2. 2
Measuring Weather:
air pressure
mercury barometer
aneroid barometer
wind
Jet Stream
pressure gradient
isobars
Highs
Lows
pressure difference
Coriolis effect
friction
air mass
cyclones
anticyclones
3. 3
Air mass- parcel of air with the same qualities
(pressure & temperature)
• Takes on characteristics of the area that formed it
Air moves in two ways:
-- VERTICALLY due to air pressure
-- HORIZONTALLY due to tendency of moving from
HIGH H to LOW L pressure areas
(In N Hemisphere, L
tends to move
north and H tends
to move SE)
4. 4
Air pressure: weight of air above, pressing down with gravity
• Changes with
elevation
50% of air
lies below
this altitude:
VIDEO: How Heavy is Air? (3 min)
5. 5
MEASURING AIR PRESSURE
Air pressure: ‘weight of air’; exerted
in all directions
• Mercury Barometer-
• Inverted tube in a bath
of Mercury;
• Air pressure pushes
down on the Mercury
bath, causing the
Mercury to go up the
tube
VIDEO: How does a barometer work? (5 min)
6. 6
Forecasting Weather
• Pressure falls: indicates storms & rain
• Pressure rises: fair weather
• Pressure steady: more of the same
7. 7
WIND
Wind: horizontal differences in air pressure; air
moves from areas of high pressure to areas of low
pressure
VIDEO: Why does the
wind blow? (3 min)
Wind speed:
anemometer- measures
wind speed
10. 10
WIND is influenced by:
Pressure difference: Solar radiation unevenly
heating atmosphere
11. 11
WIND is influenced by:
Coriolis effect: Earth’s rotation affects movement of
objects; deflects objects to the right in N
Hemisphere
Actual
Movement
Direction of
Movement
VIDEO: Coriolis Effect (3 min)
12. 12
WIND is influenced by:
Friction:
• Friction Layer: landforms slow air masses near
the surface
• Jet stream: high, fast moving air above the
friction layer; West to East direction
13. 13
How do we show Wind on a map?
Start with AIR PRESSURE.
Is it rising or falling?
• pressure gradient- spacing of isobars show
amount of pressure change over a distance
How closely are the lines spaced?
• Isobars- closer spaced isobars
= steep pressure gradient
= high winds
LOW
A
B
14. 14
H = “heavy”, high pressure:
L = “light”, low pressure
LOW
A
B
17. 17
•Big Blue H
•EXAMPLE: warm air
mass stays low to the
ground, pushing on the
ground.)
•At ground: swirling cool,
dry air
•Fair weather
•Clockwise swirl in N
Hemisphere
“Hi! How are
you?” arms
HIGH PRESSURE SYSTEMS
H
anticyclone
19. 19
•Big Red L
•EXAMPLE: warm air mass
lifts and moves vertically,
causing less pressure on
the ground.)
•At ground: warm, moist
air, strong winds
•Severe weather/
thunderstorms
•Counter clockwise swirl
in N Hemisphere
Feeling “low” running
away arms
LOW PRESSURE SYSTEMS
L
L
cyclone
21. 21
Patterns
global winds frontal wedging
Polar Air Circulation warm front
Ferrel Air Circulation cold front
Hadley Air Circulation middle latitude cyclones
Polar Easterlies occluded front
Westerlies stationary front
NE Trade winds
SE Trade winds
anemometer
prevailing winds
El Nino
La Nina
Maritime mass
Continental mass
Polar mass
Tropical mass
humidity
22. 22
Air Masses
m
Maritime: origins over the ocean
c
Continental: origins over continents
P
Polar: low humidity
T
Tropical: high humidity
27. 27
GLOBAL WINDS
Because of uneven heating of Earth, the
atmosphere acts as a giant heat-transfer system to
maintain balance
Non-rotating Earth Model Rotating Earth Model
2 cell-convection Multi-cell convection
28. 28
LOW pressure
LOW pressure
LOW pressure
HIGH pressure
HIGH pressure
Polar Air Circulation
Ferrel Air Circulation
Hadley Air Circulation
Atmo 2 WS-
complete
with lecture
29. 29
LOW pressure
LOW pressure
LOW pressure
HIGH pressure
HIGH pressure
Polar Air Circulation
Ferrel Air Circulation
Hadley Air Circulation
Polar Easterlies
Westerlies
NE Trade winds
SE Trade winds
Atmo 2 WS-
complete
with lecture
33. El Nino- part of GLOBAL
WEATHER
• Cold Peruvian current
flows toward equator,
allowing upwelling & good
fishing
• In December, warm waters
replace the current, lasting
for a few weeks, upsetting
weather patterns; blocking
upwelling; inland areas get
more rain than usual
33VIDEO: El Nino 3 min
36. 36
WARM FRONT
• Warm air moves into an area formerly covered by
cooler air
• Hard to move the cooler & denser air, so it slowly
rides on top of it… Creates HEAVY (High) pressure
• Associated with stable, fair weather, possible light rain
if front remains for a few days
Frontal Wedging
38. 38
COLD FRONT
• Cold, dense air moves into a region occupied by
warmer air
• Forceful lifting of warm, wet air causes instability;
thunderstorms & strong wind
• Faster front because it’s denser
Frontal Wedging
40. 40
STATIONARY FRONT
• When air flow on either side is neither toward cold
mass or warm mass
• Stable
• Sometimes, gentle precipitation
Frontal Wedging
42. 42
OCCLUDED FRONT
• When an active cold front moves fast & overtakes an
existing warm front
• Wedges the warm air upwards causing heavy rain
• Front moves slowly, so rains for several days
Frontal Wedging
43. 43
OCCLUDED FRONT
When a cold front
overtakes a warm front
that was moving ahead
of it.
Frontal Wedging
44. 44
Middle Latitude Cyclones
Large centers of low pressure, moving West to
East, causing stormy weather
• between 30° and 60° latitude
US WEATHER
REMEMBER:
49. 49
STORMS
Thunderstorm- storm generating lightening &b
thunder
• 2,000 thunderstorms happening at any given
time; highest number in the tropics (warm,
humid)
• Southeast US
• Warm, humid
air rises in an
unstable
environment
VIDEO: The most lightning
=struck place (3:40)
55. 55
Tornado Warning System
• Issued by National Weather Service
Watch: conditions are favorable for a tornado
Warning: a tornado has been spotted; seek
shelter immediately
VIDEO: How do tornadoes form (4 min)
56. 56
TORNADO SAFETY
• Go to low ground: basement, ditch
• Avoid windows
• Leave mobile home
• At school: follow the drill- interior hallway,
cover head
• Leave vehicle, find a low ditch, cover head
AFTER:
• Do NOT touch downed power lines or sharp
objects
• Stay out of heavily damaged buildings
58. 58
HURRICANE
Hurricane- whirling tropical cyclones producing
winds of at least 74 mph
• Name is regional:
• Hurricane
• Typhoon
• Cyclone
• Tropical cyclone
• Associated with storm surges and flooding
• Occurrences seem to be increasing with global
warming
64. 64
Hurricane Katrina
August 2005
• Extremely destructive and deadly Category 5
hurricane
• 1,800 deaths
• Struck Gulf of Mexico
• Damage from central Florida to Eastern Texas
• New Orleans: sits below sea level & was
catastrophically flooded
VIDEO Katrina (5 min)
66. 66
HURRICANE SAFETY
• Easily predicted, so pay attention to weather
forecasts
• Prepare: food, water, flashlight, first aid kit
• Stay indoors during hurricanes
• Heed local warnings; if an evacuation is
called, LEAVE before the hurricane
• Know evacuation routes
71. 71
•Big Red L
•EXAMPLE: warm air mass
lifts and moves vertically,
causing less pressure on
the ground.)
•At ground: warm, moist
air, strong winds
•Severe weather/
thunderstorms
•Counter clockwise swirl
in N Hemisphere
Feeling “low” running
away arms
LOW PRESSURE SYSTEMS
L
L
cyclone
72. 72
•Big Blue H
•EXAMPLE: warm air
mass stays low to the
ground, pushing on the
ground.)
•At ground: swirling cool,
dry air
•Fair weather
•Clockwise swirl in N
Hemisphere
“Hi! How are
you?” arms
HIGH PRESSURE SYSTEMS
H
anticyclone
