Transcript Local Weather Systems

Local Weather Systems
Ch 15 SciencePower 10
Lifting Mechanisms for Air (text p. 488)
A. Solar radiation heats ground, ground heats air
by conduction, warm air expands and rises
B. Two air masses of different temperatures meet
cooler mass slides under warmer mass
pushing it higher up where it then begins to
cool
C. Moving air mass meets mountain range, rises
to cross mountains, expands and cools, often
reaching dew point temperature
If the ground is very cool, the air above is
cooled by contact. Ground continues to
cool by emitting IR (infrared) radiation. Air
cools to dew point and clouds or fog forms
Cloud Shapes
Three main shapes- cumulus, stratus,
cirrus
Cumulus refers to puffy or billowy
Stratus means layered
Cirrus refers to thin, whispy,
Cloud Shapes continued
Stratus clouds may form when air masses
meet, when air is pushed up a mountain
range, or when conduction takes place
with a cool ground suface.
Fog is a stratus cloud
Radiation fog when rapid cooling of the
ground results in the air reaching dew
point and condensing.
Cloud Shapes continued
 If there is very little water vapour in the air, dew
point temperature is low and air must rise very
high to cool sufficiently to dew point
temperature. Water vapour undergoes a phase
change directly to solid ice crystals
(deposition).
 Cirrus clouds made up of ice crystals
 Clouds often have features that are a
combination of the three shapes and they are
named as such e.g. cirrocumulus
Rain or snow clouds have the word
nimbus included e.g. cumulonimbus,
nimbostratus
Cloud Classification (text p. 490-1)
Low (< 2 km)
Middle
(2km <x< 6 km)
High (>6 km)
All level
Stratus
Altostratus
Cirrus
Cumulus
Nimbostratus
Altocumulus
Cirrostratus
Cumulonimbus
Stratocumulus
Cirrocumulus
Formation of raindrops
 Size of water droplets in clouds determine type of
precipitation we get.
 Average water droplet ≈ 0.02 mm needs to be 0.5 mm to
fall. Avg raindrop ≈2 mm
 Cloud droplets almost always in motion due to air
currents-they coalesce and blend together forming
raindrops
 In temperate climates e.g. Canada, raindrops start out as
snowflakes which in turn begin as ice crystals. Water
vapour deposits on ice crystals which collide with other
crystals an stick together forming snowflakes. The
snowflakes melt into raindrops as they fall through
warmer air.
Rain, Snow, Sleet, Hail (text p. 494)
 If the air below the clouds is below freezing all the way to
the ground, the ice crystals in the clouds stay in the form
of snow. If the air is very cold, crystals remain small (like
powdery snow). If the air is warmer but still below
freezing, crystals stick together and form larger
snowflakes. If air near ground is cold enough, raindrops
freeze again into ice pellets called sleet. If the air is
warm enough to allow rain to fall but ground and objects
on ground are below freezing, rain freezes instantly on
contact and can cause damage and dangerous driving
conditions. Hail starts out as frozen raindrops but they
are carried back up into clouds by wind currents, they
collide with droplets in the air at or around freezing
forming hailstones. Hail can cause serious damage to
crops and property
Highs, Lows, & Fronts
An air mass is a parcel of air that spends
enough time over a region to take on the
temperature and humidity of that region.
Moving air masses interact and sometimes
“fight” with each other.
The zone between air masses is a front.
The 4 types of fronts are: cold, warm,
stationary, and occluded
Highs, Lows, & Fronts
 A cold front is the zone where a moving cold air
mass overtakes a warmer air mass. The denser
cold air moves in below the warm air pushing it
rapidly upward. A cumulus cloud forms. The
water vapour in the rising air condenses
releasing energy and slowing down the cooling
of the rising air. If the rising and condensation
continues, a cumulonimbus cloud forms and rain
usually falls. While some cold air masses move
slowly, others can travel at 100 km/h. This fast
moving front normally causes intense
thunderstorms often with hail and sometimes
tornadoes. (see text p 497)
Highs, Lows, & Fronts
A warm front is created when a warm air
mass overtakes a cooler one. The warm
air gently rides up over the training end of
the cold air mass (see p. 497). The zone
of contact between the two masses is a
gently slope. Stratus clouds usually form
and cirrus clouds higher up.Nimbostratus
clouds form close to the front bringing light
rain or snow. The temperature is warmer
after the warm front has passed.
Highs, Lows, & Fronts
 A stationary front occurs when a cold and a warm air
mass contact each other and neither moves. Some of
the warm air rises and passes over the cold air. The
rising air condenses forming stratus clouds and often
rain. If the front remains in place for a few days,the result
is a dreary, gloomy sky
 An occluded front is created when a cold front catches
up with a slow moving warm front, pushes it up and out
of the way and meets the leading cold air mass. The cold
front brings cumulus clouds while the warm front brings
stratus and cirrus clouds. Occluded fronts often cause
steady precipitation.
Highs, Lows, & Fronts
 A high pressure system occurs where air is descending (i.e exerting
more pressure on region below)
 If an air mass forms over cold ground, conduction and convection
cause the air above the ground to cool, become more dense and
settle closer to the ground. This also increases air pressure. The
high pressure system pushes air out toward the lower pressure area
at the perimeter. In the Northern Hemisphere, this air curves to the
right in all directions because of the Coriolis effect. This sets up a
clockwise rotation of air called an anticyclone. High pressure
systems form from cold descending air with little moisture hence no
cloud formation.
Highs, Lows, & Fronts
 A low pressure system forms when warmer air
rises leaving behind less dense air. As the air
rises, more air is pulled in beneath it. The
Coriolis effect causes the air to curve to the right
initiating rotation in a counterclockwise direction
in the Northern Hemisphere. This forms a
cyclone. Low pressure systems bring unstable
conditions e.g. changing weather, precipitation
and often storms.
Severe Weather
Thunderstorms
Tornadoes
Hurricanes
Monsoons
Severe Weather
 3 stages in the development of thunderstorms (p. 503)
 1. Cumulus cloud forms –warm air quickly rises
 2. Warm air condenses, latent energy released updraft
created, droplets coalesce into snowflakes then rain
which falls to Earth. Downdraft created as well as anvil
shape at top. Charge separation occurs. Electrons at
bottom, buildup of negative charge. Eventual discharge
to ground as lightning.
 3. No more warm air drawn up, rain continues with
downdraft.
Severe Weather
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Tornadoes (see p. 505)
A swirling vortex of air
Wind speeds of 60 km/h to 500 km/h
3 stages of development
1. Small cyclones in thunderclouds intensified by wind shear i.e.
blowing in different directions at different heights
2. Lower pressure in centre of cyclone and below stretches cyclone
which becomes narrower. Then spins faster. Wall cloud visible at
base.
3. Cyclone continues to stretch & spinning accelerates. Funnel cloud
develops when touches ground it is classed as a tornado.
Fujita scale F0 to F5 based on wind speed and damage caused.
Tornado alley- Oklahoma, Kansas, Missouri
Severe Weather
 Hurricanes
 Larger more destructive tornado. May be 500 km wide. Swirling,
roving thunderstorm.
 Fueled by thermal energy of warm tropical ocean water
 Need Coriolis effect to start spinning
 Directly over equator Coriolis effect is absent.
 Most hurricanes affecting North America start in the Atlantic west of
Africa. Thunderclouds form, and are drawn together by updrafts
fueled by water vapour from warm ocean water and energy released
by condensation in clouds. Coriolis starts spinning. When winds
reach 37 km/h storm is classed as a tropical depression. At 65 km/h
it is a tropical storm. At 120 km/h it is a hurricane.
Severe Weather
 Monsoons
 Systems of winds bringing torrential rain and flooding in
summer. Affects mainly Southern Asia because of
position of the land and oceans. Intense heat in summer
causes hot air to rise brings strond winds from Indian
Ocean. Warm moist air generates strong rains and
flooding. In winter, direct sunlight warms ocean, land
cools, wind blows from land to sea. It is dry and a high
pressure system from Siberia directs cold dry air toward
southern Asia.