• Air Masses
• Fronts (cold, warm, occluded, stationary)
• Pressure Systems
• Clouds
• Winds

mP cA
Air masses are large bodies of air that have similar horizontal temperature and moisture characteristics.
cP cT mT mT mP
Air masses are classified by where they originate (their
“source region”).
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.

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”).
Cold air mass
(e.g, cP)
L Cool air mass
Cool
L
Cold
Warm
B
Frontal intensity is defined by the the temperature and moisture differences on either side.
A
Warm air mass
(e.g, mT)
The stronger the differences on either side of the front, the more potential for severe weather.

• Cold
• Warm
• Occluded
• Stationary
There are four types of fronts.
Occluded
L
Cold
Stationary
Warm

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
• Wind shifts with frontal
Cool passage
• Pressure changes with frontal passage
• Unstable cumuliform
Cold
Front clouds and showery precipitation patterns Cold Warm

cold
Cold fronts are further characterized by their speed of movement - which defines their slopes.
Slow-moving cold front 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 front cold
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

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.
• Thick, stratiform clouds and steady precipitation
Cool patterns.
• Overall improvement in weather conditions with frontal passage.
Cold
Warm
Front
Warm

Warm fronts have extremely shallow slopes.
Warm front warm cool
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.

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”

There are two types of occluded fronts: warm , and cold .
cool
Warm occlusion warm cold cold
Cold occlusion warm 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.

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
North
Warm

Warm moist
Stationary Front
Overrunning
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.

L
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.
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.

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.”

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)

20
24
H
1025
16 12
96
00
04
08
L
994
1008 hPa
996 hPa
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.

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”
(time)
H
L
(Cold)
(Cool)
(Warm)
H
Cold air mass
(e.g, cP)
L
Cool air mass
As the system develops, the position of the low moves away from the cold and warm fronts.
Warm air mass
(e.g, mT)

H
Original
Low
L
New Lows frequently form at the “triple point.”
This low “fills”
(dissipates) over time
L
L
(time)
H
L L
New
Low
(time)
The “original low” fills and a new system moves off to begin the cycle again.

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

SEAS
WIND AND SEA SCALE FOR FULLY ARISEN SEA WIND Wave Height (feet)
4
5
2
3
0
1
6
7
8
9
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.
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.
Small waves, becoming larger; fairly frequent white horses.
Moderate waves, becoming larger; fairly frequent white horses.
Large waves begin to form; white foam crests more extensive everywhere, probably some spray.
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).
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.
High waves. Dense streaks of foam along direction of the wind. Sea begins to roll, visibility affected.
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.
U
1
2
3
4
5
6
7
8
9
10
Calm
Light Airs
Light
Breeze
Gentle
Breeze
Moderate
Breeze
Fresh
Breeze
Strong
Breeze
Moderate
Gale
Fresh
Gale
Strong
Gale
Whole
Gale
1
Exceptionally high waves (small and medium-sized ships may become lost to view
Storm
1 behind waves). Sea completely covered with long white patches of foam lying along 11 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 12 Hurricane
1 very seriously affected.
< 1
1 – 3
4 – 6
0
2
5
0
0.05
0.18
0
0.08
0.29
0
0.10
0.37
<
---

0.4 – 2.8
---
0.7
2
---
0.5
1.4
---
.83
6.7
---
5
8
---
18 min
39 min
7 – 10
11 – 16
17 – 21
22 – 27
28 – 33
8.5
10
12
13.5
14
16
18
0.6
0.88
1.4
1.8
2.0
2.9
3.8
1.0
1.4
2.2
2.9
3.3
4.6
6.1
1.2
1.8
2.8
3.7
4.2
5.8
7.8
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
3.4
4
4.8
5.4
5.6
6.5
7.2
2.4
2.9
3.4
3.9
4
4.6
5.1
20
27
40
52
59
71
90
9.8
10
18
24
28
40
55
19
20
22
24
4.3
5.0
6.4
7.9
6.9
8.0
10
12
8.7
10
13
16
2.8 – 10.6
3 – 11.1
3.4 – 12.2
3.7 – 13.5
7.7
8.1
8.9
9.7
5.4
5.7
6.3
6.8
99
111
134
160
65
75
100
130
24.5
26
28
30
8.2
9.6
11
14
13
15
18
22
17
20
23
26
3.8 – 13.6
4 – 14.5
9.9
10.5
4.5 – 15.5
11.3
4.7 – 16.7
12.1
7
7.4
7.9
8.6
164
188
212
250
140
180
230
280
30.5
32
14
16
23
26
29
33
4.8 – 17
5 – 17.5
12.4
12.9
8.7
9.1
258
285
290
340
1.7 hrs
2.4
3.8
4.8
5.2
6.6
8.3
9.2
34 19 30 38 9.7
322 420 5.5 – 18.5
13.6
34 – 40 36
37
21
23
35
37
44
46.7
5.8 – 19.7
14.5
10.3
363
6 – 20.5
14.9
10.5
376
500
530
34
38 25 40 50 6.2 – 20.8
15.4
10.7
392 600
37
38
30
41 – 47
48 – 55
40
42
44
46
48
50
51.5
52
54
28
31
36
40
44
49
52
54
59
45
50
58
64
71
78
83
87
95
58
64
73
81
90
99
106
110
121
6.5 – 21.7
16.1
11.4
444
7 – 23 17 12.0
492
7 – 24.2
7 – 25
7.5 – 26
17.7
18.6
19.4
12.5
13.1
13.8
534
590
650
710
830
960
1100
1250
7.5 – 27
8 – 28.2
8 – 28.5
8 – 29.5
20.2
20.8
21
21.8
14.3
14.7
14.8
15.4
700
736
750
810
1420
1560
1610
1800
42
47
52
57
63
69
73
75
81
10
12
14
15
17
20
23
24
27
56
56 – 63
59.5
64 103 130 8.5 - 31
10 – 32
22.6
16.3
910 2100 88
73 116 148 24 17 985 2500 101
64 – 71 > 64 > 80
2
> 128
2
> 164
2
(35) (26) (18) -------

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.

Wind barbs define wind direction and speed on a synoptic chart...
20
16
12
24
H
1025
L
994
96
00
04
08
…and also help define frontal boundaries

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 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 .

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)

Cumulus
Stratocumulus
L
Altocumulus
Altostratus
Cirriform
Cirrostratus
Altostratus
Nimbostratus
Stratus (fog)
Stratocumulus
Cumulus
Cumulonimbus
Altocumulus
Cirrocumulus