On-board weather forecasting

Kenn Batt, Senior Meteorologist for the Bureau of Meteorology, explains how you can use your own on-board observations to accurately predict the weather.

A question asked of me from time to time is - How has vessel-board weather forecasting changed over the years? It’s a complex answer but the simplest answer is – it has changed for the better over the years. Before I go on to elaborate, let’s take a peek at a much potted history of the art and science of meteorology. This may help us to understand where I’m coming from.

It’s safe for us to assume that our very early ancestors acquired weather wisdom by observing weather sequences and noting the foreshadowed effects of certain atmospheric conditions on objects animate or inanimate. The knowledge thus acquired was communicated to their descendants and that it was handed down, with additions and amplifications, from generation to generation.

The wisdom thus acquired has been perpetuated in the form of sayings or proverbs; some of these sayings are true, others have lost their potency by transfer to places in the world where dissimilar climatic conditions exist and some were simply superstition.

Seafarers have long been renowned for their use of weather folklore to assist them in their weather forecasting endeavours. Some examples are below:

“Mackerel scales and mare’s tails, make lofty ship’s carry low sails”

Refers to high-level cloud signaling the approach of a cold front and thus stronger winds. We know that some cold fronts move through cloud-free and some move through as very weak features and as such not a lot of wind. High-level jet stream winds can also produce high-level cloud without a cold front being anywhere near your area and light winds being experienced at the surface.

“Red sky at night sailor’s delight, red sky in the morning sailor’s warning”

Probably the one that has best stood the test of time in different parts of the world. It refers to the passage of a cold front and has more potency in temperate to high latitudes than low latitudes.

These sayings probably worked quite well in the UK and in some European waters. But once these seafarers started to move further afield, they would have (and still do) discovered that these sayings became a very much hit and miss affair. Early meteorology was very much an art.

During the 1970’s and 80’s, meteorology world-wide grew in “leaps and bounds” to become the science that it is today. Much of this growth can be attributed to computers.

This brings me back to the answer to the question that was posed in the opening paragraph, namely, vessel-board weather forecasting has changed for the better over the years. The degree of change has varied greatly from one vessel to the next. Essentially it’s a function of how much money boat owners have been prepared to fork out for technology!

OK to start with, in the early days of sail, before the telegraph and the radio. Mariners had no way of obtaining observations from other vessels at sea or locations. The shipping company owner would have paid top money in order to obtain the best captain. These captains then had no choice but to adopt single station weather forecasting techniques (SSWFT) - observing the clouds and other signs around them and then using their local knowledge and folklore - in order to derive their own forecast.

Once the telegraph, radio, robust weather forecast systems and knowledge came along, then it depended on how much money the owner could spend on receiving systems and over time, onshore human weather routers in order to receive weather bulletins, maps (via radio facsimile) and forecasts at sea. Up until recent times, this was the domain of commercial shipping. The owners had the big money required to buy these systems. Over the last ten years or so however, there has been a move to sophisticated systems (radio transceivers, computers and associated software) on cruising and racing yachts as the price of these electronic marvels has kept coming down.

Nevertheless, there is still a plethora of craft that put to sea almost daily with only a radio onboard (a lot without) in order to receive broadcast weather forecasts and warnings. The skippers of these craft have to resort to a blend of the “official” forecast and SSWFT.

For those mariners without any means of obtaining outside weather information then, they have to adopt SSWFT in much the same way as the early seafarers had to.

You will now ask - how do I find out more about this thing called single station weather forecasting?

SSWFT is essentially the regular keeping of weather observations. These observations should be performed at least every three hours. Observations should include the following:

• wind direction and speed

• cloud types

• barometric pressure (actual)

• barometric tendency  (how quickly the barometer is rising and more particularly falling) and

• sea and swell conditions.

By logging the above variables and noting any trends, one should be able to build a picture of what is going on around them. In particular it is the observation of cloud, followed by the barometric tendency that will give you an indication of any impending significant changes.

Become very familiar with various cloud forms. There are many good weather books and cloud charts that will help you. When you are happy with this aspect (it takes time) then you can start to concentrate on cloud sequences or trends. It is the sequence or trend that is very important for forecasting. An example of a must know cloud trend (outside of the tropics) is that associated with the passage of a cold front.

It goes like this, 20 to 36 hours before the arrival of the cold front you will generally observe high-level clouds, 6 to 12 hours before-hand middle-level cloud will be observed, and marking the leading edge of the front at the surface will be our low-level clouds. But remember that from the point of view of cloud and weather, each cold front will be different, and that some cold fronts are cloud free.

The aneroid barometer is your main instrument at sea that will sense significant changes. It is the tendency (rise or fall over a time interval) of the pressure that is most important and not the instantaneous value. For example, a pressure rise or fall of say 6hPa or more, over a 3 hour period will indicate that, either at least strong winds or greater are currently  being observed, or they are just around the corner. The greater the pressure change over the three hour period, the stronger the winds and faster their onset. SSWFT is good out to about 24hours, after which the accuracy drops.

Using these simple techniques, you can become a competent on-board weather forecaster.