Download Air Mass Classifications

Violent Weather
Tropical Cyclone - any low that originates over tropical oceans
- ccw (cyclonic flow) in the N.H. & cw (anticyclonic flow) in the S.H.
a) tropical depression - winds <= 33 kts, 61 kph, 38 mph
b) tropical storm - winds between 33-64 kts, 61-120 kph, 38-74 mph
c) typhoon/hurricane/cyclone/willy-willy - winds >= 64 kts, 120 kph, 74 mph
typhoon (fr: Chinese - tai feng, great wind) - western Pacific
hurricane (fr: Caribbean Taino people - Huracan, god of the wind) - N. Atlantic
cyclone - (fr: kyklon - moving in a circle) Indian Ocean
willy-willy - Australia
- to develop there must be low-level convergence, cyclonic wind shear &
horizontal divergence at high tropospheric levels. Usually originate bet. 5° & 20° latitude
in late summer & early autumn. It decays as it moves into the middle latitudes thru cooler
air flowing into it. It also decays faster over land than water. The “eye” forms in the
tropical storm stage with an average diameter of 15-20 miles; maximum wind is located
in a wall of clouds surrounding the “eye”
Favorable Factors for Tropical Storm Formation
1) high sea-surface temperature (> 27°C)
2) high R.H. in middle atmosphere
3) weak vertical shear (change of wind direction or speed) with height
4) location of low level disturbance east of an upper-level trough (found on an isobaric
pressure surface for low pressure)
5) larger than normal vorticity (spin) of low level flow
Winds blow around the storm faster near its center due to the Law of Conservation of
Angular Momentum, where the product of the tangential (rotational) velocity of a parcel
of air about an imaginary vertical axis & the distance of the parcel from this axis of
rotation is constant
If a hurricane remains over warm waters & away from cold dry air masses it may
continue indefinitely. After moving ashore, evaporation from the sea is cut off & rapid
reduction of the wind occurs. The roughness of the land also increases friction &
contributes to the wind reduction. Last from 1-3 weeks
Thunderstorm
- local storm produced by a cumulonimbus cloud & always accompanied by
lightning & thunder
- occur most frequently in the tropical regions & least frequently in the arctic
- in the U.S., they occur most frequently in the south-central & southeast &
predominate in July & August
Thunderstorm Conditions
1) sufficient water vapor in the air
2) unstable lapse rate
3) initial upward lifting (from surface heating, converging winds, sloping terrain, frontal surface
or any combination thereof)
Thunderstorm Life Cycle
1) cumulus & growing - all thunderstorms begin as a cumulus cloud with updraft as a key
feature. It varies in strength, extends from bottom to top, & can grow in excess of
3000’/min. The updraft carries water droplets above freezing level & they continue to
grow. They then fall, creating a cold downdraft
2) mature - falling of precipitation identifies this stage. The downrushing air spreads outward at
the surface giving strong gusty winds, a temperature drop & a rise in pressure. Updrafts
reach 6000’/min. Most intense period, lightning is most frequent, turbulence is most
severe. Reaches its greatest vertical development near the end of this stage
3) dissipating - characterized by downdrafts & the storm rapidly dies, occurs ~15-30 min after
the mature stage, as downdraft hits ground & spreads out it forms a gust front - miniature
cold front
Sizes
diameter: 5-30+ mi
bases: 100s - 10,000+ ft
tops: 25K - 45K ft (at times 75K ft)
Types
1) Air Mass - result from surface heating; is self-destructive because precipitation occurs within
or next to updraft & causes its reversal; life span 20 - 90 min
2) Steady State (Frontal) - associated with weather systems (ex: fronts, converging winds &
troughs aloft) which force upward motion. These often form into squall lines.
Precipitation occurs outside the updraft thereby allowing it to continue. Life span is
several hours
3) Mesoscale Convective Complex (MCC) - develops rapidly, becomes very large, moves slowly,
persists for long periods of time, usually circular or elliptical, more dramatic effects,
appears to grow in weak pressure gradient & light winds, usually very unstable with an
upper level disturbance, within 2½ hrs a scattered air mass thunderstorm can develop into
a MCC, can be the size of the Dakotas, covers several states & lasts 6-12 hrs, 100 x area
of an air mass thunderstorm
Tornadoes
- violently rotating column of air extending from the base of a thunderstorm; winds can
exceed 480 kph (300 mph); they are the strongest surface winds on Earth; can range in diameter
from a few m to few 100 m; can last from a few moments to 10s of minutes; advances in an
irregular track at 25-50 kph (15-30 mph); midwest U.S. has more tornadoes than anywhere else
in the world
a) tornado - touches land surface
b) funnel cloud - doesn’t touch land surface
c) water spout - touches water surface
Enhanced Fujita Tornado Intensity Scale - named after Theodore Fujita, from the Univ. of Chicago