Marine Weather Lab

Approach

Warm air replaces cooler air and rises gradually over it. Cloud and precipitation often spread ahead of the surface front, giving useful signs before passage, but the pattern varies with the system.

Passage

In this illustrative sequence, rain eases and temperature rises near passage. In real cases the pressure, wind, precipitation and temperature changes depend on the front’s strength and local conditions.

Behind it

Air behind the front is often warmer and more humid. Low cloud, mist or drizzle may persist; visibility depends on moisture, wind and sea temperature.

Sailor’s signs

• Barometer: pressure often falls ahead and steadies or rises after passage; the rate varies.
• Wind: in a Northern Hemisphere depression it may back before passage and often veer as the front crosses; direction and amount vary.
• Temperature: it often rises behind a warm front, but the size and timing vary.
• Visibility: it may fall in precipitation or frontal fog where warm rain moistens cooler air.
• Sea state: a swell may arrive before local wind changes, depending on the storm’s fetch.

What to do

• Prepare the vessel and crew before conditions deteriorate, using the current forecast and the vessel’s procedures.
• Fix your position and log it before visibility closes in.
• Log pressure regularly and compare its trend with the current forecast and other observations.
• Allow for a possible wind shift when reviewing the passage plan.

Risks

• Long hours of rain and closed visibility with a crew that is already tired.
• Sea fog in the warm sector, over a cold sea, right when you want to make a landfall.
• A lee shore becoming dangerous as the wind builds hours before the front arrives.

Approach

Cooler, denser air undercuts warmer air. A narrow band of showers or thunderstorms may occur near the front, but precipitation can be ahead, behind or absent.

Passage

In this illustrative sequence, passage brings a sharp wind shift and heavy showers. Real cold fronts range from weak to squall-producing.

Behind it

Air behind is often cooler and drier; showers and improved visibility may follow, but conditions depend on the air mass and sea temperature.

Sailor’s signs

• Barometer: the fall often stops and pressure begins to rise; timing and magnitude vary.
• Wind: a marked shift is common near a cold front in the Northern Hemisphere, but its direction and size depend on the pressure pattern.
• Temperature: it often falls behind the front, but the change may be small or delayed.
• Visibility: showers can reduce visibility, with clearer intervals possible between them.
• Sea state: a cross sea may develop when the new wind meets the earlier swell.

What to do

• Prepare for possible squalls before the cloud band arrives; available warning time varies.
• Secure the vessel and use the appropriate crew safety equipment before conditions worsen.
• Choose sail trim and steering for the vessel, point of sail, sea room and conditions; there is no universal squall manoeuvre.
• Allow for a possible veer, and avoid letting the new wind put you on a lee shore.

Risks

• A violent gust and wind shift together, which is how boats are knocked down and gear is broken.
• A confused cross sea for several hours after the passage.
• Hail, lightning and near-zero visibility in the squall itself.

Approach

A cold front can catch a warm front and lift the warm sector away from the surface. The air masses determine whether the occlusion has more warm-front or cold-front character.

Passage

The fronts may appear as one band. Rain, showers, wind change and pressure response vary; embedded convection can occur within the cloud and precipitation.

Behind it

Conditions often improve gradually, but an occlusion can resemble either a warm or a cold front. Cloud, rain and temperature changes depend on the air masses.

Sailor’s signs

• Barometer: pressure may be near a local minimum and then rise, but the pattern depends on the depression.
• Wind: a change may occur, with direction and timing set by the surrounding pressure pattern and air masses.
• Temperature: the change can be small when similar cool air masses meet, or more marked in another occlusion.
• Visibility: precipitation and cloud may reduce it; improvement depends on the band’s structure and wind.
• Sea state: an older swell can remain after the depression starts to fill.

What to do

• Do not assume a warm sector; a missing temperature rise can support an occlusion diagnosis when other signs agree.
• Allow for a potentially prolonged rain band and manage crew fatigue.
• Maintain a proper lookout and use radar when fitted and operational; embedded showers can contain squalls.
• Check the forecast and observations before judging that conditions are improving; a pressure rise alone is insufficient.

Risks

• A long exposure to rough conditions that wears down the crew rather than testing them briefly.
• Embedded cumulonimbus hidden inside a uniform rain shield.
• A false clearance: the sky brightens, then the next band arrives.

Approach

Neither air mass advances decisively. Winds can run along the boundary and cloud or precipitation may persist, but the structure and duration vary from case to case.

Passage

There may be no clear passage. Conditions on one side can continue while waves or disturbances develop along the boundary.

Behind it

There is no defined “behind” until the boundary moves or a wave develops. A new disturbance can change the weather quickly.

Sailor’s signs

• Barometer: it may remain nearly steady, but a wave can introduce a clear fall or rise.
• Wind: it can run parallel to the front; direction and change depend on the local pressure pattern.
• Temperature: it may remain stable on one side while contrasting air lies nearby.
• Visibility: cloud or precipitation can keep it moderate or poor along the boundary.
• Sea state: it can build with fetch and duration even without a frontal passage.

What to do

• Monitor the pressure pattern and observations; little frontal movement does not guarantee unchanged weather.
• Watch for a wave developing along the boundary; some waves deepen into depressions.
• Review the forecast and available sea room before relying on a persistent wind direction.
• Keep logging pressure; a new tendency may signal a changing pressure pattern.

Risks

• Days of rain and poor visibility with no improvement to sail towards.
• A frontal wave deepening quickly into a new low right on top of you.
• Complacency, because nothing has changed for two days.