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Severe Weather

Disaster Science & Management
Course Code
DSM 2000

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Severe Weather Part 1
Air Pressure:
-Represents the weight of the atmosphere on the surface of the Earth
-Decreases with increasing altitude
-Measured in millibars (mb) or inches of Mercury
-High Pressure system
-denoted by the letter ‘H’ on a weather map
-region of higher air pressure than the air around it
-Pressure Gradient: air pressure always flows from high pressure to low pressure ->
pressure gradient force (PGF)
-calculated as the change in pressure between two locations dividen by the
change in distance of those two locations
-Isobars: blue lines that connect points of equal air pressure
-Wind speed is directly proportional to the PGF
-If the PGF is high, the wind speed is high and vice versa
-Coriolis Effect: result of the fact that the Earth’s rotational velocity varies by latitude
therefore objects that move through the atmosphere in the northern hemisphere are
deflected to the right of their projected path, while objects that move through the
atmosphere in the southern hemisphere are deflected to the left of their projected path
-object in the NH moving S -> N will be deflected to the East
-Same object moving N -> S will be deflected to the West
-In the NH, the movement of air around a high is always clockwise and around a low it is
always counterclockwise
-Forces of friction negligible in the NH
-At the surface, friction causes the flow of air to changed into a spiral pattern
-High flow -> outward and clockwise
-Low from -> inward and counterclockwise
-Opposite in the SH
-PGF always remains the same regardless of the location
-Air Masses: a large body of air that shares similar temperature and moisture content
-“c” refers to origin over a continental land mass -> dry masses or low moisture
-“m” refers to maritime and means it has oceanic point of origin -> moist and high
moisture content
-Second letter describes the characteristics
-T -> tropical, warm air masses
-P -> polar, cold air masses
-A -> arctic
-mP: Maritime Polar
-cA: Continental Arctic
-cP: Continental Polar
-mT: Maritime Tropical
-cT: Continental Tropical
-Weather front: a boundary that separates two distinct air masses

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-All fronts over North America move west to eat with westerly winds
-Some also swing like a pendulum in the direction that their frontal symbols point
-Cold front: separates a cold air mass from a warmer air mass, the front
positioned in front of the cold air
-air is advancing in the direction the blue spikes are pointing
-warm air pushed up over the cold because its of lower density
-as it rises, cools and the water vapor condenses to form clouds
-vertical cloud, short lived rain
-NW -> SE
-Warm front: separates a cold air mass from a warmer air mass, front positioned
in front of the warm air
-warm air advancing in the direction that red symbols are pointing
-advancing warm air pushed up against cold air because it is of lower
-same cloud formation -> warm air, rises, cools, and water vapor
-horizontal cloud, light and steady rain that lasts longer because
warm fronts move slower
-Move slower because lower density air (warm) pushing
higher density air (cold)
-SW -> NE
-Stationary Front: separated a cold air mass from warm but doesnt move in the
direction that its symbols are pointing
-neither air mass is advancing
-W -> E
-Cold front symbols usually point south meaning the cold air resides
behind the blue spike symbols
-Warm front usually points north meaning the warm air resides behind the
red semi circles
-not always parallel to lines of latitude
-Most popular on southern boundary of the cP air mass -> Polar front
-Mid Latitude Cyclone: low pressure system that forms anywhere fro about 30
degrees north latitude to about 60 degrees north latitude
-Can also from in the same respective zone in the SH
-In NH, counterclockwise movement of air around a low, forces cold air
from the north to pulled southward on the west side of the low resulting in formation of a
cold front
-Opposite occurs for a warm front
-Fronts eventually collide and created a occluded front
-Occluded Front: occur when cold fronts collide with warm fronts in a mid
latitude cyclone system
-front with both warm and cold front symbols on the same side
-variable weather sometimes associated with thunderstorms
-warm air is trapped above the two cold air masses

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Thunderstorms: electrical storm associated with lightning and thunder
-typically accompanied b heavy rainfall, strong winds, and in more rare cases hails and
-most frequent in the mid latitudes where collisions between cold and warm, most air
masses are most prevalent
-In order for a thunderstorm to be formed a number of conditions must be satisfied:
-Fuel in the form of water vapor -> provided by mP or mT air masses
-Instability -> warm moist air at or near the surface with cold dry air above it
-Lifting mechanism: any mechanism that causes uplift of air in the atmosphere
(unstable air continues to rise up when it is pushed down)
-Convective lifting: air near the surface is warmed to a point where it is
warmer than the air around it
-air becomes bouyant and rises
-sub tropical and tropical regions
-Orographic lifting: air that is advected horizontally encounters a large hill
or mountainous barrier
-air pushes against barrier and forced upward
-Frontal lifting: cold air advaces along cold front and warm air forced up
over the cold air
-Convergent lifting: associated with low pressure systems, air from all
around the low is drawn toward the center of the pressure system, air
forced upward, etc
-amount of water evaporation higher over warm ocean currents and therefore puts more
moisture into the atmosphere than cold ocean currents
-highest occurrence frequency is around equator and the intertropical convergence
zone (ITCZ) where air masses of high moisture and heat energy converge
-Thunderstorms do not occur along poles
-Stages of a thunderstorm:
-starts off as a cumulus cloud
-grows into a cumulous congestus cloud (taller and wider)
-dominated by undrafts
-Mature Stage:
-Most dangerous stage
-cloud spreads out near the top of the troposphere (layer in atmosphere where
weather occurs)
-Anvil shaped cumulonimbus cloud
-40-60,000 ft
-Associated with both updraft and downdraft
-outflow winds, hit the ground and spread out in a straight line
-gust front
-Expect occurence of hail, tornadoes, or damaging winds
-Dissipating Stage:
-final stage
-dominated by downdraft
-Sometimes the anvil of the cloud remains (orphan cloud)
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