Ch' 13 The Nature of Storms

1
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?
• 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.

2

• An air-mass thunderstorm occurs when the air rose
  because of unequal heating of Earths 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.

3
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.

4

• Lighting is produced in the same fashion as
  static electricity.
• Static electricity occurs with the friction of an
  object Ex. Your feed rubbing against carpet.
• A lighting 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.

5

• Downbursts occur when violent downdrafts are
  concentrated in a local area.
• Macrobursts can cause a pather of destruction up
  to 5 km wide and wind over 200 km/h.
• Microbursts are up to 3 km wide and windsover 250
  km/h.
• They are hard to detect and prepare for thus they
  produce lots of damage.
• 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.
• Floods usually occur when wind current in the
  upper atmosphere are slow weak, thus the storm
  moves slow and dump a lot of moisture.

6
Tornadoes

• A tornado is a violent, whirling column of air
  in contact with the ground.
• Before it hits the ground its 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.

7

• Fujita tornado intensity scale ranks tornadoes
  on their path of destruction, wind speed, and
  duration.

8

• 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)

9
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
  low-pressure 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.

10

• 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.

11
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,
  high-pressure 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