Anatomy of a windy day

What keeps a cyclone or anticyclone going?

Jet StreamsGlobal Rivers of Air

Winds

• Know affected by friction • Friction is negligible a few kilometers

above the surface of the earth.• Allows for very high speed winds• Faster winds = greater Coriolis effect• Greater Coriolis effect = winds blow

PARALLEL to isobars.• Called jet streams.

Jet stream characteristics

• Defined: Fast moving river of air.• Few hundred KM wide, few KM thick• 120-140 km/hr• Found at the tropopause (10-15 km)• From West to East• Responsible for the convergence or

divergence, at the tropopause, necessary to maintain cyclones and anticyclones

Two major jet streams:

• In the North, polar jet is most important.– Marks the boundary between cold polar air

in the North and warmer air in the South– Sets up a frontal zone!

• In the South = subtropical jet• Both repeat in the Southern hemisphere

to make a global total of four.

Global Air Circulation

Severe Weather

Thunderstorms & Tornadoes

Thunderstorm

• Rainstorm that generates lightning and thunder!– Can also produce high winds– Hail– Heavy rain– Occasionally tornadoes

• Form when warm, humid air rises in an unstable environment.– Means it will rise, and quickly!

Recall. . .

• When air rises, it cools.• When air cools, its capacity decreases.• When capacity is decreased enough,

the air becomes saturated. (Dew point)• Further cooling results in condensation

(makes a cloud).

Where do they come from?• Beginning or Cumulus Stage• Unstable air means it will rise easily and quickly.

(Low pressure)– Unstable air masses are different from their surroundings

and will move vertically until they are in surroundings that are similar to them.

– Stable air masses are similar to their surroundings and resist vertical movement.

• Raising warm, moist air quickly will result in a cloud.• As air converges at the surface, water

condenses, and latent heat is released(keeps air warm and rising)

• This makes the cloud will become taller. (one hour)

Then what happens?• Mature stage• Water in the cloud becomes too much for the

updraft to keep up.– Updraft = Hot air rises– It begins to rain!– Downdrafts form as air cools inside the cloud (cold

air sinks)• Not that easy. . .

– Stronger updrafts (severe thunderstorms) can freeze the water in the cloud(freezing warms atm).

– Can also catch water (liquid/solid) as it falls and take it back upwards again. (cell)

– This is very BAD for us if it is solid! HAIL!

Hail Stone

Dissipating Stage

• Water begins to cool inside the thunderhead.• Causes downdrafts to dominate the updrafts.• As the downdrafts dominate, the storm cell

dies out.• Downdrafts are cutting off the “fuel” supply of

warm, moist air from the updraft.

Stages of a thunderstorm(PG 520)

Severe Thunderstorms• As a storm front moves, new cells may

fire along it as long as there is a supply of warm moist air. (Cloud/storm doesn’t move.)

• Severe storms form in areas with strong vertical wind shear. (Air moving fast in opposite directions, up and down.)– Has a tilt in the updrafts and downdrafts

• Allows them to be maintained for longer period of time.

– High wind speeds, hail 2” or larger– May even cause rotation. . .

Tornadoes

• Associated with severe thunderstorms• A spinning vortex, column of air,

extending downward from the thunderhead.– Called a funnel cloud when it does NOT

reach the ground.– Majority rotate counterclockwise

Funnel

Vortex 2

Tornado Formation• We don’t know everything!• Tend to happen when upper winds are

blowing at a faster speed than surface winds.– Mesocyclone– Winds roll horizontally

• Updraft of severe storms stand it upright.

Mesocyclone

Radar signature

Anatomy of a Severe Thunderstorm

Updraft = Rain Free Zone

Wall Cloud with Funnel

2nd Funnel Pic