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Fronts, Humidity, &
Severe Weather
AIR MASSES
AIR MASSES
• FRONT:
– The boundary that forms between two
colliding air masses
– The place where most changes in
weather occur
FRONTS
WARM FRONT
• Develops when warm
air mass meets cold
air mass
FRONTS
COLD FRONT
• Develops when cold
air mass invades warm
air mass
• Cold air forces warm
air up rapidly
• Clouds form and
produce rain and
thunderstorms
FRONTS
STATIONARY FRONT
• Happens when pressure differences cause an
air mass to stop moving forward
• May remain in the same place for days
• Light winds and precipitation occur all along
the frontal area
FRONTS
OCCLUDED FRONT
• Develops when two
cold air masses merge
• They force the warm
air between them rise
• Strong winds and
heavy precipitation
occur
Why is every type of front associated with
precipitation???
• They each involve cooling of warm air
• Cool air can’t hold as much moisture
as warm air
• The air becomes saturated and
precipitation falls
HUMIDITY:
Clouds and Precipitation
• Humidity is the
amount of water
vapor in the air.
• When droplets reach
0.2 mm, they are too
heavy to stay
suspended and fall
out of the clouds as
precipitation.
HUMIDITY: Precipitation
Rain:
• Precipitation
that falls in
temperatures
that are above
freezing
HUMIDITY: Precipitation
Snow:
• Water vapor
changes
directly to a
solid without
melting first
• Falls when
temperatures
are below
freezing
HUMIDITY: Precipitation
Sleet:
• Snow passes
through a
warm layer of
air
• It melts
• Then it
refreezes near
the ground
HUMIDITY: Precipitation
Hail:
• Water freezes
in layers
around a
nucleus of ice
HUMIDITY: Precipitation
Hail Formation:
SEVERE WEATHER
THUNDERSTORM
• Result of rapid upward
movement of warm, moist
air
• Where they occur:
– Inside warm moist air
masses
– At fronts
• Falling droplets create
downdraft that spreads out
at surface, causing winds
• Thunder and lightning
TORNADO
• Violent, whirling wind that
moves in a narrow path over
land
• Warm air is forced upward
quickly, causing very low
pressure at the surface
• Strong winds converge from
different directions, collide
and rotate violently
• Pressure inside the rotation
drops rapidly
• Funnel cloud appears at the
base of the cloud
• Water condenses on dirt and
debris, causing dark gray
color
TORNADO
• Wind can reach
speeds up to 500
km/hr
• Usually less than
200 meters in
diameter
• Usually only last a
few minutes
• One of the most
destructive
storms
Hurricanes
HURRICANES
• Largest and most
powerful storm
• Large swirling low
pressure system
HURRICANES
• Forms over
tropical oceans
• Winds over 74
mph or 120 km/hr
• Usually occur
between 5º and
20º north latitude
where the water is
warm
HURRICANES
• While it is over water,
the warm air will rise
and fuel the storm
• Over land, the
hurricane loses its
power
HURRICANE IKE
• Strong hurricanes
cause severe damage
• Main concerns of a
hurricane are floods
and storm surges.
HURRICANE DAMAGE