Barometer readings, weather fronts, Beaufort scale, cloud types, fog formation, and NOAA forecast warnings — everything the OUPV exam tests and every skipper needs at sea.
Weather accounts for roughly 10% of the Navigation General section. The exam tests barometer interpretation, front identification, the Beaufort scale, cloud types, and NOAA warning levels. Beyond the exam, weather judgment is one of the most critical seamanship skills — a skipper who can read a falling barometer, identify a cold front on approach, or recognize cumulonimbus build-up can keep a crew safe. Know the numbers cold.
~10%
of Navigation General exam questions cover weather topics
Beaufort 8
Gale force — 34–47 knots — the threshold every skipper must know
29.00"
Below this barometric pressure signals stormy, unsettled conditions
Atmospheric pressure is measured in inches of mercury (inHg). The trend matters as much as the number — watch rate of change, not just the current reading.
High pressure — above 30.00"
High pressure systems bring fair weather and stable conditions. Skies are clear or partly cloudy, winds are light and variable. A steady high is a green light for offshore passages.
Low pressure — below 29.00"
Low pressure systems are associated with stormy, unsettled conditions — clouds, precipitation, stronger winds. The lower the reading, the more intense the system.
Rapidly falling barometer
A fast-dropping barometer is one of the clearest storm warning signs. A drop of 0.06" or more per hour signals an incoming storm system. The faster the fall, the more severe and rapidly approaching the weather.
Rapidly rising barometer
A rising barometer signals improving weather as a system clears. However, rapid rises after a frontal passage can bring strong northwest winds. Improving does not mean immediately calm.
A front is the boundary between two air masses of different temperature and humidity. Front type determines how fast weather arrives, how severe it is, and how long it lasts.
Wind is backing when it shifts counter-clockwise (e.g., SW to S to SE). In the Northern Hemisphere, backing winds indicate worsening weather — a warm front or low pressure system approaching. If the wind backs, conditions are likely to deteriorate.
Wind is veering when it shifts clockwise (e.g., S to SW to W to NW). In the Northern Hemisphere, veering winds indicate improving weather — a cold front has passed and high pressure is building. Post-cold-front NW winds are a classic veering sequence.
Stand with the wind at your back. Low pressure is to your left. High pressure is to your right. This works because winds circulate counter-clockwise around low pressure systems in the Northern Hemisphere.
Memory anchor: "Wind on your back — Low is Left."
A wind-force scale from 0 to 12 used to estimate wind speed by observing sea conditions. Know Forces 0, 4–5, 6–7, 8–9, and 12 for the exam.
| Force | Description | Wind Speed | Sea Conditions |
|---|---|---|---|
| 0 | Calm | <1 kt | Mirror-like sea — no ripples |
| 1–3 | Light / Gentle | 1–10 kts | Small wavelets, crests do not break |
| 4–5 | Moderate / Fresh | 11–21 kts | Whitecaps common, some spray |
| 6–7 | Strong / Near Gale | 22–33 kts | Spray, white foam streaks, larger waves |
| 8–9 | Gale / Strong Gale | 34–47 kts | High waves, dense spray, reduced visibility |
| 10–11 | Storm / Violent Storm | 48–63 kts | Very high waves, sea appears white |
| 12 | Hurricane | 64+ kts | Phenomenal seas, air filled with foam and spray |
Exam anchor — Force 8 threshold
Force 8 (34 knots) is the gale threshold and triggers a Gale Warning from NOAA. Force 12 (64+ knots) is the hurricane threshold. Small craft advisories begin at Force 4–5 (21 knots or seas 4 ft). Know these breakpoints cold.
Cirrus
—Weather change likely in 24–48 hours
High altitude ice clouds — thin, feathery streaks. Often the first sign of an approaching warm front. Fair weather now, but note the trend.
Cirrostratus
—Rain likely within 24 hours
A thin, milky veil covering the sky that produces halos around the sun or moon. Classic indicator of a warm front 12–24 hours away. Halo around sun or moon = rain is coming.
Cumulonimbus
—Seek shelter immediately
Massive, anvil-topped clouds extending high into the atmosphere. These are thunderstorm cells — lightning, heavy rain, gusty and shifting winds, possible waterspouts. Get off the water or find protected anchorage immediately.
Stratus
—Fog, drizzle, poor visibility
A flat, featureless gray overcast layer hugging low altitudes. Produces drizzle and reduced visibility. Associated with fog conditions, especially coastal advection fog.
Cumulus
—Stable if small — danger if building vertically
Puffy, cotton-ball clouds with flat bases. Small cumulus in the morning typically means a fair day. If they build vertically into towering cumulus by mid-afternoon, watch for afternoon thunderstorms — especially in summer.
NOAA Weather Radio broadcasts continuously on VHF frequencies 162.400–162.550 MHz. These are the WX channels on your VHF radio:
| Warning Level | Wind Speed | Sea Height | Action |
|---|---|---|---|
| Small Craft Advisory | 21–33 kts | 4–7 ft | Consider staying in port — inexperienced mariners should not go out |
| Gale Warning | 34–47 kts | — | Gale-force winds — seek protected anchorage |
| Storm Warning | 48–63 kts | — | Storm force — all vessels should be in port |
| Hurricane Warning | 64+ kts | — | Evacuate marina if directed — life-threatening conditions |
Small craft advisory threshold: 21 knots / 4 ft seas
The NOAA warning ladder is the most tested weather fact on the OUPV exam: Small Craft Advisory 21–33 kts, Gale Warning 34–47 kts, Storm Warning 48–63 kts, Hurricane Warning 64+ kts. These match the Beaufort scale thresholds exactly — learn them together.
Backing winds = worsening; veering = improving
Counter-clockwise (backing) wind shifts signal deteriorating conditions in the Northern Hemisphere — a low or warm front is approaching. Clockwise (veering) shifts signal improvement — a cold front has passed and high pressure is building. The direction of shift tells you which way the weather is going.
Halo around sun or moon = rain within 24 hours
A halo is caused by cirrostratus clouds — thin ice-crystal sheets at high altitude. Cirrostratus is a classic warm front precursor. If you see a halo, expect rain within 24 hours. 'Halo around the moon, rain before noon' is the sailor's rhyme — exaggerated but useful.
Rapidly falling barometer = storm approaching — act now
A barometer dropping more than 0.06" per hour is a critical warning. Don't wait for the weather to arrive — seek protected anchorage, reef early, or delay departure. The exam often presents scenarios where the correct action is based on the rate of pressure change, not just the current reading.
The Beaufort scale is a numerical wind-force scale from 0 to 12 used to describe wind speed and its visible effects on the sea surface. Force 0 is calm with a mirror-like sea; Force 4–5 (11–21 knots) produces whitecaps; Force 6–7 (22–33 knots) brings spray and qualifies as a strong breeze or near gale; Force 8–9 (34–47 knots) is gale-force with high waves; and Force 12 is hurricane force at 64+ knots. Mariners use the Beaufort scale to estimate wind conditions by observation when instruments are unavailable, and it appears on the OUPV Navigation General exam.
A barometer measures atmospheric pressure in inches of mercury (inHg). Readings above 30.00 inHg indicate high pressure and generally fair, stable weather. Readings below 29.00 inHg indicate low pressure and stormy or unsettled conditions. A rapidly falling barometer means a storm system is approaching — the faster the drop, the more severe the coming weather. A rapidly rising barometer signals improving conditions, but can also bring strong northwest winds after a frontal passage. The trend (direction and rate of change) matters as much as the absolute reading.
A cold front moves fast and brings a line of storms, rapid pressure drop then rise, and a clockwise wind shift (veering) after passage — skies clear quickly. A warm front moves slowly and produces extended periods of rain and fog ahead of it with gradual clearing. A stationary front moves little and causes persistent precipitation over the same area. An occluded front forms when a cold front overtakes a warm front, creating complex and often severe weather. Boaters should monitor VHF weather channels and be particularly alert to cold front passage, which can bring sudden squalls.
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