Lecture 10: Thunderstorms and Tornadoes

🌍 1. Frequency & Location of Thunderstorms

Global

Your lecture shows that thunderstorms occur mostly over land between 45°N and 45°S.

“Most thunderstorms occur over land, 45ºN to 45°S.”

Reason: land heats faster → stronger convection.

United States

  • Most frequent in spring.

  • Highest frequency in Florida, Gulf Coast, and Central U.S.

  • Thunderstorm days per year exceed 80 in Florida.

🌡 2. Thunderstorm Formation (Including Lifting Mechanisms)

Thunderstorms require three ingredients (your lecture lists these explicitly):

âś” 1. Moist Air

Source for U.S. storms: Gulf of Mexico (Critical Thinking slide).

âś” 2. Lifting Mechanism

Your lecture covers four lifting mechanisms:

a. Frontal Lifting

Cold front forces warm air upward.

“Advancing cold front forces warm, moist air up.”

b. Gust Fronts

Downdrafts spread out → lift warm air → new storms form.

“Downdrafts… spread out and force air up.”

c. Orographic Lifting

Mountains force air upward.

d. Convergence & Convection

  • Convergence: air masses collide

  • Convection: ground heating causes rising warm air

âś” 3. Atmospheric Instability

“Rising air is less dense… continues to rise, forming an updraft.”
Moist air cools slower → becomes buoyant → strong updrafts.

🌩 3. Types of Thunderstorms & Their Details

Your lecture covers three major types:

A. Ordinary (Single‑Cell) Thunderstorms

Characteristics:

  • No rotation

  • One updraft + one downdraft

  • Form in warm afternoons or mountains

  • Last < 1 hour

Stages (from your diagrams):

1. Developing Stage

  • Warm, moist air rises

  • Updraft dominates

  • Anvil forms at tropopause

2. Mature Stage

  • Rain falls → downdraft forms

  • Evaporative cooling strengthens downdraft

“Evaporative cooling makes air denser, intensifies the downdraft.”

3. Dissipating Stage

  • Downdraft overtakes updraft → storm collapses

B. Squall Lines

“Develop when ordinary thunderstorms coalesce and form a large group.”

  • Long line of storms along cold front or gust fronts

  • Produce heavy rain, hail, lightning, straight‑line winds

  • Shelf cloud forms at leading edge (first visible sign)

C. Supercell Thunderstorms

Most important for tornado formation.

Characteristics:

  • Long‑lasting

  • Rotating updraft (mesocyclone)

  • 30% produce tornadoes

  • ~50 km wide

“Supercell thunderstorms are long-lasting with a rotating updraft.”

Wind Shear

  • Different wind speeds/directions with height

  • Creates horizontal rolling tube → lifted by updraft → rotation

Structure

  • Overshooting top

  • Anvil

  • Wall cloud (tornadoes form here)

  • Forward‑flank downdraft (FFD)

  • Rear‑flank downdraft (RFD)

🌬 4. Derechos

Your lecture includes a full section on derechos:

“Derecho may develop following the shelf cloud… severe winds moving in a straight line.”

Key points:

  • Long‑lived, fast‑moving straight‑line windstorm

  • Forms behind squall lines

  • Can produce 80–100+ mph winds

  • Example: Houston, May 16, 2024 — 90 mph derecho (in your lecture)

⚡ 5. Lightning & Thunder

Charge Separation

“Electrons jump from tiny ice crystals to large ice particles… ice crystals get (+), ice particles get (–).”

  • Updraft carries (+) crystals upward

  • Bottom of cloud becomes (–)

Types of Lightning

  • Cloud‑to‑cloud (most common)

  • Cloud‑to‑ground (least common)

Stepped Leader → Return Stroke

“Electrons surge from the cloud base… as a stepped leader.”
“Main discharge is the return stroke.”

Thunder

“Temperature of lightning is up to 30,000°C… instantly heats air causing explosive expansion.”
Shockwave = thunder.

🌪 6. Tornado Frequency & Locations

Where Tornadoes Occur

“70% of Earth’s tornadoes occur in the central U.S.”

Tornado Alley:
Texas → Oklahoma → Kansas → Nebraska → Iowa

Peak Season

  • Spring (April–June)

🌪 7. Tornado Formation

Your lecture gives a 4‑step process:

Step 1 — Horizontal Roll

RFD creates a rotating horizontal tube of air.

“Rear flank downdraft creates a tube of rotating air.”

Step 2 — Updraft Tilts Tube

Updraft pulls rotation vertical → mesocyclone.

Step 3 — Stretching

Air stretches upward → rotation speeds up.

“Air stretches, thins, and wind speed increases.”

Step 4 — Tornado Forms

If the vortex touches the ground.

Where in the Storm?

  • Tornado forms near the rear‑flank downdraft (RFD)

  • In the wall cloud