Mariner’s Web

Mares’ tails and mackerel scales, make tall ships trim their sails

Or, as a variation, Make lofty ships carry low sails.

In the absence of instruments, clouds are the most useful aid to weather forecasting as they are visible clues as to what is going on aloft.  Mares’ tails are hooked-shaped cirrus clouds and mackerel scales are cirrocumulus which appear as bands of fish scales, both high altitude clouds above 20,000 ft.  When both are seen together it is a sign to the mariner of a storm front bringing in a weather change within 24 hours.  A common fair weather cloud that should be familiar with their cauliflower-like appearance of flat bases and domed tops is the vertically formed cumulus cloud.  If they begin to build into mountainous cumulonimbus clouds upwards of 50,000 ft then this is a sure sign of atmospheric instability and likely thunderstorms.  An anvil head may form at the top which indicates the edge of the troposphere and points in the direction of movement of the storm.

If you have trouble identifying clouds try to observe their shape and determine if they are “low, middle, high, or vertical” -  with the help of a cloud chart from NOAA or a good weather book.  There is a fabulous online training course from the National Weather Service (NWS) on the web that presents weather in an easy to understand format with excellent graphics known as Jetstream.

An uncommon but extremely useful instrument that mariners might keep onboard is a sling psychrometer that will measure temperature and dew point which will allow you to calculate the cloud base – or elevation at which clouds are forming in the sky.  It is also invaluable, when anchoring in some regions, for predicting impending fog.

Every hour or so, while you cruise, there is an additional reason for looking aloft.  It is single station forecasting or weather in real time.  Don’t forget to trim you sails…