Frank Singleton - Skippers Cockpit Weather Guide

Contents

All water activities carry risk and sailing, cruising in particular, is no exception. Royal Yachting Association training courses and publications emphasise the need for preparation before going to sea. In doing so, they place weather first in a list that includes vessel and crew suitability and ability, navigation, pilotage, tides and currents. This book will follow broadly, and expand upon, the Yachtmaster and Coastal Skipper weather syllabus. As concisely as possible, it covers terminology, air masses, cloud types, weather patterns, sources of weather information, use of forecasts, land and sea breezes, sea fog, use of the barometer and other observations. A section has been added on sea state and water levels, omissions from the RYA course syllabus. There are few absolutes in meteorology and words like might, could and can happen will frequently be used rather than will happen. That is the nature of weather.

Science and technology have combined to make large-scale weather prediction far better than even ten years ago. It is significant that weather is not now a prime cause of Royal National Lifeboat Institution call-outs. That may not be surprising, but there are still limitations in weather predictability. Sailors have to be aware of short-lived, small-scale details that are not usefully predictable. No marine weather forecast will be totally accurate. Using forecasts is really about risk assessment.

General weather terms

The troposphere is where our weather occurs, up to 912 km above the surface in mid-latitudes, higher in the tropics.

The tropopause is the top of the troposphere where temperature stops decreasing with height in a temperature inversion; it may become warmer above the tropopause.

Depression and low (pressure) are synonymous.

Winds, anticlockwise around a low, are cyclonic.

But the word cyclone usually refers to a tropical storm.

Anticyclone and high (pressure) are also synonymous.

Winds, clockwise around a high, are anticyclonic.

Fronts are in low pressure troughs, which may or may not be well marked.

Troughs of low pressure do not always have fronts.

Air is unstable if warming low down creates convection.

Precipitation is an omnibus word describing rain, drizzle, sleet, snow or hail.

These words are a useful source of information because they describe why the precipitation is occurring

rain, sleet or snow are usually from frontal cloud

drizzle is light, usually steady precipitation from low level, layered cloud, often in warm sectors of frontal lows

showers fall from convective cloud and can be rain, hail, sleet or snow.

Global NWP models run by the international meteorological community

ECMWF High resolution, usually just ECMWF

GFS, NOAA

ICON, DWD

UK UM, Unified Model, a multipurpose model

ARPEGE, France

GEM, Canada

NAVGEM, US Navy

NEMS, NOAA multipurpose modelling system.

Some regional and national area models

ICON-EU Regional version of ICON

UK4 Regional version of UM

UKV British Isles version of UM

COAMPS US Navy regional

COSMO German waters version of ICON

AROME French waters

WRF US Research and development model, freeware

NMM/NAM/HRRR US detailed models

HIRLAM European consortium NWP model

HARMONIE and ALARO Derivatives of HIRLAM

Weather forecast terminology

Some terms are agreed internationally, those describing wind, visibility and sea state for example. Words listed here are as used by the UK Met Office. Other countries will use similar words.

Marine forecasts describe winds in Beaufort forces, knots (kt) or metres/second (m/s). Some unofficial marine forecasts may use kilometres/hour (km/h). Douglas scale descriptions are used for sea state.