Thunderstorm Formation
Must have 3 conditions:
• An abundant source of moisture in the lower
levels of the atmosphere
• A mechanism must lift the air so that
moisture can condense and produce latent
heat
• The portion of the atmosphere in which the
cloud is growing must be unstable
Types of Thunderstorms
•Air-Mass: Caused by the unequal heating of
the Earth’s surface within a single air mass.
Most common during late afternoons
– Mountain: a result of orographic lifting
– Sea-breeze: a result of the extreme
difference in temperature of air over land
and air over water
•Frontal: Mostly due to advancing cold fronts,
rarely by advancing warm fronts. Can persist
into the night since they are not dependant on
daytime heating.
Stages of Development
• Cumulus: Air begins to rise nearly vertical,
creating updrafts.
• Mature: Updrafts and downdrafts form a
convection cell.
• Dissipation: Warm, moist air eventually used
up and downdrafts cool surface.
SEVERE WEATHER
Severe Thunderstorms
•Supercells: Characterized by intense, rotating
updrafts
Lightning
•Stepped leader: Negatively charged ions from
a cloud forming an invisible channel towards
Earth
•Return stroke: Positively charged ions from
the ground moving up the channel to meet the
negative ions
• Lightning: The actual illumination of the
channel when the return stroke meets the
stepped leader.
• Thunder: Air is heated to about 30,0000C, 5
times hotter than the sun. The rapid
expansion and contraction of the
superheated air creates thunder. Since
sound travels slower than light, we see
lightning sooner than we hear the thunder
associated with it.
• Downbursts: Violent downdrafts
concentrated in a small area. Categorized as
microbursts or macrobursts
• Hail: Precipitation on the form of balls or
lumps of ice. Most common during the
spring.
• Floods: Caused by more rain falling than the
ground can absorb
Tornadoes
•Formation: Wind speed within a thunderstorn
suddenly changes (wind shear). This horizontal
rotation is “stood up” by the storm’s updraft
•Most occur in the spring during late afternoon
and evening.
•“Tornado Alley” – Region extending from
Northern Texas through Oklahoma, Kansas,
and Missouri where many tornadoes occur
each year.
Classification:
•Enhanced Fujita Scale
Original Fujita Scale
EF0: 65 – 85 mph
F0: (45-78)
EF1: 86 – 110 mph
F1: (79-117)
EF2: 111 – 135 mph
F2: (118-161)
EF3: 136 – 165 mph
F3: (162-209)
EF4: 166 – 200 mph
F4: (210-261)
EF5: 200+ mph
F5: (262-317)
TROPICAL STORMS
•Tropical Cyclone – Large, rotating, lowpressure storm
•Two basic conditions to form:
1. An abundant supply of very warm ocean
water
2. A disturbance to lift warm air and keep it
rising
•Do not occur in South Atlantic or South
Pacific (W of South American coast) – Ocean
waters too cool
Stages of a tropical cyclone:
•Tropical Disturbance – can originate from a weak lowpressure area called tropical waves. Common in
summer and early fall.
•Tropical Depression – when stage 1 acquires a
cyclonic circulation around a center of low pressure
•Tropical Storm – when wind speeds around the lowpressure center reach 65 km/h (40 mph).
•Hurricane – wind speeds reach 120 km/h (74 mph) and
develop a calm center (eye). Strongest winds are
concentrated in a band surrounding the eye (eyewall).
Can be called by one of three names:
Atlantic – Hurricane
Pacific – Typhoon
Indian – Cyclone
Saffir-Simpson Hurricane Scale:
Category 1 – winds 74-95 mph
Category 2 – winds 96-110 mph
Category 3 – winds 111-130 mph
Category 4 – winds 131-155 mph
Category 5 – winds in excess of 155 mph
Storm Surge – Occurs when hurricane-force winds
drive a mound of ocean water towards coastal areas,
where it washes over the land.
RECURRING WEATHER
•Drought – Extended period of well-below-normal
rainfall
•Heat Wave – Extended period of above-normal
temperatures
•Heat Index – A measurement of “apparent”
temperature calculated by using actual
temperature and relative humidity
•Cold Wave – Extended period of below-normal
temperatures
•Wind-Chill Index – A measurement calculated by
using the wind speed and actual temperature