Meteorology

Pressure systems:

- High pressure (anticyclone): descending air, clear skies, fair weather. In the Northern Hemisphere, winds rotate clockwise around a high (anticyclonic). - Low pressure (cyclone/depression): ascending air, cloud formation, precipitation, strong winds. In the Northern Hemisphere, winds rotate counter-clockwise around a low (cyclonic). - In the Southern Hemisphere, these rotation directions are reversed.

Isobars:

Lines on a weather chart connecting points of equal barometric pressure. Closely spaced isobars = steep pressure gradient = strong winds. Widely spaced isobars = gentle gradient = light winds.

Buys Ballot's Law:

If you stand with your back to the wind in the Northern Hemisphere, the low pressure is to your LEFT. In the Southern Hemisphere, the low is to your RIGHT. Useful for determining the location of a low and predicting wind shifts. This is the single most-tested meteorology fact on the USCG exam.

Pressure tendency:

A falling barometer indicates approaching low pressure or a frontal system — deteriorating weather. A rising barometer indicates improving weather. A rapidly falling barometer (more than 3 mb/hour) indicates an approaching storm. A rapid rise after a fall may indicate a passing storm and improving conditions.

Station model (weather map):

Standard weather observations on synoptic charts include: current pressure (3-digit, e.g., 156 = 1015.6 mb), pressure change over 3 hours, pressure tendency (rising/falling/steady), temperature, dew point, wind direction and speed (barbs), sky cover, weather present.

Cold front:

A cold air mass advancing and displacing warmer air. Cold fronts are steeper and move faster than warm fronts (20–35 knots). Weather: narrow band of heavy precipitation, thunderstorms, rapid wind shift from SW to NW (Northern Hemisphere) as front passes, sudden pressure rise, temperature drop, visibility improvement after frontal passage. Depicted on charts as a blue line with triangles pointing in the direction of movement.

Warm front:

A warm air mass advancing over retreating colder air. Warm fronts are shallow and slow-moving (10–25 knots). Weather: widespread stratiform precipitation well ahead of the surface front (sometimes 600+ miles). Sequence as warm front approaches: cirrus → cirrostratus → altostratus → nimbostratus. Visibility poor in rain and drizzle. Wind shifts from SE/E to S/SW as front passes. Depicted as a red line with semicircles.

Occluded front:

Formed when a cold front overtakes a warm front (the fast-moving cold front catches the slower warm front). The warm air is lifted off the surface — the occlusion. Weather can be complex — a mix of cold and warm front characteristics. Depicted as a purple line with alternating triangles and semicircles.

Stationary front:

A front with little or no movement. Persistent cloudiness, precipitation, and poor visibility along the front. Depicted with alternating blue triangles (cold side) and red semicircles (warm side) on opposite sides of the line.

Pre-frontal weather sequence (approaching warm front from the east):

1. Cirrus clouds (≥6 hours ahead) 2. Cirrostratus (halo around Sun or Moon) 3. Altostratus (Sun visible through thin overcast) 4. Nimbostratus (continuous rain begins) 5. Frontal passage: rain stops, wind shifts, temperature rises 6. Post-frontal: stratus/fog possible in warm sector

Radiation fog (ground fog):

Forms over land on calm, clear nights when the surface radiates heat rapidly. The air near the surface cools to the dew point. Conditions: clear sky (allows maximum radiation), calm winds (light mixing would disperse fog; gale-force winds prevent it), high relative humidity. Typically forms after midnight and burns off mid-morning as solar heating increases. Does NOT form over water (water does not cool as rapidly as land). Often drifts offshore in river valleys.

Advection fog:

Forms when warm, moist air moves over a cooler surface. The most common type of sea fog. Conditions: warm moist air + cool sea surface. Can persist with wind, unlike radiation fog. The Grand Banks and California coasts are notorious for advection fog (warm Gulf Stream air over cold Labrador Current; or warm Pacific air over the California Current). Advection fog can occur day or night and may persist for days.

Steam fog (Arctic sea smoke):

Cold air moves over relatively warmer water. The water evaporates and the vapor immediately condenses in the cold air above — producing wisps of "steam." Common in polar regions and fall when cold air masses first move over still-warm water. Usually shallow (a few feet to tens of feet). Not typically a navigation hazard unless combined with other conditions.

Frontal fog:

Forms when warm rain falls through cooler air below a warm front, evaporating and supersaturating the cold air mass. Common ahead of warm fronts.

Upslope fog:

Moist air forced upward along rising terrain cools adiabatically to the dew point. Primarily a coastal and river-valley concern when onshore flow meets rising ground.

Fog prediction indicators:

- Dew point depression (temperature − dew point) less than 3°F: fog likely. - Rapidly falling dew point depression after sunset: radiation fog forming. - Warm front approach: watch for frontal fog. - Onshore flow over cold water: advection fog probable.

High clouds (above 20,000 ft):

- Cirrus (Ci): thin, wispy, ice crystals. "Mare's tails." First sign of approaching warm front. - Cirrostratus (Cs): thin sheet of ice-crystal cloud covering the sky; produces halos around Sun and Moon. Precedes warm front rain by 12–24 hours. - Cirrocumulus (Cc): small white puffs in rows. "Mackerel sky." Associated with fair weather or approaching change.

Middle clouds (6,500–20,000 ft):

- Altostratus (As): gray or blue-gray sheet, blocks the Sun ("ground glass" effect, no distinct shadow). Continuous rain or snow. - Altocumulus (Ac): gray puffs or rolls in patches or waves. "Mackerel sky" at middle level. Altocumulus castellanus (towers) in the morning = afternoon thunderstorms.

Low clouds (below 6,500 ft):

- Stratus (St): low, featureless gray layer, like fog that doesn't touch the ground. Drizzle possible. Associated with prolonged overcast and fog. - Stratocumulus (Sc): rolls or patches of low gray cloud. Most common cloud worldwide. Usually benign. - Nimbostratus (Ns): dark, featureless rain cloud. Continuous, moderate to heavy rain or snow. Associated with warm fronts.

Vertical development clouds:

- Cumulus (Cu): heap clouds with flat bases and rounded tops. Fair weather cumulus = benign (flat tops). Towering cumulus = precursor to cumulonimbus. - Cumulonimbus (Cb): "king of clouds." Anvil-shaped tops reaching the tropopause. Thunderstorms, hail, lightning, microbursts, waterspouts. Avoid on all sides. Steer 40–60 nm clear of a mature Cb.

Practical cloud reading for mariners:

- Halos around Sun/Moon → cirrostratus → warm front approaching - "Red sky at night, sailor's delight; red sky in morning, sailor's warning" → valid for temperate western-wind-belt weather systems moving west to east - Rapidly building cumulus → instability, afternoon thunderstorms likely - Low, dark, fast-moving clouds → frontal system close

Global wind patterns:

The general circulation of the atmosphere produces predictable large-scale wind belts: - 0°–30° N/S: Trade winds. Northeast in the Northern Hemisphere, Southeast in the Southern Hemisphere. Consistent, reliable winds used by square-riggers for centuries. - 30°–60° N/S: Prevailing westerlies. Variable, often stormy. The "roaring forties," "furious fifties," "screaming sixties" of the Southern Ocean. - 60°–90° N/S: Polar easterlies. Cold winds from the poles. - 30° N/S: Horse latitudes (subtropical highs). Light, variable winds; calms. - 0°: Doldrums (intertropical convergence zone, ITCZ). Calms and squalls.

Sea breeze / land breeze:

A diurnal (daily) cycle driven by differential heating: - Daytime: land heats faster than water → warm air rises over land → cool sea air flows onshore = sea breeze (from sea to land, afternoon phenomenon). - Nighttime: land cools faster → air flows from land to sea = land breeze (light, overnight). Sea breezes can reach 15–20 knots by late afternoon along open coasts.

Katabatic and anabatic winds:

- Katabatic: cold, dense air drains downhill from elevated terrain. Can be violent (Santa Ana, Bora, Williwaw). Williwaws in Alaskan fjords can gust to 100 knots. - Anabatic: warm air rises along sun-heated slopes; lighter and more predictable.

Thermal wind:

Created by horizontal temperature gradients. Common in coastal zones and channels (e.g., San Francisco Bay funnel effect, channeled winds in straits).

Wind speed definitions:

- Calm: 0–1 knot - Light breeze: 1–6 knots - Moderate breeze: 11–16 knots - Fresh breeze: 17–21 knots - Near gale: 28–33 knots - Gale: 34–47 knots - Storm: 48–63 knots - Hurricane force: ≥64 knots

Development stages:

1. Tropical disturbance: area of thunderstorm activity, no closed circulation 2. Tropical depression: closed circulation, sustained winds ≤33 knots 3. Tropical storm: organized circulation, sustained winds 34–63 knots 4. Hurricane/typhoon: sustained winds ≥64 knots (Category 1–5 on Saffir-Simpson scale)

Structure:

- Eye: calm center, 20–40 nm diameter, light winds, descending air, partly clear sky - Eyewall: most intense winds and rain, immediately surrounding the eye - Rainbands: spiral bands of heavy rain extending hundreds of miles from the center

The dangerous semicircle (Northern Hemisphere):

The right side (northeast quadrant) of a hurricane is more dangerous than the left side because: - Cyclonic wind speed adds to the forward speed of the storm - Vessels in the right semicircle are pushed toward the storm track by the wind - Sea conditions are rougher The left (southern) side is the "navigable semicircle" — conditions are still severe but more survivable.

Navigating near a tropical cyclone (Northern Hemisphere):

The classic mariner's rules: - If wind veers (shifts clockwise) → you are in the right (dangerous) semicircle. Put wind on starboard quarter, make best speed away from the storm. - If wind backs (shifts counter-clockwise) → you are in the left (navigable) semicircle. Put wind on starboard bow. - If wind holds steady with increasing force → you are near the track (bow or stern). Put wind on starboard quarter immediately and make all speed possible.

In the Southern Hemisphere:

Left semicircle is the dangerous one; directions are reversed.

Hurricane avoidance distance:

Modern guidance: remain at least 250–300 nm from the center of a Category 3+ hurricane. The danger radius extends well beyond the eyewall.

NOAA Weather Radio (NWR):

Continuous 24/7 marine weather broadcasts on VHF channels WX-1 (162.400 MHz), WX-2 (162.425 MHz), WX-3 (162.450 MHz), and others. Broadcasts include: marine forecasts, coastal forecasts, offshore forecasts, high seas forecasts, and special marine warnings.

Forecast zones:

- Coastal waters forecast: nearshore waters, typically to 25 nm offshore - Offshore waters forecast: 25–250 nm offshore - High seas forecast: beyond 250 nm, by ocean zone (Atlantic, Pacific, Gulf) - Special marine warning (SMW): sudden hazardous conditions — thunderstorms, waterspouts, winds ≥34 knots (not covered by regular forecast)

NAVTEX:

Automated printout of maritime safety information on 518 kHz (international) and 490 kHz (U.S. domestic). Includes weather warnings, navigational warnings, search and rescue information. Range approximately 400 nm. Required equipment on SOLAS vessels.

SafetyNET:

Inmarsat-based broadcast of maritime safety information, including weather, for offshore and ocean areas. Part of GMDSS.

Hurricane forecasts (NHC):

National Hurricane Center issues: tropical weather outlooks, tropical storm/hurricane watches and warnings, advisories (every 3–6 hours during active storms), and the official forecast track (cone of uncertainty).

Reading a marine forecast:

Standard format: location/zone, synopsis, wind, sea state (significant wave height), visibility, swell direction/period. "SEAS 8 TO 12 FT" means significant wave height (the average of the highest one-third of waves) is 8–12 feet — individual waves can be twice this height.

Significant wave height:

The average height of the highest 1/3 of all waves. The occasional wave may be up to 1.86 times the significant wave height. A vessel in 12-foot significant seas may encounter individual waves of 20+ feet.