Weather - Forecasting from the cloudsI am often asked by seasoned old mariners about forecasting from the clouds. This surprises me as mariners who are quite happy using GPS, depth sounders, autohelms, marine radios and various other hi-tech equipment on their vessels want to use the very old and basic technology for a very important part of their boating safety … the weather.

It is true that certain weather systems and atmospheric conditions can give indications of potential future weather. However, misidentification of some cloud types is common by the casual observer. Also some clouds give an indication of broad weather systems being in the vicinity but not of the effects on your area.

There are even books that teach you to forecast the weather by the clouds. Many of these books are for a target audience in northern Europe. This area is a lot higher northern latitude than Australia is south, even places like Cannes and Nice in the south of France are further north than southern Tasmania is south.

The clouds are the same in the southern hemisphere but often the systems that bring the sequences of clouds are located hundreds of nautical miles south of Tasmania, so are of little use to the Australian mariner.

Satellite picture of cyclone Yasi. The “bright white” is mostly dense cirrus cloud. The border of the cirrus cloud can be clearly seen over inland Queensland a long way from the cyclone’s centre. Photos NASA.Here are the first two of my top five weather indicators by cloud type for Australian conditions. The other three will appear next month.

1. Cirrus (dense)

For areas that experience tropical cyclones, during cyclone season.

A visible sign of a tropical cyclone approaching can be a large, dense mass of very white cirrus cloud starting to fill the horizon. This cirrus canopy covers the cyclone.

To the mariner in the middle of the ocean this cloud would not be the first indication of a cyclone. A building short period swell coming from a direction where the only generator of this swell must be a cyclone would be an indicator long before you see the cloud.

Cyclones can vary in size. Yasi was 650km across with a 100km wide eye. Cyclone Tracy was 50km across with a 12 kilometre wide eye. Due to the size of Yasi a mariner sailing towards the eye would have seen a gradual fall in air pressure beginning a long way away from the eye.

Altocumulus Castellanus.Cyclones such as Tracy are termed midgets by meteorologist due to their small size. A mariner approaching a ‘midget’ cyclone may in fact get a small pressure rise on the periphery of the storm followed by a very rapid decrease in pressure and increase in wind speed. This pressure rise is caused by the air that has risen in the centre of the cyclone sinking on the cyclone’s extremities, raising the air pressure.

2. Altocumulus Castellanus

This cloud gives an indication of ‘instability’ in the middle layers of the atmosphere.

If these clouds are seen during the morning it is possible, with the heating of the ground during the day, that any air rising that forms cumulus cloud that moves into the middle layers will tap into the instability and thunderstorms may form.

This cloud also indicates that should a large bushfire occur, the hot air and smoke rising into this layer will be more effectively ‘exhausted’ away which in turn increases the inflow of air/oxygen into the fire.

 

Next month Cirrostratus, Cumulonimbus and Cirrus (mare’s tales).

 

*Malcolm Riley is the Public and Marine Officer for the Bureau of Meteorology in Hobart. He has worked in all States with the exception of Qld and is a Master V. He gives education courses on Marine Meteorology.