Meteorology at Sea Flashcards

What is the difference between a cold front and a warm front, and how do they affect weather conditions at sea?

Answer: A cold front is a boundary where cold air undercuts warm air causing rapid weather changes with heavy showers and squalls; a warm front is where warm air rises over cold air causing prolonged rain and gradually lowering cloud

A cold front occurs where advancing cold air undercuts warmer air, forcing rapid uplift. This produces cumulonimbus clouds, heavy showers, squalls, possible thunderstorms, and a sharp wind veer. A warm front is where advancing warm air gradually rises over retreating cold air, producing a sequence of high cirrus, then stratus, then nimbostratus with steady, prolonged rain and gradually deteriorating visibility.

What causes sea fog (advection fog) and in which conditions is it most commonly encountered in UK waters?

Answer: Warm, moist air moving over a colder sea surface, common in spring and early summer when sea temperatures are still low

Sea fog (advection fog) forms when warm, moist air moves over a colder sea surface. The air is cooled below its dew point by contact with the cold water, causing condensation at or near the surface. In UK waters, this is most common in spring and early summer when air temperatures have risen but sea surface temperatures remain low from winter cooling. It can persist for days and is not dispersed by wind alone.

What is the significance of the 'diurnal variation' of barometric pressure for a mariner in tropical waters?

Answer: A regular daily oscillation of about 1-3 hPa — disruption of this pattern is an early warning of tropical cyclone approach

In tropical waters, the barometer shows a regular daily (diurnal) variation: pressure rises around 1000 and 2200 local time, and falls around 0400 and 1600. This oscillation is typically 1-3 hPa. When this regular pattern is disrupted or absent, it is one of the earliest and most reliable indicators that a tropical cyclone may be approaching, often detectable before any other signs.

A vessel in the North Atlantic observes the wind backing from west to south-west and the barometer falling steadily. What weather system is most likely approaching?

Answer: A depression (low-pressure system) with its warm front approaching from the west

A backing wind (anticlockwise shift) combined with a steadily falling barometer in the North Atlantic indicates the approach of a depression from the west. The warm front precedes the depression centre, and as it approaches, the wind typically backs and the barometer falls. After the warm front passes, conditions temporarily improve before the cold front arrives.

What is the difference between 'wind speed' and 'gust speed' as reported in marine weather forecasts?

Answer: Wind speed is the sustained mean speed over 10 minutes; gust speed is a brief maximum lasting 3-5 seconds that can exceed the mean by 40% or more

In marine meteorology, the reported wind speed is the mean speed averaged over 10 minutes, measured at 10 metres above sea level. Gusts are brief increases in wind speed, typically lasting 3-5 seconds, that can exceed the mean by 40% or more in unstable conditions. Squalls are longer-lasting increases. The ratio of gust to mean speed is higher in unstable conditions and near land.

What are 'isobars' on a weather chart and what can a mariner deduce from their spacing?

Answer: Lines of equal pressure — closer spacing indicates stronger wind (steeper pressure gradient)

Isobars are lines joining points of equal atmospheric pressure on a weather chart, typically drawn at 4 hPa intervals. Closely spaced isobars indicate a steep pressure gradient, which produces stronger winds. Widely spaced isobars indicate a slack pressure gradient and lighter winds. The geostrophic wind blows parallel to the isobars (in the absence of friction), and surface wind is typically 60-70% of geostrophic speed, backed about 15° over the sea.