Weather & Climate, Exam 4

Thunderstorms

• a storm that generates lightning and thunder

• form when warm, humid air rises in an unstable environment

• cumulonimbus clouds

Thunderstorm Formation

• form in maritime tropical air masses

• air mass thunderstorms

Thunderstorm Occurrence

• Southeast Regions = highest frequency of thunderstorms → due to mT air masses

• West Coast = few thunderstorms → due to mP air masses and cold ocean currents → increases air stability and suppresses thunderstorms

• Colorado and New Mexico Slopes = slopes are warmer due to areas facing the sun → absorb direct solar radiation → which creates strong updrafts to form thunderstorms

Three Stages of Thunderstorm Development

• Cumulus Stage

• Mature Stage

• Dissipating Stage

Cumulus Stage

• dominated by updrafts

• continuous release of latent heat

• expands the cloud vertically into color air

  • rising air cools adiabatically to its dew point temperature and condensation releases latent heat → making the air in cloud warmer than surrounding air

  • the warmer air in cloud is less dense than surrounding air and causes cloud to grow vertically upwards

Mature Stage

• updrafts and downdrafts are present

• storm produces heavy rain, lightning and thunder, and occasionally hail

• downdrafts pull in cold, dry air from outside the cloud in a process called entrainment

• Entrainment intensifies downdraft

  • the air is colder and denser → denser air tends to sink and increase the motion of air

  • the air is drier & causes some air to evaporate (cooling process), the cooling of the air makes it denser and sink more rapidly within the downdrafts

Dissipating Stage

• downdrafts take over the updrafts

• upward supply of humid air is blocked

• precipitation decreases

• cloud dissipates

Severe Thunderstorms

produce heavy down pours, very strong winds, frequent lightning, and perhaps tornadoes

Severe Thunderstorm Types

• Supercells

• Squall Lines

Supercells

• a single, very powerful cell

• vertical wind profile may cause the updraft to rotate (mesocyclone)

Squall Lines

• long lines of individual storm cells

• common in advance of a cold front or a dry line

Dryline

separates maritime tropical and continental tropical air masses

Downbursts (Microburst)

strong localized downdrafts

Thunder and Lightning

• lighting equalizes electrical difference

• thunder is the sound head when air expands quickly due to the intense heating by lightning

Lightning has Different Components

• Flash

• Strokes

Hail

• produced in large cumulonimbus clouds with strong updrafts

• large or small hard clumps of ice

Tornadoes

violent wind storms that take the form of a rotating column of air (vortex), and extend downward from a cumulonimbus cloud

Tornado Facts

• the pressure within tornadoes can be 10% lower than surrounding area

• maximum wind speeds can exceed 300 miles per hour

Tornado Development

• tornadoes are most commonly associated with severe thunderstorms

• associated with the development of a mesocyclone

Tornado Occurrence

• typically found in spring and summer

• peak number of tornadoes in spring

Average Tornado

• about 500 to 2000 feet wide

• winds on the order of 90 mph

• travels about 30 mph

• short-lived

• most move toward the northeast

Extreme Tornadoes

• persists for a few hours

• travels at speeds over 60 mph

• damage paths longer than 90 miles and a mile wide

• maximum wind speeds exceeding 300 mph

Tornado Destruction

• greatest portion of tornado damage is caused by the winds

• most tornado injuries and deaths result from flying debris

• average annual death toll is about 60 people in the United States

T. Theodore Fujita

developed a guide to tornado intensity

Tornado Forecasting

• they are among the most difficult weather features to forecast

• tornado watch

• tornado warning

Doppler Radar

• a type of radar that has the capacity of detecting motion directly

• Doppler Effect

These weather systems are known by different names:

• Hurricanes

• Tropical Storms

• Typhoons

• Cyclones

Individual Naming of Tropical Cyclones:

• winds exceed 39 mph

• assigned sequentially each year

• male/female names alternated

• names are reused every 6 years

• names of big storms are retired

Hurricanes

• winds over 74 mph

• no fronts

• rain bonds

• eye wall

• wall

• Saffir Simpson Hurricane Wind Scale

Ideal Hurricane Formation Conditions

• require warm water, greater than 80°F

• form in late summer and early autumn

• tend to originate between 5°-30° latitude

Tropical Disturbance

a group of thunderstorms with a weak rotation

Tropical Depression

a low pressure center is formed

Tropical Storms

• winds increase to 39 mph

• storm is given a name

Hurricane Formation

• warm, humid air spirals upwards around eye wall

• air rises, and loses moisture

• air flows out the top of the storm

Hurricane Decay : Hurricanes Diminish in Intensity when…

• move over cooler water

• move onto land

• reach a location where large-scale flow of air aloft is unfavorable

Hurricane Decay - Moves Over Cooler Water

the cooler water supply cannot supply the warm, humid tropical air required to fuel the hurricane

Hurricane Decay - Moves Onto Land

a storm moving over land would also cut off the source of warm humid air. the increased friction of the land surface also slows the winds of the hurricane

Hurricane Decay - Reaches a Location where Large-Scale Flow of Air Aloft is Unfavorable

strong upper-level winds tend to rip apart hurricanes

Hurricane Paths

• Westward or Northwestward direction in the Northern Hemisphere

• North Eastward direction once poleward of the Subtropical Highs

Damage caused by Hurricanes can be divided into 3 categories:

• Storm Surge

• Wind Damage

• Inland Flooding

Storm Surge

• a storm surge is produced by waves of water being pushed toward the shore by the force of winds moving around the shore.

• would be over the high or low tides; and if the waves are low in the open ocean, they eventually gain height as they approach the coast

Right of the Hurricane Eye

• has the strongest winds and greatest storm surge

• the winds blowing counterclockwise around the hurricane are in the same direction of the forward motion of the hurricane, adding strength of the winds, creating faster winds

Left of the Hurricane Eye

• the winds blowing counterclockwise around the hurricane are in the opposite direction as the forward motion of the hurricane, reducing the strength of the winds, creating slower winds

Costliest Hurricanes

• Katrina 2005: $196 billion

• Harvey 2017: $156 billion

• Ian 2022: $116 billion

• Maria 2021: $112 billion

• Ida 2012: $83 billion

• Irma 2017: $62 billion

Estimating Hurricane Intensity

• surface observations are rarely available

• Global Positioning System

Forecasting and Warnings

• Forecasts

• Watch vs. Warning

Hurricane Watch

issued when a hurricane containing sustained winds of 74 mph or higher poses a possible threat within 48 hours

Hurricane Warning

issued when a hurricane containing sustained winds of 74 mph or higher poses a possible threat within 36 hours or less

Tropical Cyclone Forecasts

• can be an error

• this forecasts uncertainty is conveyed by the track forecasts Cone of Uncertainty

Cone of Uncertainty

• the solid white shaped cone and stippled white areas around the hurricane

• gets larger the further into the future because of increased uncertainty in how the hurricane will travel

Hurricane Prediction and the Future

• deaths have decreased dramatically

• coastal populations are increasing