Thunderstorms/tornadoes - Chapter 14

what is the first stage in the lifecycle of a thunderstorm?

cumulus - vertical growth of cloud, releases large quantities of latent heat, evaporates at top due to dry air but moistens with time, updrafts as cloud is growing

• entrainment draws drier air into the cloud

what is the second stage in the lifecycle of a thunderstorm?

Mature stage: downdrafts develop, reaches stable part of atmosphere - falling precipitation evaporates due to entrainment of drier air, cooling drives descent

• gust front forces warm, humid air up into the storm which enhances the cloud’s updraft

what is the third/final stage in the lifecycle of a thunderstorm?

dissipating: updraft weakens as gust front moves on; downdraft dominates throughout much of the cloud

squall line

line of thunderstorms along cold front or in it

mesoscale convective complex

thunderstorms organized in large circular weather system

• number of individual multicellular thunderstorms may occasionally grow in size and organize into a large, roughly circular convective weather system

microburst

when a downdraft can become localized so that it hits the ground and spreads horizontally in a radial burst of wind - winds spanning less than 4km

• associated with severe thunderstorms that produce strong, damaging winds

  • also can occur with ordinary cell thunderstorms

what is the 5 second rule with lightning?

it is 5 seconds/mile for thunder

ex. if thunder occurs 15 seconds after the strike, the lightning is 3 miles awayw

what is the development of a lightning stroke?

• A. negative charge near the bottom of cloud becomes enough to overcome air resistance

  • flow of electrons (stepped ladder) rushes towards the Earth approach

    • cloud to ground

• as electrons rush towards the ground, region of positive charge moves up through the air through any conducting object

• when downward flow meets upward charge, bright return stroke carries positive charge upwards into the cloud

  • ground to cloud

what are the conditions for thunderstorms?

• warm moist air in a rising, conditionally unstable environment

  • buoyant force acting on the air

wind shear

change in wind speed/direction with height in the atmosphere

• separates updrafts/downdrafts, makes storm stronger & last longer

ordinary cell thunderstorms

form in regions with limited vertical wind shear

• form as parcels of air are lifted from the surface by turbulent overturning in the presence of wind

• often form along shallow zones where surface winds converge

gust front

when cold downdraft reaches Earth’s surface, it pushes outward in all directions producing a strong gust front that represents the leading edge of the cold outflowing air

• represents that of a cold front - temperature drops, wind shifts, becoming strong and gusty

severe thunderstorms

large hail, strong downdrafts, or tornadoes

• moderate shear: winds aloft push precipitation downward so it does not fall into the updraft and does not suppress it, allowing hail to grow larger

supercell thunderstorms

with strong vertical wind shear, may form that the outflow of cold air from the downdraft never undercuts the updraft

• intense, long-lasting thunderstorm with a single violintly rotating updraft

mesocyclone - formation of tornado from supercell

rotating tube on south side of storm, usually 5-10km across

produced by rear flank downdraft interacting with inflow at the ground

• area of lower pressure

• rotating updraft is so strong that precipitation cannot fall through

  • clouds form with updraft but inflowing air cools and condenses, causing descent to surface

what are the conditions that form a tornado?

warm humid air at the surface overlain by cooler, dry air - very unstable, strong wind shear

what are properties of tornadoes?

rapidly rotating columns of air about an intense low circulation that reaches the ground

• most are CCW in the Northern Hemisphere, 100-600km in diameter, 20-40 knots

• form from highly conditionally unstable conditions

• most commonly occur March-July

• cyclostrophic force - centrifugal force that acts outward, balancing the inward pull

what is tornado alley?

region in US where tornados most frequently occur

• central plains, stretches from central Texas to Nebraska