Atmospheric Pressure & Barometric Tendency
The barometer is the mariner's most important weather instrument. The rate of change — barometric tendency — tells you more than the absolute pressure.
Standard Pressure
29.92 in Hg
1013.25 hPa / mb at sea level
Low Pressure Center
< 29.50 in Hg
Stormy — counterclockwise flow in N. Hemisphere
High Pressure Center
> 30.20 in Hg
Fair weather — clockwise flow in N. Hemisphere
| Barometric Change | What It Means | Mariner Action |
|---|---|---|
| Rising rapidly (> 0.06 in/hr) | Storm clearing quickly, strong high approaching | Conditions improving — remain vigilant |
| Rising slowly (0.02–0.06 in/hr) | High pressure building, improving weather | Conditions improving |
| Steady | Stable weather pattern | Current conditions likely persist |
| Falling slowly (0.02–0.06 in/hr) | Low pressure approaching, weather deteriorating | Monitor closely — prepare for worsening |
| Falling rapidly (> 0.06 in/hr) | Storm or intense low approaching | Seek shelter — severe weather imminent |
Buys Ballot's Law: In the Northern Hemisphere, stand with your back to the wind — the center of low pressure is to your left and slightly ahead. Winds flow counterclockwise (cyclonically) around lows and clockwise (anticyclonically) around highs. In the Southern Hemisphere, these rotations are reversed.
Global Wind Patterns
Large-scale wind belts are driven by differential solar heating and the Coriolis effect. Understanding them is essential for offshore passage planning.
Trade Winds
0°–30° N and S — NE in N. Hemisphere, SE in S. Hemisphere
Steady, reliable winds driven by air descending from subtropical highs and flowing equatorward toward the ITCZ. Deflected by Coriolis. Historically essential for sailing routes.
Exam may test direction and latitude band — NE trades blow in the tropics N. of equator.
Prevailing Westerlies
30°–60° N and S — From SW in N. Hemisphere, from NW in S. Hemisphere
Air flowing poleward from subtropical highs is deflected to the right (N. Hemisphere). Responsible for much of the mid-latitude weather systems and the westward-to-eastward movement of storms.
Important for North Atlantic and North Pacific passage planning.
Polar Easterlies
60°–90° N and S — From NE in N. Hemisphere
Cold, dense air flows from polar highs toward lower latitudes. Deflected by Coriolis to produce easterly flow. Less significant for most mariners but relevant to Arctic operations.
Less commonly tested, but complete the three-cell circulation model.
Doldrums (ITCZ)
Near 0° — equatorial zone — Variable, light — calms frequent
The Intertropical Convergence Zone (ITCZ) is where NE and SE trade winds meet. Air rises, producing frequent calms, squalls, and thunderstorms. Historically feared by sailing vessels.
Calms and squalls. Not a belt of steady wind.
Horse Latitudes
~30° N and S — Variable, light — high pressure dominates
Subtropical high-pressure belts where air descends, creating calm or light variable winds. Both the North Atlantic High and North Pacific High generate the surrounding trade winds and westerlies.
Calm belt — origin of the name from becalmed sailing vessels.
Roaring Forties
40°–50° S — Strong westerlies — unobstructed by land
In the Southern Hemisphere, the prevailing westerlies blow with great strength between 40° and 50°S where little land interrupts the fetch. Relevant to circumnavigations and Southern Ocean passages.
Southern Hemisphere westerlies — extreme conditions, relevant to offshore racing.
Frontal Systems — Characteristics & Weather Sequences
Fronts separate air masses of different temperature and humidity. Each front type produces a distinct cloud sequence, wind shift, and precipitation pattern. This is heavily tested on the USCG exam.
Warm Front
Semicircles →Approach / Speed
24–48 hrs of cloud buildup
Cloud Sequence
Cirrus → Cirrostratus → Altostratus → Nimbostratus
Weather
Prolonged steady rain or drizzle, reduced visibility, halo around sun/moon
Wind Shift at Passage
SE → S → SW (veering)
Pressure
Falls steadily ahead of front
Cold Front
Exam FavoriteTriangles →Approach / Speed
Fast-moving — squall line may precede by 50–100 nm
Cloud Sequence
Cumulonimbus (thunderstorms), cumulus, rapid vertical development
Weather
Heavy showers, thunderstorms, hail, gusty squalls — brief but intense
Wind Shift at Passage
SW → W → NW (sudden veer after passage)
Pressure
Falls sharply then rises rapidly after passage
Occluded Front
Alt. triangles & semicirclesApproach / Speed
Combined warm/cold front characteristics
Cloud Sequence
Mix of stratiform and cumuliform clouds
Weather
Prolonged precipitation, fog, deteriorating conditions
Wind Shift at Passage
Complex — depends on type of occlusion
Pressure
May rise slowly after passage
Stationary Front
Alt. red semicircles / blue trianglesApproach / Speed
Little or no movement — may persist for days
Cloud Sequence
Stratus, stratocumulus along front boundary
Weather
Prolonged cloudiness, drizzle, or rain along the frontal zone
Wind Shift at Passage
Minimal — winds shift at the boundary, flow parallel to front on each side
Pressure
Little change on either side
Warm Front Cloud Sequence — Memorize This
Cirrus
20,000+ ft
Mares' tails — first sign, 24–48 hrs out
Cirrostratus
18,000–20,000 ft
Halo around sun/moon
Altostratus
6,500–18,000 ft
Sun visible as dim disk
Nimbostratus
Below 6,500 ft
Steady rain begins
Air Masses
Air masses are large bodies of air with uniform temperature and moisture properties at any given level. They are classified by source region.
| Symbol | Name | Source Region | Temperature | Moisture | Typical Weather |
|---|---|---|---|---|---|
| cP | Continental Polar | Canada, Siberia | Cold | Dry | Cold, clear, dry — winter outbreaks |
| mP | Maritime Polar | North Pacific / North Atlantic | Cool | Moist | Cloudy, fog, drizzle, cool temps |
| cT | Continental Tropical | Southwestern US, Mexico | Hot | Dry | Hot, dry — dust, haze |
| mT | Maritime Tropical | Gulf of Mexico, Caribbean, Pacific | Warm | Very Moist | Warm, humid — thunderstorms, fog |
| A | Arctic | Arctic ocean, ice sheets | Extremely cold | Very dry | Extreme cold, blizzards, arctic sea smoke |
Fog Types — Formation & Recognition
Fog reduces visibility below 1/4 nautical mile. Knowing how each type forms determines where you'll encounter it, how long it will last, and how to predict its dissipation.
Advection Fog
Exam Freq: HighFormation
Warm, moist air moves horizontally over cooler water surface and is chilled to its dew point
Location
Open ocean, coastal waters — especially Pacific Coast over cold currents
Persistence
Can last days — not dependent on time of day
Dissipation
Wind shift, warmer water, rain
Most common at sea — the exam expects you to identify this as the primary offshore fog type.
Radiation Fog
Exam Freq: MediumFormation
Rapid nighttime radiational cooling of ground surface chills overlying air to dew point
Location
Land — valleys, calm nights with clear skies
Persistence
Burns off after sunrise — typically morning fog
Dissipation
Solar heating after sunrise
Primarily a land phenomenon. Can drift short distances offshore but rarely extends far to sea.
Sea Fog (Marine Advection)
Exam Freq: HighFormation
Specific form of advection fog — warm moist air over cold sea surface
Location
Grand Banks, Pacific Coast (California Current), Gulf of Maine
Persistence
Extremely persistent — notorious in Grand Banks fishing grounds
Dissipation
Shift to drier or warmer air mass
The Grand Banks off Newfoundland are famous for persistent sea fog — common exam context.
Arctic Sea Smoke
Exam Freq: LowFormation
Very cold, dry arctic air moves over relatively warm open water — evaporation and instant condensation
Location
Polar regions, Great Lakes in winter, coastal areas after arctic outbreak
Persistence
Lasts as long as cold air mass persists over open water
Dissipation
Warming of air mass or change in wind direction
Also called steam fog — the sea appears to 'smoke.' Can be locally dense but thin layer.
Precipitation Fog
Exam Freq: LowFormation
Rain or drizzle falling through cooler air evaporates and raises humidity to 100%
Location
Ahead of warm fronts — coincides with warm front precipitation zone
Persistence
Duration of precipitation
Dissipation
Cessation of rain
Often associated with warm front approach — explains why warm front passage can bring fog along with steady rain.
Beaufort Wind Scale — Complete Reference
The Beaufort scale (Force 0–12) describes wind speed by observed sea state. The USCG exam frequently tests the knot ranges and descriptive conditions for each force number, particularly Forces 6–10.
| Force | Name | Knots | MPH | Wave Ht | Sea State Description |
|---|---|---|---|---|---|
| 0 | Calm | < 1 | < 1 | 0 ft | Mirror-smooth sea |
| 1 | Light Air | 1–3 | 1–3 | 0–1 ft | Ripples, no foam crests |
| 2 | Light Breeze | 4–6 | 4–7 | 1–2 ft | Small wavelets, glassy crests |
| 3 | Gentle Breeze | 7–10 | 8–12 | 2–4 ft | Large wavelets, scattered whitecaps |
| 4 | Moderate Breeze | 11–16 | 13–18 | 3.5–6 ft | Small waves, frequent whitecaps |
| 5 | Fresh Breeze | 17–21 | 19–24 | 6–10 ft | Moderate waves, many whitecaps, some spray |
| 6 | Strong Breeze | 22–27 | 25–31 | 10–15 ft | Large waves, whitecaps everywhere, spray |
| 7 | Near Gale | 28–33 | 32–38 | 14–19 ft | Sea heaps up, white foam blown in streaks |
| 8 | Gale | 34–40 | 39–46 | 18–25 ft | Moderately high waves, crests break into spindrift |
| 9 | Strong Gale | 41–47 | 47–54 | 23–32 ft | High waves, dense foam, reduced visibility |
| 10 | Storm | 48–55 | 55–63 | 29–41 ft | Very high waves, sea appears white, heavy rolling |
| 11 | Violent Storm | 56–63 | 64–72 | 37–52 ft | Exceptionally high waves, reduced visibility |
| 12 | Hurricane Force | ≥ 64 | ≥ 73 | ≥ 46 ft | Phenomenal sea, air filled with foam and spray |
Wave heights are significant wave heights — average of the highest one-third of waves. Individual waves may be considerably higher.
Thunderstorms, Squalls & Line Squalls
Thunderstorm Life Cycle
Cumulus Stage
Strong updrafts build the towering cumulus. No precipitation yet. Cell grows rapidly vertically.
Mature Stage
Cumulonimbus with anvil top. Both updrafts and downdrafts present. Lightning, heavy rain, hail, gusty surface winds.
Dissipating Stage
Downdrafts dominate, cutting off updrafts. Precipitation diminishes. Cell collapses.
Cumulonimbus Warning Signs
- ▸Rapid vertical development — flat base, cauliflower top
- ▸Anvil (cirrus) top spreading downwind — sign of mature cell
- ▸Darkening base, greenish hue — possible hail
- ▸Sudden wind shift and gust front ahead of cell
- ▸Static on AM radio — lightning indicator
- ▸Rapid pressure drop followed by sharp rise as cell passes
Actions When a Squall Approaches
- 1.Reduce sail immediately — reef early, not late
- 2.Don life jackets — all crew on deck
- 3.Secure all gear and hatches
- 4.Maintain a 30°–45° angle off the wind if possible
- 5.Note the direction of the squall's movement — does not always follow apparent wind
- 6.Monitor VHF WX for any warnings already broadcast
- 7.Stay low — avoid masts and metal rigging during lightning
Line Squalls
A line squall is an organized band of thunderstorm cells arranged in a line, often associated with a cold front or prefrontal trough. They can extend hundreds of miles and produce the most severe, sudden wind shifts and gusts.
Approach: Look for a long, dark, shelf-like cloud (arcus) along the horizon advancing toward you. Wind may drop to calm briefly just before the line passes — then hits suddenly with extreme gusts. Radar is the best tool for detecting and tracking line squalls.
Tropical Cyclones — Formation & Navigation Rules
Formation Requirements
- ▸Sea surface temperature at least 80°F (26.5°C) to a depth of 150+ feet
- ▸Atmosphere that cools rapidly with altitude (lapse rate supporting deep convection)
- ▸High humidity in the lower to middle troposphere
- ▸Low vertical wind shear — strong shear tears storms apart
- ▸Pre-existing weather disturbance with some rotation (e.g., tropical wave)
- ▸Sufficient Coriolis effect — generally 5°+ from equator
Development Stages
Dangerous vs. Navigable Semicircle
In the dangerous semicircle (right side of storm track in N. Hemisphere), the storm's forward motion adds to the wind speed and the curved track may bring the vessel directly into the storm's path. Always avoid this semicircle.
In the navigable semicircle (left side of track), storm forward motion partially offsets the wind, and vessels are blown away from the track. Conditions are still severe but comparatively less dangerous.
1-2-3 Rule: Account for 100, 200, and 300 nm of storm position uncertainty over 24, 48, and 72 hours to establish a safe avoidance zone.
Saffir-Simpson Scale
Marine Weather Broadcasts — NOAA VHF & NAVTEX
NOAA Weather Radio — VHF WX Channels
NOAA broadcasts continuous weather information 24/7. Your VHF radio's WX button scans all channels to find the strongest local signal. The specific channel varies by geographic area.
| Channel | Frequency | Notes |
|---|---|---|
| WX1 | 162.550 MHz | Primary — most areas |
| WX2 | 162.400 MHz | Secondary coverage |
| WX3 | 162.475 MHz | Secondary coverage |
| WX4 | 162.425 MHz | Secondary coverage |
| WX5 | 162.450 MHz | Secondary coverage |
| WX6 | 162.500 MHz | Secondary coverage |
| WX7 | 162.525 MHz | Secondary coverage |
SAME Alert System: Specific Area Message Encoding allows your WX radio to trigger an alarm only for weather warnings in your specific county or marine zone — preventing false alarms from distant areas.
NAVTEX
NAVTEX is an international automated medium-frequency broadcast system that delivers navigational warnings, meteorological forecasts, and SAR information in printed (text) form via a dedicated receiver.
- ▸518 kHz — International NAVTEX (English, offshore — up to 400 nm)
- ▸490 kHz — Coastal NAVTEX (national language, nearshore)
- ▸4209.5 kHz — HF NAVTEX for expanded range
- ▸Messages are printed automatically — no continuous watch required
- ▸GMDSS-required on SOLAS vessels — tested on Master exam
Weather Fax (Radiofax / HF Fax)
Radiofax broadcasts synoptic weather charts, surface analysis, 24/48-hr forecasts, and wave height charts via HF radio. NOAA broadcasts on multiple HF frequencies; the schedule and frequencies are published in NOAA's Marine Weather Service Charts.
- ▸Common NOAA fax frequencies: 4316, 8502, 12788, 17151.2 kHz
- ▸Read isobars, frontal positions, and H/L pressure centers
- ▸Isobar spacing = wind strength — closer = stronger gradient winds
Reading Synoptic Charts & Weather Symbols
Chart Symbols Quick Reference
Reading Wind Barbs
Wind barbs indicate both direction and speed. The line shows the direction the wind is blowing from (not toward). Feathers on the tail indicate speed:
Pressure Gradient & Wind Speed
The pressure gradient force drives wind from high to low pressure. The tighter the isobar spacing on a surface analysis chart, the greater the pressure gradient, and the stronger the wind.
Gradient wind approximately follows isobars (not across them), with slight inflow toward the low due to friction. The geostrophic wind (theoretical frictionless) is exactly parallel to isobars at altitude — used to estimate upper-level flow direction from chart analysis.
Meteorology Exam Tips
Warm front cloud sequence is guaranteed
Cirrus → Cirrostratus (halo) → Altostratus → Nimbostratus. The halo around the sun or moon is the classic 'warm front approaching' indicator. The exam tests this sequence in multiple question formats.
Cold front = sudden, violent, fast
Cold fronts move faster than warm fronts, produce brief but intense precipitation, cumulonimbus clouds, and a sharp wind shift from SW to NW in the Northern Hemisphere. Rising pressure after passage = cold front cleared.
Advection fog dominates offshore
When an exam question asks about the primary fog type at sea, the answer is advection fog. Radiation fog is a land phenomenon. Sea fog on the Pacific Coast (California Current) is a classic exam scenario.
Beaufort 8 = Gale (34–40 kt)
The USCG exam expects you to know: Force 6 = 22–27 kt, Force 7 = 28–33 kt, Force 8 = 34–40 kt Gale, Force 10 = 48–55 kt Storm, Force 12 = 64+ kt Hurricane. Focus on the key transition points.
Buys Ballot's Law locates the low
Back to wind, low is to your left (N. Hemisphere). This is tested directly. In the Southern Hemisphere, low is to the right. Knowing the direction to the low helps determine whether you are in the dangerous semicircle.
NAVTEX frequencies: 518 and 490
518 kHz = international NAVTEX in English. 490 kHz = coastal NAVTEX in national language. These are tested on the Master exam. VHF WX channels are tested on both OUPV and Master.
Dangerous semicircle is on the right
In the Northern Hemisphere, the right (eastern) semicircle of a tropical cyclone relative to storm track direction is the dangerous semicircle. Storm motion adds to wind speed; the track curves can trap a vessel in the path. Always maneuver to avoid the right side.
Barometer falling rapidly = seek shelter
A rapid fall greater than 0.06 in/hr signals an intense low or approaching storm. Standard pressure is 29.92 in Hg. Values below 29.50 in Hg typically indicate strong low-pressure systems. The exam tests both the standard value and the significance of rapid change.
Frequently Asked Questions
What does a rapidly falling barometer indicate, and how fast is considered significant?
A falling barometer indicates approaching low pressure, which typically brings deteriorating weather, increasing winds, and precipitation. A drop of 0.02 to 0.06 inches of mercury (0.7–2.0 hPa) per hour is considered a moderate fall. A drop greater than 0.06 in/hr is rapid and signals an approaching storm. A rise in pressure generally indicates improving weather and the approach of a high-pressure system with clear skies. The rate of change — barometric tendency — is as important as the absolute reading.
What are the four types of fronts and what weather does each bring?
A warm front forms when warm air advances over retreating cold air. Clouds develop in sequence: cirrus → cirrostratus → altostratus → nimbostratus. The result is prolonged, steady rain or drizzle, falling barometer, and winds veering from SE to SW as the front passes. A cold front forms when cold air undercuts warm air sharply. It brings rapidly developing cumulonimbus clouds, heavy showers, thunderstorms, and gusty winds that shift suddenly from SW to NW. Passage is quick but violent. An occluded front forms when a fast-moving cold front catches a warm front, lifting warm air aloft. It combines characteristics of both. A stationary front is a front with minimal movement — it produces prolonged cloudiness and precipitation along a fixed boundary.
What Beaufort wind force corresponds to gale conditions, and what sea state results?
Beaufort Force 8 is a Gale, with winds of 34–40 knots. The sea state shows moderately high waves (18–25 feet) with crests beginning to break into spindrift. Visibility is reduced. Force 9 is a Strong Gale at 41–47 knots with high waves, dense foam streaks, and spray reducing visibility. Force 10 is a Storm with winds 48–55 knots — very high waves, the sea appears white due to dense foam, and the rolling is heavy. The USCG exam frequently tests the knot ranges and descriptive sea state for Beaufort Forces 6 through 10. Force 12 is a Hurricane with winds 64+ knots and phenomenal seas.
What is the difference between advection fog and radiation fog, and which is more common at sea?
Radiation fog (ground fog) forms over land when the surface cools rapidly on clear, calm nights, chilling the air above to its dew point. It burns off after sunrise and rarely extends far offshore. Advection fog forms when warm, moist air moves horizontally over a cooler water surface and is chilled to its dew point — this is the dominant fog type at sea and can persist for days. Sea fog is a type of advection fog common on the Pacific Coast where warm Pacific air moves over the cold California Current. Arctic sea smoke (steam fog) forms when very cold air moves over relatively warm water, causing rising wisps of vapor.
What NOAA VHF weather channels should a mariner monitor, and on what frequency?
NOAA Weather Radio broadcasts continuously on VHF frequencies: WX1 at 162.550 MHz, WX2 at 162.400 MHz, WX3 at 162.475 MHz, WX4 at 162.425 MHz, WX5 at 162.450 MHz, WX6 at 162.500 MHz, and WX7 at 162.525 MHz. Most VHF marine radios have dedicated WX buttons that scan all NOAA channels. NAVTEX is a separate automatic digital telex system operating on 518 kHz (international) and 490 kHz (coastal) that delivers printed weather, navigational warnings, and SAR information up to 200–400 nautical miles from shore. Both systems are tested on the USCG exam.
How do trade winds and prevailing westerlies form, and why do they matter to mariners?
Trade winds form because air rising at the equator (ITCZ) flows poleward and descends at roughly 30°N and 30°S, creating subtropical high-pressure belts. As surface air flows back toward the equator to replace rising air, the Coriolis effect deflects it — right in the Northern Hemisphere, left in the Southern — producing the NE trade winds (N. Hemisphere) and SE trade winds (S. Hemisphere). Between approximately 30° and 60° latitude, the prevailing westerlies dominate, blowing from the southwest in the N. Hemisphere. Mariners planning offshore passages rely on these large-scale patterns to select efficient routes and avoid headwinds.
What conditions lead to tropical cyclone formation, and what are the stages?
Tropical cyclones require sea surface temperature of at least 80°F (26.5°C) to depth, an atmosphere that cools fast enough with altitude to sustain convection, sufficient moisture through the mid-troposphere, low vertical wind shear, and a pre-existing weather disturbance with some rotation. Stages: a Tropical Disturbance is an organized area of convection. A Tropical Depression forms when a closed circulation develops with sustained winds to 38 mph (33 kt). A Tropical Storm develops when winds reach 39–73 mph (34–63 kt) and the system receives a name. A Hurricane (Atlantic/E. Pacific) or Typhoon (W. Pacific) forms when winds exceed 74 mph (64 kt). The Saffir-Simpson scale rates hurricanes Category 1–5 based on wind speed.
How do you read isobars and frontal symbols on a synoptic weather chart?
Isobars are lines of equal atmospheric pressure. Closely spaced isobars indicate strong pressure gradients and strong winds; widely spaced isobars mean light winds. The wind flows nearly parallel to isobars, slightly inward toward low pressure in the N. Hemisphere (counterclockwise around a Low, clockwise around a High). On synoptic charts: a warm front is shown as a red line with solid semicircles on the side toward which the front is advancing. A cold front is a blue line with solid triangles pointing in the direction of movement. An occluded front is purple with alternating triangles and semicircles. A stationary front uses alternating red semicircles and blue triangles pointing in opposite directions.
What cloud types signal an approaching warm front, in order of appearance?
As a warm front approaches, clouds appear in this sequence from high to low altitude: cirrus (mares' tails) first, appearing 24–48 hours before the front at 20,000+ feet — thin, wispy, ice crystal clouds. Next, cirrostratus creates a milky veil causing a halo around the sun or moon — a classic warm-front indicator. Then altostratus thickens the sky and the sun becomes a vague disk. Finally, nimbostratus produces continuous, steady rain or drizzle. The exam frequently tests this sequence. Cumulonimbus is the thunderstorm cloud associated with cold fronts — towering vertically to 40,000+ feet with an anvil top.
What is the buys ballot law and how is it used to locate a storm center?
Buys Ballot's Law states: in the Northern Hemisphere, if you stand with your back to the wind, the center of low pressure is to your left and slightly ahead. (In the Southern Hemisphere it is to the right.) This allows a mariner to estimate the direction to the storm center using only observed wind. To avoid a Northern Hemisphere tropical storm, use the 1-2-3 rule for storm track uncertainty and the navigable semicircle concept: the right semicircle (relative to storm track) has the most dangerous winds because storm motion adds to wind speed — dangerous semicircle. The left (navigable) semicircle has somewhat less wind. Always maneuver to maintain distance and avoid the dangerous semicircle.
Related Study Guides
Navigation & Chart Reading
Chart symbols, compass deviation, tide calculations, and piloting for the USCG exam.
Rules of the Road
COLREGS lights, shapes, sound signals, and right-of-way rules for inland and international waters.
VHF Marine Radio Guide
Channel assignments, MAYDAY procedure, DSC, GMDSS, and FCC licensing.
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