail produces almost $1 billion in damage every year.
– Due to water droplets freezing in the cumulonimbus cloud.
As they form and fall they may encounter a nearby updraft to
lift them up and form more ice.
• Video
• Floods usually occur when wind current in the upper
atmosphere are slow & weak, thus the storm moves
slow and dump a lot of moisture.
• Video
Tornadoes
• A tornado is a violent, whirling column of air in contact
with the ground.
– Before it hits the ground it’s a funnel cloud
– Usually associated with supercells.
• Tornadoes form when wind speed and direction
change suddenly with height  wind shear.
• A horizontal rotation starts and if the rotation is near
the storms updraft it can shift its rotation to a vertical
rotation.
– As updrafts accelerate the rotation, air is removed from the
center of the column.
•
•
•
•
Video
Video
Video
Video
• Fujita tornado intensity scale – ranks tornadoes
on their path of destruction, wind speed, and
duration.
• Video
• Video
• Most tornadoes occur in late spring during the
late afternoon and evening, when the
temperature contrasts between polar air and
tropical air.
– This occurs more commonly in the Central United
States – “Tornado Alley”.
• An average of 80 deaths & 1500 injuries due to
tornadoes.
• Tornado Safety  Table 13-3 (pg. 340)
Tropical Storms
• Tropical cyclones are large, rotating, low-pressure
storms.
– The strongest of these occur in the U.S. and other parts of
the Atlantic Ocean as hurricanes.
• They move counter-clockwise in the northern
hemisphere due to the Coriolis effect around a lowpressure system.
• They need an abundant supply of very warm ocean
water and some sort of disturbance to lift warm air and
keep it rising.
– Fig. 13-10 (were they form)
• These tropical cyclones move according to the wind
currents. They move steadily toward the west, then
eventually turn poleward when they reach the far
edges of the high-pressure system.
• Video
• The first stage of the tropical cyclone is called a tropical
depression.
• A tropical storm occurs when the winds exceed 65 km/h.
• It is classified as a hurricane when the winds exceed 120 km/h.
• An eye of the storm forms a calm center.
• The strongest winds occur in the area immediately surrounding
the eye  eyewall.
• Saffir-Simpson hurricane scale classifies hurricanes according
to wind speed, air pressure in the center, and potential for
property damage. Table 13-4.
• Energy to run a hurricane is lost once it moves over land.
• Storm surges are produced when hurricane-force winds drive
tides of ocean water toward coastal areas, where it washes
over the land.
• Video
Recurring Weather
• Droughts occurred during the 1930s in the Central
U.S., which produced the Dust Bowl.
– Fig. 13-13
– Droughts are usually due to global wind pattern that produce
high-pressure systems (lack of rain) for several weeks or
months.
• Heat waves are also associated with these patterns.
The high-pressure systems blocks cooler air masses
from moving into the area.
• Cold waves are also brought on by large, highpressure systems. They occur however by arctic or
polar air masses.
• Wind chill – the temp the body feels with winds.
– less than 15 degrees F wind chill may cause
frostbite.
– Ex. 0 degrees F & 30 mph wind = -49
– Ex. 0 degrees F & 10 mph wind = -22