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General circulation of the
atmosphere
and climatic zones
Radiation
Conduction
Shallow layer heated by contact
Convection
Clear day
Overcast day
Clear night
Terrestrial
Long wave
Clear night
Terrestrial
Long wave
Absorption
by water vapour
and CO2
Large net loss
Overcast night
Solar heating
2%
absorbed
20%
absorbed
CLOUDS
AND DUST
35%
reflected
CLOUDS, ICE
SNOW, WATER
43%
reaches earth
Why is it coldest at the
‘poles’.....
... and warmest in the tropics ?
Area heated
Depth of atmosphere
N Pole
Equator
‘Still’ earth
‘Still’ earth
40N
20N
0
20S
40S
Rotating earth
Polar
Disturbed Temperate
Warm Temperate
Arid Sub-Tropical
40N
20N
Tropical
0
Equatorial
Tropical
20S
Arid Sub-Tropical
Warm Temperate
40S
Disturbed Temperate
Polar
Rotating earth
Any questions?
Air Masses
Air Masses
 Definition:
 A large body of air (covering thousands of
square miles) in which, at a given level,
temperature, lapse rates and humidity
characteristics are almost uniform
Air Mass - Origins
Polar
Tropical
Maritime Track
 Evaporation
 moistens air
 raises dew point
Maritime Track
 Heating (from below)
 raises temperature
 makes air unstable
Maritime Track
 Cooling (from below)
 lowers temperature
 makes air stable
Continental Track
 Evaporation
 has little effect
Continental Track
 Heating (from below)
 increases temperature
 makes air unstable
Continental Track
 Cooling (from below)
 lowers temperature
 makes air stable
 traps pollutants
Tracks into Britain
Polar Maritime (Arctic)
Tropical Maritime
Polar Continental
Tropical Continental
Returning Polar Maritime
Air Mass - Tracks
Polar
Maritime
Polar
(or Arctic)
Maritime
Polar
Continental
Returning Polar
Maritime
Polar
Continental
Tropical
Continental
Tropical
Maritime
Tropical
Continental
Polar Maritime
Cold air over warm
sea Very Long Sea Track
Polar Maritime
Iceland
Scotland
Polar Maritime
Cold
and dry
Warming
from below
-05 /-10
Increasingly
unstable
Moistening
-02/-07
00/-03
Iceland
Scotland
06
07
08
09
08
Polar Maritime
Iceland
Scotland
Polar Maritime
•
•
•
•
Unstable air mass
Sct Cu/Cb cloud
Showers of rain or snow
Visibility very good, except in showers
Iceland
Scotland
Tropical Maritime
Warm air over cold
seaVery Long Sea Track
Tropical Maritime
Azores
SW England
Tropical Maritime
Warm
Rather moist
Cooling
from below
23/15
Increasingly
stable
Moistening
19/16
16/16
Azores
SW England
22
20
18
17
16
Tropical Maritime
•Stable
•Ovc StSc
•Drizzle, hill/coast fog
Azores
•Poor
Polar Continental
Summer
Warm air over warm
Short Sea Track
land
Polar Continental
Summer
24/06
25/05
16
Kent
English
Channel
25/05
26
North France
27
Germany
Poland
Polar Continental
Warm air over cold
seaLong Sea Track
Summer
Polar Continental
Summer
16/14
14
Lincolnshire
21/09
North Sea
17
25/05
Baltic
North Germany
16
Latvia
Polar Continental
Stable
Ovc StSc
Drizzle, hill/coast fog
Moderate/poor visibility
North Sea
Lincolnshire
Summer
Baltic
North Germany
Latvia
Polar Continental
Winter
Cold air over cold
Short Sea Track
land
Polar Continental
Winter
-09/-17
-12/-20
07
Kent
English
Channel
-12/-20
-15
North France
-15
Germany
Poland
Polar Continental
Stable
Nil cloud
Haze
Moderate visibility
Kent
English
Channel
North France
Winter
Germany
Poland
Any questions?
Wind
Terminology
• Veering
– Direction changing in a clockwise sense
• Backing
– Direction changing in an anti-clockwise sense
260 deg
180 deg
VEERING
110 deg
200 deg
BACKING
Pressure gradient
low pressure
high pressure
By sloshing water in a bath a horizontal pressure gradient
is created on the bottom of bath, and as a result a flow is
established from high to low pressure (this is analogous to
pressure gradients in the atmosphere)
‘Still’ earth
100 W
100 W
60N
500 mph
60N
30N
870 mph
30N
Imagine a southerly airflow
blowing over the coordinate
system
As a result of the rotating
coordinate system the southerly
airflow veers
PGF = pressure gradient force
GF = geostrophic force (or coriolis force)
PGF
PGF
986mb
PGF
V
988mb
990mb
B
PGF
GF
992mb
994mb
996mb
GF
GF
PGF
GF
998mb
1000mb
A
At B, PGF and GF are equal but opposite in direction,
The resultant wind (V) is the geostrophic wind
Buys Ballot’s Law
In the northern Hemisphere standing with your
back to the wind, the area of low pressure is on
your left
In which direction
does the wind
blow ?
L
H
984
988
992
1032
1028
1024
Surface Wind Speed
To determine the surface wind speed over the land:
Multiply the geostrophic wind speed by 0.5 (by day)
Multiply the geostrophic wind speed by 0.3 (by night)
To determine the surface wind speed over the sea:
Multiply the geostrophic wind speed by 0.75
Surface Wind Direction
To determine the surface wind direction over the land
or sea:
Subtract 20o from the geostrophic wind direction (over land)
Subtract 10o from the geostrophic wind direction (over sea)
Gusts
To calculate gusts, multiply the surface wind by one
of the following factors:
Open Sea
Isolated hill tops
Flat open country
Rolling country (few wind breaks)
Rolling country (numerous wind breaks,
1.3
forests, towns, outskirts of large cities)
1.9
Centres of large cities
2.1
1.4
1.6
1.7
Beaufort Wind Scale
Force
Speed (knots)
Description
0
<1
Calm
1
1-3
Light air
2
4-6
Light breeze
3
7-10
Gentle breeze
4
11-16
Moderate breeze
5
17-21
Fresh breeze
6
22-27
Strong breeze
7
28-33
Near gale
8
34-40
Gale
9
41-47
Severe gale
10
48-55
Storm
11
56-63
Violent storm
12
>64
Hurricane
Practical
• Calculate surface winds at different locations
– Measure isobaric spacing
– Use geostrophic wind scale to estimate geostrophic
wind
– Correct for friction
Geostrophic
wind scale
Diurnal variation of wind
Day
Night
H
(ft)
2000
Surf
H
geostrophic= 30 kt
10
20 30
(ft)
2000
Surf
V (kt)
geostrophic= 30 kt
10
20 30
V (kt)
Summary
• Friction:– leads to the surface wind blowing across the isobars
towards low pressure
– is negligible above 2000 ft (i.e. above the friction layer)
– varies daily, as a result of thermal, turbulent mixing
Local winds
• The cause of a local irregularity in the
pressure pattern might be ?
– Topography (shape of the land)
– Heating/cooling
– Combination of the above
LAND AND SEA (LAKE) BREEZE
CIRCULATIONS ACROSS A SHORELINE
Day
DURING ANTICYCLONIC WEATHER
Night
Clear night and a light wind (i.e. slack pressure
gradient)
Air in contact with ground cools by conduction
and its density increases, causing it to flow
down the slopes of the valley
Katabatic wind
Met Office
Anabatic wind
The reverse argument for katabatic wind
Pressure pattern
Air funnels through valley at greater speed
Summary
• The causes of local winds are:– Topography
– Heating or cooling
– A combination of the above
• Examples of local winds are:–
–
–
–
Katabatic
Anabatic
Valley
Sea breeze
Any questions?
Fronts and
Frontal
Depressions
Low
988
Front and frontal surface
Frontal
Surface
Tropical Air Mass
Polar Air Mass
North
Front
Development of a Frontal
Depression
Cold Air
Low
Cold Air
Warm Sector
Frontal depression, with warm/cold fronts
Development of a Frontal
Depression
Polar Air
Falling Pressure
Polar Front
Tropical Air
New Low
Low
Day 4
Day 3
Day 2
Low
988
995
Low
979
Low
984
Low
Day 5
Occlusion
disappearing
Well occluded
(slow moving low)
Low 1011
Day 1
Frontal wave
(developing)
Warm Sector
Depression
(deepening)
Partly occluded
(low starting to fill)
Life cycle of warm sector depression.
Frontal slopes and surfaces
Frontal slopes and surfaces
Direction of Movement
30,000 FT
Average Slope 1:150
20,000 FT
Warm Air
10,000 FT
Cold Air
0 NM
Warm
Front
200NM
400NM
600NM
Earth’s Surface
Frontal slopes and surfaces
Direction of Movement
30,000 FT
Average Slope 1:50
20,000 FT
Warm Air
10,000 FT
Cold Air
150NM 0 NM
Cold
Front
Earth’s Surface
Frontal slopes and surfaces
Direction of Movement
30,000 FT
Average Slope 1:50
Average Slope 1:150
20,000 FT
Warm Air
10,000 FT
Cold Air
Cold Air
150NM 0 NM
Cold
Front
0 NM
Warm
Front
200NM
400NM
600NM
Earth’s Surface
Pressure changes
Pressure changes
1000
1000
1002
1004
Movement
1002
1004
Pressure changes
Plan View
1000
1000
1002
1002
A
1004
B
1004
Cross Section
B
A
1004
1002
1001
1001
1002
1004
Wind changes at a front
Wind changes at a front
Front moves
Warm front
Warm front
40,000 ft
Tropopause
30,000 ft
Tropical Air
20,000 ft
Slope: 1:150
0 nm
200nm
Polar Air
400nm
10,000 ft
600nm
Warm front
40,000 ft
Ci
30,000 ft
Cs
As
20,000 ft
Ns
10,000 ft
Cu
Sc
St
0 nm
200nm
400nm
600nm
Warm front
40,000 ft
Tropopause
Tropical Air
Ci
Cs
30,000 ft
Jet Stream
As
20,000 ft
Polar Air
Ns
10,000 ft
Cu
Sc
St
0 nm
O deg C isotherm
200nm
400nm
600nm
Cold front
Cold front
40,000 ft
Tropopause
40,000 ft
30,000 ft
Tropical Air
20,000 ft
Slope: 1:50
10,000 ft
Polar Air
400nm
200nm
0 nm
Cold front
40,000 ft
40,000 ft
Ci
Cs
30,000 ft
As
20,000 ft
Cb Ns
Cb
10,000 ft
Cu
Sc
St
400nm
200nm
0 nm
Cold front
40,000 ft
40,000 ft
Ci
Tropical Air
Jet Stream
Cs
Tropopause
30,000 ft
As
20,000 ft
Polar Air
Cb Ns
Cb
400nm
Cu
O deg C isotherm
200nm
10,000 ft
Sc
St
0 nm
Describe the weather likely to be
produced by:
 Warm front
 Cold front
Low
988
When a warm front passes?
Low
Wind: Backs ahead
Veers astern
Pressure: Falls ahead, Steadies astern
Temperature:
Rises
Dew Point:
Rises
Cloud Base:
Falls
Weather:
Rain to drizzle
Visibility:
Deteriorates
When a cold front passes?
Wind: Backs ahead veers astern
Pressure: Falls ahead rises astern
Temperature:
Falls
Dew Point:
Falls more
Cloud Base:
Rises
Weather:
Rain dies out
Visibility:
Improves
Low
Mobile anticyclone and frontal
depression
Any questions?
Visibility and
Fog
Upslope (Hill)
Radiation Fog Formation
•Clear skies
•Light winds
•Moist air
•Long cooling period
•Low-lying ground
Met Office
Advection Fog Formation
Requires:
Warm, moist air
Cold surface
Some wind
Occurs:
Over sea and windward coasts
Late spring and early summer
Advection Fog Formation
Occurs:
Overland
In winter
After severe frost and/or snow
Any Questions?
SOURCES OF WEATHER
INFORMATION
Inshore Waters Forecast
Inshore Waters Forecast to 12 miles offshore from 0500 UTC
to 1700 UTC.
From Cape Wrath to Rattray Head including Orkney.
12 hour forecast:
Wind: variable 2 to 4 becoming southwest 5 or 6, possibly
7 around Cape Wrath.
Weather: fair then patchy rain and drizzle.
Visibility: moderate or good.
Sea State: moderate or rough, but slight Inner Moray Firth.
Outlook for the following 24 hours:
Wind: west or southwest 5 or 6, occasionally 4 Inner
Moray Firth.
Weather: patchy light rain and drizzle, mainly north.
Visibility: moderate or good.
Weather on the Web
The UK Met Office
http://www.meto.govt.uk
Has satellite pictures from around the globe, pressure
charts (00, 24,48hrs) but without a wind scale. The aviation
chart F215 has good weather, cloud and visibility forecasts
(need to register (free))
Bracknell Fax Charts
http://weather.noaa.gov/fax/otherfax.shtml
http://www.wetterzentrale.de/topkarten/tknfax.html
Both sites have Met Office charts with wind scale for
00, 24, 36,48,60,72,96,120 hours
ECMWF Charts
http://www.ecmwf.int/services/forecast/index.html
Surface pressure charts to 6 days but without a
wind scale, covers N. Atlantic, northern and southern
hemispheres
Satellite Picture
http://www.met.fu-berlin.de/wetter/meteosat/D2.jpg
The latest infra-red picture, covers Europe and south
to Dakar
Any Questions?
Chart interpretation
Interpret surface meteorological
charts
Movement of lows
• Developing depressions move at near 100%
and move parallel to warm sector isobars
• Developed depressions move at 80%
• Occluded depression will slow down and
curve polewards
Movement of Anticyclones
• Anticyclonic movement is difficult to
calculate past history is a useful guide.
• Mobile ridges (those between fronts) tend to
move at right angles to their axis
Movement of fronts
• Warm fronts
– 65% over sea
– 50% over land
• Cold fronts and troughs
– near 100%
• Occlusions become slow moving
• Fronts move 90º to their orientation
• Measure along axis of front apply to geostrophic
scale at the correct latitude then apply correction
Measurement of winds
• Take a mean between isobars apply to geostrophic
scale then apply correction:
– Over the sea
• Direction back by 10º, speed 75% of geostrophic
– Over the land
• Direction back by 20º, speed 50% , 30% at night
Visibility terms
• Good
– 10 - 20km
• Moderate
– 4 - 10km
• Poor
– 1 - 4km
• Fog
– < 1km
Any Questions?
Summary
Temperature=8.6C
Intermittent
moderate rain
Visibility=3000M
086
Pressure=1012.2 mb
122
30
28
Past weather =
rain and drizzle
082
Dew point=8.2C
Pressure tendency
= rising strongly
then falling,
at 2.8mb in3 hr
8/04
Wind=140 10 KT
Low cloud=Stratus
8/8 at 400 FT