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Naval Meteorology and Oceanography
Professional Development Center
Fundamentals of Weather
Contents
• Air Masses
• Fronts (cold, warm, occluded, stationary)
• Pressure Systems
• Clouds
• Winds
Air Masses
Air masses are large bodies of air that have similar
horizontal temperature and moisture characteristics.
Air masses are classified by
where they originate (their
“source region”).
cA
cP
mP
mP
cT
mT
mT
First letter:
c - continental
m - maritime
Second letter:
P - polar
T - tropical
Air masses move from their source regions.
As they do, distinct boundaries form between them.
Fronts
A “front” is defined as the boundary between two air masses.
Air mass characteristics on either
side of a frontal boundary can be
very different (point “A”) or more
similar (point “B”).
Cool
L
Cold
Warm
B
Cold
air mass
(e.g, cP)
L
A
Warm
air mass
(e.g, mT)
Cool
air mass
Frontal intensity is defined by the
the temperature and moisture
differences on either side.
The stronger the differences on
either side of the front, the more
potential for severe weather.
Fronts
There are four types of fronts.
Occluded
• Cold
L
• Warm
Warm
• Occluded
• Stationary
Cold
Stationary
Cold Front
A cold front defines the boundary between an advancing
cold air mass and a warm air mass.
Cold fronts are associated with:
• Sharp temperature changes over relatively short distances
• Changes in air moisture content
(moist before, dry after)
• Reduced visibility in showers
Cool
• Wind shifts with frontal
passage
• Pressure changes with
frontal passage
Cold
Front
• Unstable cumuliform
clouds and showery
precipitation patterns
Warm
Cold
Cold Front
Cold fronts are further characterized by their speed of
movement - which defines their slopes.
Slow-moving cold front
cold
Fast-moving cold front
cold
Slow moving cold fronts:
• Most clouds and weather are at and
behind the advancing cold front
• Longer periods of rain/snow, less
thunderstorm activity
Fast moving cold fronts (steeper slope):
• Most clouds and weather are near and
ahead of the advancing cold front
• Rain/show showers (sometimes heavy),
more thunderstorm activity
• Thunderstorms often form ahead of front
Warm Front
A warm front defines the boundary between a retreating
cool air mass and an overriding warm air mass.
Warm fronts are associated with:
• Extensive cloud activity ahead of the front.
• Temperature rises with frontal passage.
• Wind shifts with frontal passage.
• Poor visibility at and ahead
of the frontal boundary.
Cool
• Thick, stratiform clouds
and steady precipitation
patterns.
• Overall improvement
in weather conditions
with frontal passage.
Cold
Warm
Front
Warm
Warm Front
Warm fronts have extremely shallow slopes.
Warm front
cool
warm
Fog
• Clouds and weather are at and ahead the advancing warm front.
• Precipitation consists of steady rain or snow and usually no
thunderstorm activity - although thunderstorms may be embedded
within the frontal area and hard to discern on satellite pictures.
• Fog is frequently found in the cooler air ahead of the warm front.
Occluded Front
An occluded front defines the portion of frontal area where the
cold front has overtaken the warm front and pushed it aloft.
Occluded fronts are associated with:
• Both warm front and cold front
weather characteristics
• The worst weather with
an occluded front is
located where the cold
and warm fronts meet
at the surface: the triple point.
“Triple point”
Occluded Front
There are two types of occluded fronts: warm, and cold.
Warm occlusion
warm
cool
cold
Cold occlusion
warm
cold
cool
Warm occlusions:
• Milder maritime polar (mP) air overtakes
colder continental polar (cP) air.
• Warm occlusion weather is similar to
that of a warm front.
• More steady, less showery precipitation.
Cold occlusions:
• Colder cP air overtaking milder mP air.
• Cold occlusion weather resembles warm
frontal weather before the front passage,
and cold frontal weather during and after
passage.
Stationary Front
A stationary front has essentially no movement (the
advancing cold front has “stalled out”).
Stationary fronts are associated with:
• East-west orientation.
• Normally clear to partly
cloudy skies.
• Normally little or no
precipitation.
Cool
Warm
North
Stationary Front
Stationary Front
Overrunning
Warm
moist
Stationary fronts:
• Normally have “good” weather
associated with them.
Exceptions:
• If a new pulse of cold air moves in from
the north, the cold front can begin to
advance and a new low can form on the
frontal boundary.
• If warm, moist air overruns the frontal
boundary, widespread cloudiness and
light precipitation can cover a vast area.
Pressure Systems
There are two types of pressure systems: Highs and Lows
H
A “high,”or anticyclone, is an area of high pressure
around which the winds blow clockwise in the
northern hemisphere (counterclockwise in the
southern hemisphere.) High pressure is associated
with sinking, more dense air.
L
A “low,”or cyclone, is an area of low pressure
around which the winds blow counterclockwise in
the northern hemisphere (clockwise in the southern
hemisphere.) Low pressure is associated with rising,
less dense air.
Pressure Measurements
The amount of force exerted by air molecules over a given area of
the earth’s surface is called atmospheric pressure (or “air
pressure.”)
When the density of the air increases, pressure goes up. When
density decreases, pressure goes down.
Barometers are used to measure pressure in different units:
• Hectopascals (hPa) - Measured to the 1/10 (ex. 1018.8 hPa)
(1 Hectopascal = 1 millibar)
• Inches of mercury - measured to the 1/100 (ex. 29.92 ins)
The most common type of barometer is called an “aneroid
barometer.”
Pressure Systems
High pressure is normally associated with “good” weather:
• Clear or clearing skies, no precipitation, light winds (away
from terrain effects).
Low pressure is normally associated with “unsettled” weather:
• Cloudy skies, precipitation, gusty winds.
Barometers are used to measure pressure in different units:
• Hectopascals (hPa) - Measured to the 1/10 (ex. 1018.8 hPa)
(1 Hectopascal = 1 millibar)
• Inches of mercury - measured to the 1/100 (ex. 29.92 ins)
Pressure Systems: Isobars
20
16
12
04
24
96
H
L
994
1025
08
1008 hPa
00
Lines of equal pressure
are called “isobars.”
Isobars are usually drawn in 4 hPa increments.
• Denoted by a solid black line, labeled as shown.
• The highest and lowest pressure values within highs and
lows are are depicted next to the “H” or “L” label.
996 hPa
Pressure Systems and Fronts
Pressure systems and fronts have a direct relationship
A low pressure area forms where the
cold and warm front meet.
High pressure
defined by the air
mass “moving in”
(Cool)
L
H (Cold)
(Warm)
(time)
H
Cold
air mass
(e.g, cP)
L
Warm
air mass
(e.g, mT)
Cool
air mass
As the system develops, the position
of the low moves away from the cold
and warm fronts.
Pressure Systems
New Lows frequently form at the “triple point.”
L
This low “fills”
(dissipates) over time
L
(time)
H
Original
Low
H
L
L L
New
Low
(time)
The “original low” fills and
a new system moves off to
begin the cycle again.
Wind
Wind is air in motion relative to the earth’s surface. In
meteorology, wind is the observed effect of horizontal transport of
air masses over the Earth’s surface. It is caused by temperature
differential between 2 areas.
Wind speeds are plotted on meteorological charts as follows:
Flag = 50 kts
Long line = 10 kts (8-12 kts)
Half line = 5 kts (3-7 kts)
5 kts
10
20
50
65
100
Wind Speed Scales - Beaufort Scale
SEAS
Significant Range of
Periods (Seconds)
(Period of Maximum
Energy of Spectrum)
Average Period
(Seconds)
Average Wave Length
(Feet)
Minimum Fetch
(Nautical Miles)
Minimum Duration
(Hours)
3.4
4
4.8
5.4
5.6
6.5
7.2
7.7
8.1
8.9
9.7
9.9
10.5
11.3
12.1
12.4
12.9
13.6
14.5
14.9
15.4
16.1
17
17.7
18.6
19.4
20.2
20.8
21
21.8
22.6
24
2.4
2.9
3.4
3.9
4
4.6
5.1
5.4
5.7
6.3
6.8
7
7.4
7.9
8.6
8.7
9.1
9.7
10.3
10.5
10.7
11.4
12.0
12.5
13.1
13.8
14.3
14.7
14.8
15.4
16.3
17
20
27
40
52
59
71
90
99
111
134
160
164
188
212
250
258
285
322
363
376
392
444
492
534
590
650
700
736
750
810
910
985
9.8
10
18
24
28
40
55
65
75
100
130
140
180
230
280
290
340
420
500
530
600
710
830
960
1100
1250
1420
1560
1610
1800
2100
2500
1.7 hrs
2.4
3.8
4.8
5.2
6.6
8.3
9.2
10
12
14
15
17
20
23
24
27
30
34
37
38
42
47
52
57
63
69
73
75
81
88
101
(35)
(26)
(18)
---
---
---
Average 1/10 Highest
0.8 – 5.0
1.0 – 6.0
1.0 – 7.0
1.4 – 7.6
1.5 – 7.8
2.0 – 8.8
2.5 – 10
2.8 – 10.6
3 – 11.1
3.4 – 12.2
3.7 – 13.5
3.8 – 13.6
4 – 14.5
4.5 – 15.5
4.7 – 16.7
4.8 – 17
5 – 17.5
5.5 – 18.5
5.8 – 19.7
6 – 20.5
6.2 – 20.8
6.5 – 21.7
7 – 23
7 – 24.2
7 – 25
7.5 – 26
7.5 – 27
8 – 28.2
8 – 28.5
8 – 29.5
8.5 - 31
10 – 32
Significant
--18 min
39 min
0
0.05
0.18
0
0.08
0.29
0
0.10
0.37
1.2
<
7 – 10
0.6
0.88
1.4
1.8
2.0
2.9
3.8
4.3
5.0
6.4
7.9
8.2
9.6
11
14
14
16
19
21
23
25
28
31
36
40
44
49
52
54
59
64
73
1.0
1.4
2.2
2.9
3.3
4.6
6.1
6.9
8.0
10
12
13
15
18
22
23
26
30
35
37
40
45
50
58
64
71
78
83
87
95
103
116
1.2
1.8
2.8
3.7
4.2
5.8
7.8
8.7
10
13
16
17
20
23
26
29
33
38
44
46.7
50
58
64
73
81
90
99
106
110
121
130
148
> 80
Small waves, becoming larger; fairly frequent white horses.
4
Moderate
Breeze
11 – 16
Moderate waves, becoming larger; fairly frequent white horses.
5
Fresh
Breeze
17 – 21
Large waves begin to form; white foam crests more extensive everywhere,
probably some spray.
6
Strong
Breeze
22 – 27
Sea heaps up and white foam from streaking waves begins to be blown
in streaks along the direction of the wind (spindrift begins to be seen).
7
Moderate
Gale
28 – 33
Moderately high waves of greater length; edges of crests break into spindrift. Foam
blown in well marked streaks along direction of wind. Spray affects visibility.
8
Fresh
Gale
34 – 40
High waves. Dense streaks of foam along direction of the wind. Sea begins to roll,
visibility affected.
9
Strong
Gale
41 – 47
Very high waves with long overhanging crests. Resulting foam in great patches and
is blown in dense white streaks along direction of wind. Sea surface takes on white
appearance. Rolling of sea becomes heavy and shock-like. Visibility affected.
10
Whole
1
Gale
48 – 55
11
Storm
1
56 – 63
8.5
10
12
13.5
14
16
18
19
20
22
24
24.5
26
28
30
30.5
32
34
36
37
38
40
42
44
46
48
50
51.5
52
54
56
59.5
12
Hurricane
64 – 71
> 64
The Beaufort Scale defines
seas (and sea state) according
to wind speeds
9
Exceptionally high waves (small and medium-sized ships may become lost to view
behind waves). Sea completely covered with long white patches of foam lying along
direction of wind. Edges of wave crests are blown into froth. Visibility affected.
Air filled with foam and spray. Sea completely white with driven spray. Visibility
very seriously affected.
--5
8
0
2
5
6
8
--.83
6.7
<1
1–3
4–6
5
7
--0.5
1.4
Calm
Light Airs
Light
Breeze
Gentle
Breeze
3
4
0.4 – 2.8
--0.7
2
Average
3
---
Wind Velocity (kts)
2
U
1
2
Range (kts)
1
SEA – GENERAL CONDITION
Sea like a mirror
Ripples with appearance of scales formed; without foam crests
Small wavelets, short but more pronounced; crests have glassy appearance,
do not break.
Large wavelets, crests begin to break. Foam of glassy appearance. Perhaps
scattered white horses.
Description
0
Notes: 1 – For hurricane winds (and often whole gale and storm winds)
Required durations and fetches are rarely attained.
Seas are therefore not fully arisen.
2 – For such high winds, the seas are confused. The wave crests blow
aft and the water and air mix.
Wave Height (feet)
WIND
Beaufort Wind Force
Sea State
WIND AND SEA SCALE FOR FULLY ARISEN SEA
1
2
> 128
2
> 164
2
Wind Speed Scales
Wind speeds not associated with tropical systems (World
Meteorological Organization).
4 - 27 kts = breeze (light, gentle, moderate, fresh, strong).
28 - 33 kts = near gale.
34 - 47 kts = gale (gale, strong).
48 - 63 kts = storm (storm, violent).
64 kts and greater = hurricane force.
Wind speeds associated with tropical systems.
Less than 34 kts = Tropical Depression.
34 - 63 kts = Tropical Storm.
64 - 129 kts = Hurricane.
Greater than 130 kts = Super Hurricane.
Pressure Systems: Isobars
Wind barbs define wind direction and speed on a synoptic chart...
20
16
12
04
24
H
1025
08
00
96
L
994
…and also help define frontal boundaries
Pressure Systems, Fronts, Isobars, Winds: Example
Clouds
Clouds are:
• Water molecules suspended in the atmosphere.
• Three things are required for cloud formation:
- Moisture
- Cooling
- Condensation nuclei (something for
the moisture to condense on)
There are three general types of clouds:
• Cumuliform
• Stratiform
• Cirriform
Cumuliform Clouds
Cumuliform clouds are unstable, vertically developed, and have
generally distinct edges. They are formed either by convective
action (daytime heating) or mechanical lifting (cold front).
Showery precipitation is associated with cumuliform clouds.
Cumulonimbus clouds are clouds with extreme vertical extent and
are associated with heavy precipitation and thunderstorms.
Stratiform clouds are stable and form indistinct layers. Steady,
light precipitation is associated with stratiform clouds. A particular
form of stratiform clouds, nimbostratus, is associated with heavy,
steady precipitation. Fog is nothing more than a form of
straitiform clouds (stratus) that has reached the ground.
Cirriform clouds are located at higher altitudes and are composed
completely of ice crystals.
Clouds and the Atmosphere
For meteorological purposes, the atmosphere is divided into three
levels (“etages”).
• Low etage - Surface to 6500 ft (middle latitudes)
• Middle etage - 6500 ft to 23,000 ft
• High etage - 16,00 ft to 43,000 ft
Different cloud types are associated with the low, middle, and high
etages. More common cloud types are shown below. Some “stay”
in their etages, some extend through one or more (*):
Low
Cumulus, Cumulonimbus *
Stratocumulus
Stratus
Middle
Altostratus
Altocumulus
Nimbostratus *
High
Cirrus
Cirrocumulus
Cirrostratus
(Cumulonimbus begins in the low etage and builds into the mid and high etages)
(Nimbostratus frequently begins as a “mid” cloud and descends into the low etage)
Clouds and Fronts - Example
L
Cirrostratus
Altostratus
Nimbostratus
Stratus (fog)
Altocumulus
Altostratus
Cirriform
Stratocumulus
Cumulus
Stratocumulus
Cumulus
Cumulonimbus
Altocumulus
Cirrocumulus
The End…